AN  INTRODUCTION  TO  GEOLOGY 


THE  MACMILLAN  COMPANY 

NEW  YORK  •    BOSTON  .   CHICAGO 
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MACMILLAN  &  CO.,  LIMITED 

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THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


The  Navajo  Church,  New  Mexico ;  wind  sculpture  of  cross-bedded  sandstone 


AN  INTRODUCTION  TO 
GEOLOGY 

SECOND  EDITION    REVISED  THROUGHOUT 


BY 

WILLIAM    B.   SCOTT,  PH.D.,  LL.D. 

BLAIR  PROFESSOR  OF  GEOLOGY  AND   PALAEONTOLOGY 
IN  PRINCETON   UNIVERSITY     ',      ',    . 


There  rolls  the  deep  where  grew  the  tree. 

O  earth  what  changes  hast  thou  seen ! 

There  where  the  long  street  roars,  hath  been 
The  stillness  of  the  central  sea. 

1  The  hills  are  shadows,  and  they  flow 
From  form  to  form  and  nothing  stands; 
They  melt  like  mists,  the  solid  lands, 

Like  clouds  they  shape  themselves  and  go." 


With  numerous  illustrations  from  drawings  by  BRUCE  HORSFALL., 
and  from  many  new  photographs. 


gorfe 
THE    MACMILLAN   COMPANY 

1921 

111  rights  resentd 


«'«  •t*1  COPYRIGHT,  1897,  J9°7» 

BY  THE  MACMILLAN  COMPANY. 


New  edition  revised.      Set  up  and  electrotyped. 
Published  November,  1907. 


TO 

A.  A.  P.  S. 

Book  10  ©etitcateti 

IN   GRATEFUL  RECOGNITION   OF  AN   EVER   READY 
AND  INSPIRING  SYMPATHY 


FROM  THE  PREFACE  TO  THE  FIRST 
EDITION 

THIS  book  had  its  origin  in  the  attempt  to  write  an  introductory 
work,  dealing  principally  with  American  Geology,  upon  the  lines 
of  Sir  Archibald  Geikie's  excellent  little  "  Class-Book."  In  spite 
of  vigorous  efforts  at  compression,  it  has  expanded  to  its  present 
size,  though  the  difference  from  the  "  Class- Book,"  in  this  respect, 
lies  not  so  much  in  the  quantity  of  matter  as  in  the  larger  size  of 
the  type  and  illustrations. 

The  book  is  intended  to  serve  as  an  introduction  to  the  science 
of  Geology,  both  for  students  who  desire  to  pursue  the  subject 
exhaustively,  and  also  for  the  much  larger  class  of  those  who  wish 
merely  to  obtain  an  outline  of  the  methods  and  principal  results 
of  the  science.  To  the  future  specialist  it  will  be  of  advantage  to 
go  over  the  whole  ground  in  an  elementary  course,  so  that  he 
may  appreciate  the  relative  significance  of  the  various  parts,  and 
their  bearing  upon  one  another.  This  accomplished,  he  may 
pursue  his  chosen  branch  much  more  intelligently  than  if  he  were 
to  confine  his  attention  exclusively  to  that  branch  from  the  begin- 
ning of  his  studies. 

Students,  and  only  too  often  their  instructors,  are  apt  to  prefer 
a  text-book  upon  which  they  can  lean  with  implicit  confidence, 
and  which  never  leaves  them  in  doubt  upon  any  subject,  but  is 
always  ready  to  pronounce  a  definite  and  final  opinion.  They 
dislike  being  called  upon  to  weigh  evidence  and  balance  proba- 
bilities, and  to  suspend  judgment  when  the  testimony  is  insufficient 
to  justify  a  decision.  This  is  a  habit  of  mind  which  should  be 
discouraged  ;  for  it  deludes  the  learner  into  the  belief  that  he 
knows  the  subject  when  he  has  only  acquired  sorre  one's  opinions 


Viii         FROM  THE  PREFACE  TO  THE   FIRST  EDITION 

and  dogmas,  and  renders  further  progress  exceedingly  difficult  to 
him.  In  no  science  are  there  more  open  questions  than  in 
Geology,  in  none  are  changes  of  view  more  frequent,  and  in  none, 
consequently,  is  it  more  important  to  emphasize  the  distinction 
between  fact  and  inference,  between  observation  and  hypothesis. 
An  open-minded  hospitality  for  new  facts  is  essential  to  intellectual 
advance. 

******* 

In  preparing  this  book,  I  have  of  course  availed  myself  of 
material  wherever  it  was  to  be  found,  but  I  wish  to  acknowledge 
my  special  obligations  to  the  text-books  of  Dana,  Le  Conte, 
Geikie,  Green,  Prestwich,  Credner,  Kayser,  Neumayr,  Koken,  de 
Lapparent,  and  Jukes-Brown.  From  the  last-named  writer  is 
taken  the  arrangement  of  the  Dynamical  Agencies,  which  expe- 
rience in  the  class-room  has  led  me  to  consider  as  the  best. 

PRINCETON,  N.J., 
Jan.  15,  1897. 


PREFACE  TO   THE  SECOND  EDITION 

THE  ten  years  that  have  passed  since  the  first  publication  of  this 
book  have  been  years  fruitful  of  results  in  geological  knowledge. 
Some  departments  of  the  subject  have  been  fairly  revolutionized 
and  in  all  there  has  been  great  progress,  so  that  any  text-book 
ten  years  old  is  necessarily  left  far  behind  in  the  general  advance. 
Revision,  indeed  rewriting,  had  become  imperative  to  incorporate 
the  most  important  and  significant  parts  of  the  newer  results,  as 
well  as  to  remove  as  many  of  the  defects  as  I  might  be  able  to  do. 
The  increase  in  size  is  an  extremely  regrettable  feature,  but  I  have 
not  seen  my  way  to  avoid  it,  for  it  is  largely  due  to  the  much  greater 
number  of  illustrations,  and  these  were  needed  in  the  interests  of 
clearness. 

While  many  minor  changes  have  been  made,  the  general  plan 
of  the  book  remains  the  same,  for  experience  has  convinced  me 
that  the  somewhat  rigidly  conventional  arrangement  of  topics, 
which  has  sufficiently  evident  drawbacks,  is  of  actual  assistance 
to  the  beginner.  It  avoids  confusing  him  by  any  premature 
attempt  to  point  out  the  infinitely  ramifying  relations  of  every 
fact  of  nature.  One  of  the  keenest  pleasures  of  intellectual  growth 
is  the  continual  discovery  of  these  unsuspected  relations,  but  for 
ihe  beginner  the  simpler  and  more  obvious  line  of  reasoning  is 
the  more  profitable. 

The  labour  of  revision  has  been  greatly  lightened  by  those 
admirable  store-houses  of  geological  learning  :  the  second  edition 
of  Kayser's  "  Lehrbuch,"  the  fourth  edition  of  Geikie's  "  Text- 
Book,"  and  the  "  Geology  "  of  Chamberlin  and  Salisbury.  To  all 
of  these  my  obligations  are  great,  especially  for  the  bibliographies 
which  they  contajn,  ancl  which  have  rendered  the  collection  of  the 


X  PREFACE  TO  THE   SECOND   EDITION 

newer  technical  literature  of  monographs  and  papers  a  much  less 
onerous  task  than  it  could  otherwise  have  been. 

In  this  new  edition  I  have  introduced  a  very  considerable  num- 
ber of  brief  quotations,  at  the  request  of  some  of  those  who  have 
employed  the  book  as  a  convenient  work  of  reference  and  who 
desire  to  know  the  authority  upon  which  the  more  novel  or  less 
familiar  statements  have  been  made. 

It  gives  me  great  pleasure  to  express  my  thanks  to  the  many 
friends  who  have  assisted  me  in  my  undertaking.  To  Mr.  C.  W. 
Hayes  and  Mr.  Bailey  Willis,  of  the  United  States  Geological  Sur- 
vey, I  am  under  particular  obligations  for  the  kindness  which  en- 
abled me  to  profit  by  the  magnificent  collection  of  photographs 
which  the  Survey  has  gathered.  It  so  happened  that,  but  for  this 
kindness,  obstacles  of  a  temporary  nature  would  have  prevented 
my  enjoyment  of  this  privilege.  Mr.  Willis  was  also  kind  enough 
to  give  highly  valued  assistance  and  counsel  in  many  other  direc- 
tions. 

Professor  W.  H.  Hobbs  sent  me  proofs  and  manuscript  of  un- 
published books  and  papers  on  seismological  subjects,  a  service 
which  it  is  difficult  to  describe  adequately.  Professor  Bumpus, 
director  of  the  American  Museum  of  Natural  History,  New  York, 
Professor  Osborn,  Professor  R.  B.  Young  of  Johannesburg,  and 
Professor  R.  W.  Brock  of  Kingston,  Ontario,  have  all  been  most 
liberal  in  supplying  me  with  photographs  and  other  means  of 
illustration.  My  colleagues  in  the  Geological  Department  of 
Princeton  University  have  rendered  assistance  that  was  literally 
invaluable ;  Professor  C.  H.  Smyth,  Jr.,  has  read  the  proofs  and 
has  made  very  many  useful  and  timely  suggestions,  and  Dr.  W.  J. 
Sinclair  took  many  photographs  especially  -for  the  book  and  has 
given  me  the  benefit  of  his  experience  in  using  it.  Greatest  of  all 
are  my  obligations  to  Mr.  Gilbert  van  Ingen,  to  whom  the  book 
owes  much  of  whatever  good  it  may  possess ;  he  made  a  large 
number  of  the  photographs,  prepared  the  maps,  selected  the  in- 
vertebrate fossils  for  the  plates,  and  supervised  the  admirable 
drawings  upon  which  Mr.  Horsfall  has  expended  such  pains  and 
skill,  and  gave  much  useful  assistance  in  the  stratigraphical  part. 


PREFACE  TO  THE   SECOND   EDITION  xi 

As  to  the  figures  in  the  plates,  a  word  of  explanation  is  required. 
A  few  only  are  original ;  the  great  majority  are  taken  from  mono- 
graphs by  well-known  writers,  but  almost  all  have  been  so  modified 
by  restoration  or  otherwise  that  it  did  not  seem  proper  to  put  the 
responsibility  upon  the  original  authority. 

During  the  past  ten  years  I  have  received  many  letters  contain- 
ing criticisms  of  the  book  and  suggestions  for  its  improvement  in 
one  or  other  particular.  So  far  as  lay  in  my  power,  I  have  en- 
deavoured to  profit  by  these  criticisms  and  suggestions,  and  I 
wish  to  thank  those  who  have  taken  the  trouble  to  write  them  for 
my  benefit. 

Finally,  I  venture  to  express  the  hope  that  the  new  edition  may 
find  a  place  of  usefulness  in  a  crowded  field,  notwithstanding  the 
defects  of  which  I  am  very  well  aware  but  have  not  been  able  to 
remedy  in  the  time  at  my  disposal. 

PRINCETON,  N.J., 
Oct.  10,  1907. 


CONTENTS 


PAGE 

INTRODUCTION i   . 

CHAPTER  A 
THE  ROCK-FORMING  MINERALS     .       .        .       ...        .        .        6 

PART   I 

DYNAMICAL    GEOLOGY 
SECTION  I 

SUBTERRANEAN    OR    IGNEOUS   AGENCIES 

CHAPTER   I 
DIASTROPHISM  —  EARTHQUAKES     .        . 28 

CHAPTER  II 
VOLCANOES 52 

CHAPTER  III 
VOLCANOES  (Cont?)  —  INTERNAL  CONSTITUTION  OF  THE  EARTH  .        .      69 

SECTION  II 
SURFACE   AGENCIES 

CHAPTER   IV 

DESTRUCTIVE  PROCESSES  —  THE  ATMOSPHERE 100 

xiii 


XIV  CONTENTS 

CHAPTER  V 

PAGE 

DESTRUCTIVE  PROCESSES  —  RUNNING  WATER 124 

CHAPTER  VI 

DESTRUCTIVE  PROCESSES — SNOW  AND  ICE,  THE  SEA,   LAKES,  ANI- 
MALS AND  PLANTS 148 

CHAPTER   VII 

RECONSTRUCTIVE  PROCESSES  —  CONTINENTAL  DEPOSITS,  LAND,  SWAMP 

AND  RIVER i  • .        .        .        .181 

CHAPTER   VIII 
RECONSTRUCTIVE  PROCESSES  —  CONTINENTAL  DEPOSITS,  LAKE  AND  ICE    215 

CHAPTER   IX 

RECONSTRUCTIVE  PROCESSES  —  MARINE  AND  ESTUARINE  DEPOSITS      .    243 

PART   II 

STRUCTURAL    OR    TECTONIC   GEOLOGY 

CHAPTER  X 

THE  ROCKS  OF  THE  EARTH'S  CRUST  —  IGNEOUS  ROCKS       .        .        .281 

CHAPTER  XI 
THE  SEDIMENTARY  ROCKS 302 

CHAPTER  XII 
THE  STRUCTURE  OF  ROCK  MASSES  —  STRATIFIED  ROCKS     .       •.'.'•    318 

CHAPTER  XIII 
FRACTURES  AND  DISLOCATIONS  OF  ROCKS    ......    338 

CHAPTER  XIV 
JOINTS  —  STRUCTURES  DUE  TO  EROSION        .       .       .       .       .       .    369 

CHAPTER  XV 
UNSTRATIFIED  OR  MASSIVE  ROCKS        ........    385 


CONTENTS  XV 

CHAPTER  XVI 

PAGE 

METAMORPHISM  AND  METAMORPHIC  ROCKS  ...        ,  406 

CHAPTER   XVII 
MINERAL  VEINS  AND  ORE  DEPOSITS 423 

PART   III 

GE  OMORPHOL  OGY 

CHAPTER  XVIII 
THE  GEOGRAPHICAL  CYCLE 435 

CHAPTER   XIX 
LAND  SCULPTURE 451 

CHAPTER  XX 

TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS         .       .       .    463 

CHAPTER  XXI 
ADJUSTMENT  OF  RIVERS .    477 

CHAFFER  XXII 
SEA-COASTS 492 

CHAPTER  XX.III 
MOUNTAIN  RANGES .       .       .       .    503 

PART   IV 

HISTORICAL    GEOLOGY 

CHAPTER   XXIV 
FOSSILS  —  GEOLOGICAL  CHRONOLOGY 516 

CHAPTER  XXV 
ORIGINAL  CONDITION  OF  THE  EARTH  —  PRE-CAMBRIAN  PERIODS         .    532 


xv  CONTENTS 

CHAPTER   XXVI 

PAGE 

PALAEOZOIC  ERA — CAMBRIAN  PERIOD 545 

CHAPTER   XXVII 
THE  ORDOVICIAN  (LOWER  SILURIAN)  PERIOD      .....    560 

CHAPTER   XXVIII 
THE  SILURIAN  (UPPER  SILURIAN)  PERIOD '.    578 

CHAPTER   XXIX 
THE  DEVONIAN  PERIOD .        .        .    590 

CHAPTER   XXX 
THE  CARBONIFEROUS  PERIOD         .        .        .        ._,      .'       .        «        .    609 

CHAPTER   XXXI 
THE  PERMIAN  PERIOD .        .637 

CHAPTER  XXXII 

MESOZOIC  ERA  —  TRIASSIC  PERIOD 655 

'/ 

CHAPTER  XXXIII 

THE  JURASSIC  PERIOD .       .        .677 

CHAPTER  XXXIV 
THE  CRETACEOUS  PERIOD      .  ' .    700 

CHAPTER   XXXV 

CENOZOIC  ERA — TERTIARY  PERIOD       .        .  ,        .        .        .    722 

CHAPTER  XXXVI 

THE  QUATERNARY  PERIOD  —  (OR  PLEISTOCENE)  .        .         .    r    .        .  768 

APPENDIX ...  789 

INDEX •• 793 


LIST   OF   ILLUSTRATIONS 

The  Navajo  Church,  New  Mexico  ;   wind  sculpture  of  cross-bedded 
sandstone Frontispiect 

FIG.  PAGE 

1.  Columns  of  the  "  Serapeum,"  Pozzuoli,  Italy 31 

2.  Magnified  model,  showing  the  movements  of  a  surface  particle  of 

.   earth  from  the  2oth  to  the  4Oth  second  of  shock  ....  37 

3.  Seismographic  record  of  the  San  Francisco  earthquake  of  1906, 

U.S.  Coast  Survey  observatory,  Cheltenham,  Md.           ...  38 

4.  Earthquake  regions  of  the  Eastern  Hemisphere        ....  39 

5.  Earthquake  regions  of  the  Western  Hemisphere      ....  40 

6.  Earthquake  fissure  in  limestone,  Arizona          .         .         .         .         .  43 

7.  Fault-scarp  in  the  Neo  Valley,  Japan,  earthquake  of  1891        .         .  46 

8.  Fence  broken  and  shifted  horizontally  15  feet,  San  Francisco  earth- 

quake of  1906 47 

9.  Horizontal  shifting  of  the  ground,  San  Francisco  earthquake,  1906  .  48 

10.  Pompeii,  showing  depth  of  volcanic  accumulations  .         .         .  56 

11.  Profiles  of  Krakatoa .  57 

12.  Crater  Lake,  Oregon 58 

13.  Gorge  200  feet  deep  filled  by  ash  from  La  Soufriere,  S,t.  Vincent, 

eruption  of  1902 59 

14.  Spine  of  Mt.  Pelee 60 

15.  Crater-floor  of  Kilauea,  showing  the  lava  lake,  Hale-mau-mau          .  62 

1 6.  Crater  of  Vesuvius  in  moderate  eruption 64 

17.  Monte  Nuovo,  near  Pozzuoli,  formed  in  1538 66 

1 8.  Another  view  of  the  crater-floor  and  walls  of  Kilauea       ...  67 

19.  Edge  of  Hale-mau-mau,  showing  the  ropy  forms  of  the  highly  fluid 

lava,  when  cooling 69 

20.  Ropy  lava,  Vesuvius 70 

21.  Sunset  Butte,  Arizona.     An  extinct  volcano,  with  scoriaceous  block- 

lava  in  foreground 71 

22.  Lava-tunnel  and  "  Spatter-cone  "  formed  by  escaping  steam,  Kilauea  72 

23.  Lava  stalactites  and  stalagmites  in  lava-tunnel,  Kilauea   ...  73 

24.  A  hand-specimen  of  obsidian,  showing  the  glassy  lustre  and  fracture  75 

25.  Stream  gorge,  island  of  Hawaii ;   displaying  modern  columnar  lava  77 

xvif 


xvill  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

26.  Obsidian  Cliff,  Yellowstone  Park.     Hexagonal  jointing    ...  78 

27.  Volcanic  bomb,  showing  scoriaceous  texture 80 

28.  Mauna  Loa,  seen  from  a  distance  of  40  miles 83 

29.  Mt.  Shasta,  California 84 

30.  Vesuvius  and  Monte  Somma 85 

31.  Mt.  Wrangel,  Alaska 86 

32.  Truncated  tuff  cone,  island  of  Oahu 87 

33.  Boulders  of  weathering,  Eldon  Mt.,  Arizona 104 

34.  Soil  originating  in  place  by  the  decomposition  of  sandstone     .         .  106 

35.  Bad  lands  of  South  Dakota 109 

36.  Bad  lands  near  Adelia,  Nebraska      . no 

37.  Bad  lands  in  Wyoming,  with  talus  slopes in 

38.  Monument  Park,  Colorado        .         .         .         f        .         .         .         .112 

39.  Shales  "  creeping  "  under  the  action  of  frost 114 

40.  Cliff  and  talus  slope,  Delaware  Water  Gap,  N.J 115 

41.  Smooth  exfoliated  surface  of  granite,  Matopos  Hills,  Rhodesia,  South 

Africa  .         .         . 117 

42.  Slope  of  exfoliating  granite,  Matopos  Hills       .....     118 

43.  Exfoliation  of  glaciated  granite,  Sierra  Nevada         .         .         .         .119 

44.  Exfoliating  granite  dome,  Yosemite  Valley,  California      .         .         .120 

45.  Wind-sculptured  sandstone,  Black  Hills,  South  Dakota   .         .         .121 

46.  Honey-combed  rock,  due  partly  to  wind  erosion  and  partly  to  the 

solvent  power  of  water 122 

47.  Sink -hole  in  limestone,  near  Cambria,  Wyoming      .         .         .         .127 

48.  Cafion  and  lower  tails  of  the  Yellowstone  River        .         .         .         .128 

49.  Profile  of  Turtle  Mt.,  showing  the  amount  of  material  removed  in 

the  Frank  -rock-slide 1 29 

50.  Rock-slide  of  1903  at  Frank,  Alberta 130 

51.  Arrangement  of  strata  which  causes  hillside  springs          .         .         .132 

52.  Diagram  of  fissure-spring 133 

53.  An  artesian  well 134 

54.  The  "  Bottomless  Pit,"  Arizona 135 

55.  Undermined  pot-hole,  Little  Falls,  N.Y 138 

56.  Pot-hole  in  stream,  Mill  Creek,  Oklahoma 139 

57.  A  meandering  stream  ;   ox-bow  lakes  at  the  right ;   Alashuk  River, 

Alaska 141 

58.  The  Au  Sable  Chasm,  N.Y 142 

59.  Old,  high-level  channel  of  the  Niagara  River,  below  the  present 

falls 143 

60.  Summit  of  Mt.  Blanc,  Switzerland 148 

61.  Two  valley  glaciers  descending  Mt.  Blanc 150 


LIST  OF  ILLUSTRATIONS  xix 

FIG.  PAGE 

62.  A  hanging  glacier,  Cascade  Pass,  Wash. 153 

63.  Moraine-covered  surface  of  the  Malaspina  Glacier,  Alaska     .         -154 

64.  Nunatak  rising  through  the  ice-cap,  Greenland       .         .         .         .  155 

65.  Edge  of  the  Greenland  ice-sheet,  with  a  glacier  descending  from  it  156 

66.  The  Columbia  Glacier,  Alaska 157 

67.  Glaciated  surface,  Sierra  Nevada,  Cal 158 

68.  Steeply  inclined  strata,  with  edges  roughened  by  glacial  plucking, 

overlaid  by  glacial  drift,  Iron  Mt.,  Mich 159 

69.  Glacial  striae  on  limestone,  overlaid  by  drift ;   Pillar  Point,  Lake 

Ontario 160 

70.  Ancient  glacial  striae   (Permian) ;    Riverton  on  the  Vaal  River, 

South  Africa .161 

71.  Glacial  grooves  on  sandstone  cliff ;   Delaware  Water  Gap,  Pa.        .  162 

72.  U-shaped  glacial  valley  ;   Kern  Canon,  Cal 163 

73.  Front  of  Bowdoin  Glacier,  Greenland 165 

74.  Wave  erosion ;   Etretat,  France 168 

75.  Wave-cut  arch,  coast  of  California 169 

76.  Wave  erosion,  strata  dipping  seaward  ;   Orkney  Islands,  Scotland  .  170 

77.  Wave  erosion,  strata  dipping  landward;   Duncansby  Head,  Orkney 

Islands        .         .         , 171 

78.  Joint-block,  partly  dislodged  by  the  surf  on  wave-cut  terrace,  Wick, 

Orkney  Islands 172 

79.  Igneous  rock,  corroded  by  sea- water,  about  ^  natural  size     .         .  173 

80.  Wave-cut  bluff  on  Lake  Ontario 175 

81.  Erosion  following  removal  of  forest ;   Great  Smoky  Mts.,  Tenn.      .  177 

82.  Soil  destruction  due  to  removal  of  forest ;    Mitchell  Co.,  N.C.         .  178 

83.  Diagram  showing  the  relation  between  height  and  area  of  land 

above  sea-level  and  of  water  in  ocean  basins       .         .         .         .185 

84.  Loess  deposits ;   North  China          .         .         .         .         .         .         .188 

85.  Sand  dune,  with  wind-ripples,  River  Terraces  in  distance  ;   Biggs, 

Oregon iSg 

86.  Sand  dune  ;   Beaufort  Harbor,  N.C 190 

87.  Ideal  section  through  Mammoth  Hot  Springs,  showing  the  water 

rising  through  limestone 191 

88.  Travertine   terrace  of  the   Mammoth    Hot   Springs,  Yellowstone 

Park ^ I92 

89.  Crater  of  Castle  Geyser,  Yellowstone  Park 193 

90.  Great  Dismal  Swamp,  Virginia 198 

91.  Manti  Creek,  Utah  ;   flood  of  August,  1901 200 

92.  Effects  of  flood  ;   Black  Hills,  S.D. 201 

93.  Sand  deposits,  North  Platte  River,  Nebraska  .                                     ,  202 


XX  LIST  OF   ILLUSTRATIONS 

FIG.  PAGE 

94.  Alluvial  cone  trenched  by  stream,  with  secondary  cone  below         .  203 

95.  Flood  plain,  Genesee  River,  N.Y 204 

96.  Sun  cracks  in  Newark  shale,  about  ^  natural  size           .         .         .  206 

97.  River  terraces,  Chelan  River,  Wash 208 

98.  Terrace  on  deserted  channel,  central  New  York      ....  209 

99.  Delta  of  Rondout  Creek  in  Hudson  River,  Rondout,  N.Y.      .         .211 

100.  Settling  of  clay  in  salt  and  fresh  water  after  24  hours      .         .         .212 

101.  Gravel  beach,  Lake  Ontario 217 

102.  Outlet  of  Lake  Bonneville,  Utah 218 

103.  Terraces  of  Lake  Bonneville,  Utah 219 

104.  Mountains  nearly  buried  under  old  lake  deposits ;  plain  of  Salt 

Lake,  Utah 221 

105.  Island  of  calcareous  tufa,  Pyramid  Lake,  Nevada   .        .         .  >  224 

1 06.  Salt  deposit,  El  Paso,  Texas 225 

107.  Deposits  in  an  "  alkali "  lake  ........  226 

108.  Fluted  ground  moraine,  Columbia  Glacier,  Alaska          .         .         .  228 

109.  Glacial  moraine,  Montauk  Point,  L.I 229 

no.   Glacial  pebble 230 

in.   Striated  glacial  boulders  from  Permian  of  South  Africa  .         .         .  231 

112.  Kettle  moraine,  Alaska 231 

113.  Perched  block,  near  the  Yellowstone  Cafion,  National  Park   .         .  232 

114.  Glacial  drift,  Bangor,  Pa 233 

115.  Glacial  drift  in  Permian  of  South  Africa 233 

1 1 6.  Glacio-fluvial  deposits,  Yahtse  River,  Alaska 234 

117.  Gravel  flood  plain  of  glacial  stream,  Alaska 235 

1 1 8.  Esker,  central  New  York 236 

119.  Kame  moraine,  central  New  York •   .         .  236 

1 20.  Drumlin,  near  Newark,  N.Y 237 

121.  Drift-covered  surface  of  the  Malaspina  Glacier,  Alaska  .        .        .  238 

122.  River  issuing  from  the  Malaspina  Glacier 239 

123.  Deposits  partly  made  by  stranded  ice,  west  coast  of  Greenland       .  240 

124.  Basin  of  the  Gulf  of  Mexico 244 

125.  Gravel  beach  and  wall,  Conception  Bay,  Newfoundland  .         .         .  246 

126.  Gravel  beach,  Long  Island,  N.Y .         .  247 

127.  Ripple-marked  sands,  low  tide  ;   Mont  St.  Michel,  France      .         .  248 

128.  Ripple-marked  sandstone 249 

129.  Steeply  inclined  beds  of  ripple-marked  shale,  near  Altoona,  Pa.      .  250 

130.  Wave  mark  and  rain  prints,  modern  sandy  beach   .         .         .         .251 

131.  Rill  marks  on  modern  sandy  beach 252 

132.  Sun  cracks  in  limestone  ;   Rondout,  N.Y 253 

133.  Cross-bedded  sands,  Bennett,  Nebraska 254 


LIST  OF  ILLUSTRATIONS  xxi 

FIG.  PAGE 

134.  Cross-bedded  sandstone,  New  Mexico 255 

135.  Markings  by  marine  worms,  modern 256 

136.  Diagram  showing  dove-tailed  deposition  on  the  sea-floor        .         .  257 

137.  Modern  shell  limestone  (coquina),  Florida 258 

138.  Rock  from  Pourtales  Plateau 260 

139.  Corals  on  the  Great  Barrier  Reef  of  Australia         ....  262 

140.  Various  forms  of  modern  coral  limestone 263 

141.  Map  of  marine  deposits  .         .         .         .     , 268 

142.  Foraminiferal  ooze,  X  20        .         .         .         .         ,         .         .         .  270 

143.  Pteropod  ooze,  X  4 .         .  271 

144.  Slab  of  polished  porphyry,  natural  size 286 

145.  Hand  specimen  of  granite,  natural  size 287 

146.  Hand  specimen  of  conglomerate,  natural  size          ....  305 

147.  Piece  of  banded  travertine,  polished,  natural  size   .....  308 

148.  Chalk  from  Kansas,  X  45 311 

149.  Section  in  coal  measures  of  western  Pennsylvania  ....  320 

150.  Parallel  sections  near  Colorado  Springs,  Col 322 

151.  Concretions  in  Laramie  sandstone,  exposed  by  weathering     .         .  323 

152.  Ironstone  concretion,  split  open  to  show  the  fossil  leaf  which  forms 

the  nucleus ;   Mazon  Creek,  Illinois 324 

153.  Symmetrical  folds,  anticline  on  left,  and  syncline  on  right       .         .  327 

154.  Model  of  anticline 328 

155.  Model  of  syncline 328 

156.  Anticline  near  Hancock,  Md. 329 

157.  Synclinorium,  Mt.  Greylock,  Mass. 330 

158.  Diagrams  of  folds 331 

159.  Asymmetrical  open  fold,  High  Falls,  N.Y 332 

1 60.  Overturned  sharp  fold,  Big  Horn  Mts.,  Wyoming  ....  333 

161.  Closed  recumbent  fold,  East  Tennessee 334 

162.  Plicated  gneiss,  Montgomery  Co.,  Pa.     .         .        ..         .         .         .  334 

163.  Inclined  isoclinal  folds,  eroded 335 

164.  Diagram  of  monoclinal  fold 335 

165.  Monoclinal  fold,  Selmas  Valley 336 

1 66.  Monoclinal  fold,  Mead  River,  Alaska      .         .         .         .         .         .  336 

167.  Normal  fault,  fault-plane  hading  against  dip  of  beds       .         .         .  339 

1 68.  Normal  fault  hading  with  dip  of  beds 340 

169.  Fault-breccia  of  limestone 341 

170.  Vertical  slickensides,  Rondout,  N.Y 342 

171.  Limestone  faulted  on  bedding- planes,  with  vertical  slickensides     .  343 

172.  Minute  vertical  fault  of  recent  date,  interrupting  glacial  striae          .  344 

173.  Trough-fault  of  very  small  throw 346 


xxii  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

174.  Small  faults  in  the  roof  of  a  mine,  Idaho.    Near  the  right  end,  a 

tiny  Horst 347 

175.  Effect  of  strike-fault  on  outcrop 348 

176.  Effect  of  step-faults  in  repeating  outcrops        .  348 

177.  Model  showing  stratum  offset  by  dip-fault 349 

178.  Drag  of  strata  near  fault-plane 350 

179.  Model  illustrating  horizontal  faulting,  with  hanging  wall  moved 

against  the  dip 352 

180.  Model  illustrating  horizontal  faulting,  with  hanging  wall  moved  in 

direction  of  dip 352 

181.  Horizontal  slickensides,  Oklahoma 353 

182.  Model  illustrating  pivotal  faulting 354 

183.  Fold-thrust,  near  Highgate  Springs,  Vt. 355 

184.  Steep  fold-thrust,  Big  Horn  Mts.,  Wyoming 356 

185.  Surface-thrust  of  small  displacement 357 

186.  Surface-thrust,  Holly  Creek,  Georgia 358 

187.  Folded  and  fractured  iron  ore  and  jaspilite,  Lake  Superior  region. 

About  Yz  natural  size 360 

1 88.  Plicated  beds  on  unfolded  ones,  Mineral  Ridge,  Nevada         .         .361 

189.  Plicated  limestone  with  sheet  of  igneous  rock,  ne,ar  Rockland, 

Maine 362 

190.  Model  showing  the  slip  of  folded  beds  upon  one  another        .         .  363 

191.  Model  showing  effects  of  lateral  compression          ....  363 

192.  Model  illustrating  the  development  of  a  fold-thrust         .         .         .  365 

193.  Platy  jointing  in  diabase  ;   above,  spheroidal  weathering  and  tran- 

sition to  soil.    Rocky  Hill,  N.J 370 

194.  Regular  jointing  in  gneiss,  near  Washington 371 

195.  Irregular  jointing  in  gneiss,  Little  Falls,  N.Y.          ....  372 

196.  Jointing  in  shale,  Cayuga  Lake,  N.Y 373 

197.  Jointing  in  limestone,  Black  Hills,  South  Dakota  ....  375 

198.  Joints  dying  away  downward,  shown  by  pinching  out  of  white 

calcite  veins 376 

199.  Unconformity  with  change  of  dip,  or  angular  unconformity    .         -377 

200.  Angular  unconformity,'  Grand  Canon  of  the  Colorado      .         .         .  378 

201.  Angular  unconformity,  old  gravels  on  hard  shale  ;   Kingston,  N.J.  379 

202.  Angular  unconformity,  west  of  Altoona,  Pa 380 

203.  Unconformity  without  change  of  dip,  and  overlap  ....  381 

204.  Contemporaneous  erosion,  channel  in  wall  of  Niagara  Gorge          .  383 

205.  Volcanic  neck,  Colorado          .                  386 

206.  Diamond  mine,  showing  circular  form  of  volcanic  pipe  in  sand- 

stone, Kimberley,  South  Africa 387 


LIST  OF  ILLUSTRATIONS  xxiil 

FIG.  PAGE 

207.  Irregularly  and  columnar-jointed  lava  flow  on  sandstone,  Island  of 

Staffa,  Scotland 388 

208.  Lava  flow  on  sandstone,  Upper  Montclair,  NJ 389 

209.  Pumice,  natural  size .  390 

210.  Parallel  dykes,  Cinnabar  Mt.,  Montana  .         .         .         .         .         .  392 

211.  Dyke  trenched  by  weathering  faster  than  country  rock  .         .         .  393 

212.  Veins  of  granite  in  cliff  near  Gunnison,  Col 394 

213.  Granite  veins  intrusive  in  diorite  and  both  cut  by  a  small  dyke  of 

aplite  ;   coast  of  Maine 395 

214.  The  Palisades,  seen  from  Hastings,  N.Y. 396 

215.  Contact    of  diabase   sill  with   shales   below.     Base  of  Palisades, 

Weehawken,  NJ 397 

216.  Diagrammatic  vertical  section  of  a  laccolith 398 

217.  Eroded  laccolith,  with  many  sills  and  apophyses  ;   Colorado  .         .  398 

2 1 8.  Vertical  section  through  laccolith,  shown  in  Fig.  217,  before  de- 

nudation       399 

219.  Little  Sun-Dance  Hill,  South  Dakota 400 

220.  Bear  Butte,  South  Dakota 401 

221.  Mato  Tepee,  South  Dakota 402 

222.  Inclusions  (xenoliths)  of  schist  in  granite 404 

223.  Oblique   synclinal   fold   in  slate,  showing  cleavage   planes  at   all 

angles  to  the  bedding-planes 410 

224.  Fissile  quartzite,  California      .         .         .         .         .         .         .         .411 

225.  Plicated  gneiss,  Montgomery  Co.,  Pa 419 

226.  Boulder  of  gneiss,  displaying  its  conglomeratic  nature  on  weath- 

ered surface 420 

227.  Mica  schist,  with  garnets.     Nearly  natural  size       ....  421 

228.  Dykes  of  sandstone  in  shales,  Northern  California          ,         .         .  426 

229.  Volcanic  topography,  Northern  Arizona           .....  437 

230.  Glacial  Topography,  Eastern  Washington 438 

231.  Peneplain,  with  residual  mountain,  Southern  California  .         .         .  445 

232.  The  Mohave  Desert,  Cal.        .                  447 

233.  Escarpments  and  dip  slopes,  Montana 453 

234.  Hog-backs,  east  side  of  Laramie  Mts.,  Wyo.  .....  456 

235.  Anticlinal  ridge,  Big  Horn  Mts.,  Wyo.,  hard  beds  in  relief     .         .  457 

236.  Truncated  anticlinal  ridge,  Montana 458 

237.  Palisade-sill,  Fort  Lee,  NJ 461 

238.  Abert  Lake,  Oregon.     The  line  of  cliffs  is  a  fault-scarp  .         .         .  464 

239.  Sierra  Nevada  fault-scarp  ;    Mono  Lake,  Cal.          ....  465 

240.  Normal  fault,  Au  Sable  Chasm,  N.Y 468 

241.  Great  thrust,  near  Highgate  Springs,  Vt 471 


xxiv  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

242.  Gorge  of  the  Zambesi  below  Victoria  Falls,  S.  Africa      .         .         .  473 

243.  The  Chasm,  Victoria  Falls  of  the  Zambesi       .  474 

244.  Granite  dome,  Yosemite  Valley,  Cal 475 

245.  Limestone  cliffs,  Black  Hills,  S.D 476 

246.  Stream  cutting  through  a  ridge,  Middle  Park,  Col.          .         .         .  477 

247.  Two  very  young  gulches,  Colorado 479 

248.  Evolution  of  a  river  system,  first  stage 489 

249.  Evolution  of  a  river  system,  second  stage 489 

250.  Branching  fjord,  Lynn  Canal,  Alaska 497 

251.  Fjord,  Wrangel,  Alaska  . 498 

252.  Artificial  external  mould  of  clam-shell  and  original  shell          .         .  518 

253.  Artificial  internal  cast  of  clam-shell  and  inner  view  of  original  shell  519 

254.  Petrified  logs,  exposed  by  weathering  of  tuffs,  Arizona  .         .         .520 

255.  Geological  map  of  Central  New  York,  showing  the  faunal  provinces 

of  the  Upper  Devonian  (Portage  stage) 522 

256.  East-west  section  through  area  given  in  Fig.  255,  showing  changes 

of  facies 523 

257.  Fossil  shells  (Miocene)  lying  on  a  modern  beach,  York  River,  Va.  524 

258.  Map  of  known  pre-Cambrian  surface  exposures  in  North  America  537 

259.  Map  of  known  Cambrian  outcrops  in  the  United  States  and  Canada  550 

260.  Generalized  map  of  North  America  in  the  Ordovician  period          .  562 

261.  Generalized  map  of  North  America  in  the  Silurian  .         .         .  580 

262.  Eurypterus fischeri,  Eichw.,  Island  of  Oesel,  Russia        .         .         .  587 

263.  Lanarkia  sp. ;  a  primitive  Ostracoderm  .         .         .         .         .         .  588 

264.  Upper  figure,  Birkenia  elegans  Traq.  X  3/2  >   lower  figure,  Lasanius 

problematicus  Traq.     Both  figures  restored 589 

265.  Map  of  North  America  in  the  Devonian  period      ....  592 

266.  Heliophyllum  halli  E.  &  H.,  X  YZ  —  Hamilton  of  Michigan    .         .  602 

267.  Acervularia  davidsoni  E.  &  H.,  X  % —  Hamilton  of  Michigan       .  602 

268.  Pterichthys  testudinarius.     Restored.     Old  Red  Sandstone    .         .  605 

269.  Cladoselache  newberryi.     Restored.     Ohio  Shale    ....  606 

270.  Dipterus  valenciennesi  Sedgw.  and  Murch.     Restored.     Old  Red  606 

271.  Coccosteus  decipiens  Ag.     Restored.     Old  Red        ....  607 

272.  Holoptychius  nobilissimus  Ag.     Restored.     Old  Red      .         .         .  607 

273.  Map  of  North  America  in  the  Lower  Carboniferous         .         .         .611 

274.  Map  of  North  America  in  the  Upper  Carboniferous        .         .         .  616 

275.  Roche  moutonee  exposed  by   removal   of  Dwyka  boulder   clay, 

Riverton,  Cape  Colony 644 

276.  Callipteris  conferta  Brngn 649 

277.  Sphenopteris  coriacea,  Fontaine  and  White     .....  649 

278.  Glossopteris  browniana  Brngn.     Newcastle,  Australia    .         .         .  650 


LIST   OF   ILLUSTRATIONS 


XXV 


FIG.  PACK 

279.  Pleuracanthus  decheni 652 

280.  Permian  Stegocephalian,  Eryops  megacephalus  Cope,  X  l/-\.     Skull 

seen  from  the  side       . 653 

281.  Permian  Pelycosaurian,  Naosaurus  claviger  Cope   ....  653 

282.  Map  of  North  America  in  the  Triassic  and  Jurassic  periods    .         .  662 

283.  A  Triassic  Fern,  Macrotceniopteris  magnifolia  Rogers.     Restored  668 

284.  Leaf  of  a  Triassic  Cycad,  Otozamites  latior  Saporta         .         .         .  669 

285.  Triassic  Conifer,   Voltzia  heterophylla 669 

286.  Diplurus  longicatuiatiis  Newberry 674 

287.  Skull  of  Belodon  kapffi,  v.  Meyer 675 

288.  Slab  of  Trigonia  davellata  Sowerby,  from  the  English  Oxfordian  .  688 

289.  Slab  of  Beletnnites  cotnpressus  Blainv.,  from  the  Lias  of  England    .  689 

290.  Dapedius  politus 690 

291.  Aspidorhynchus  acutirostris  Ag.      .         t         .         .         .         .         .691 

292.  Hypsocormus  insignis  Wagner 691 

293.  Ichthyosaurus  q^ladriscissus  Quenstedt.     Restoration      .         .         .  693 

294.  Plesiosaurus  macrocephalus  Owen,  X  l/20 694 

295.  Allosaurus  agilis  Marsh,  a  carnivorous  Dinosaur.     Restoration       .  695 

296.  Restoration  of  Pterosaurian,  Rhamphorhynchus      ....  696 

297.  Restoration  of  Archceopteryx  lithographica  v.  Meyer        .         .         .  698 

298.  Map  of  North  America  in  the  Cretaceous  period     ....  701 

299.  Cretaceous  leaves,  Dakota  series 714 

300.  Tylosaurus  dyspelor  Marsh.     A  Mosasaurian  in  pursuit  of  Sauro- 

dont  Fishes 719 

301.  Skull  of  Triceratops  flabellatus  Marsh,  X  l/zo         .         .         .         .  720 

302.  Skull  of  Diclonius  mirabilis  Cope,  X  */i9       .....  720 

303.  Map  of  North  America  in  the  Tertiary  period         ....  725 

304.  Flabellaria  sp.  x  l/\%.     Green  River  Shales 736 

305.  Eobasileus  cormitus  Cope.     Restoration 740 

306.  Brontotherium  dolicocheras  Scott  &  Osborn.     Restoration      .         .  746 

307.  Idealized  section  of  the  great  Bad  Lands  of  South  Dakota      .         .  751 

308.  Rhus  sp.  Lesq.     A  sumach  from  the  Florissant  Shales    .         .         -755 

309.  Ecphora  quadricostata  Say,  x  %.     Miocene,  Yorktown,  Va.           .  756 

310.  Theosodon   lydekkeri   Amegh.      One   of  the  Litopterna   from  the 

Santa  Cruz  beds 760 

311.  North  America  in  the  time  of  maximum  glaciation          .        .         .  773 


PLATES 


PAGE 


I.     CAMBRIAN  FOSSILS.  . 554 

II.    CAMBRIAN  TRILOBITKS 556 

III.  CAMBRIAN  CRUSTACEA 557 

IV.  ORDOVICIAN  SPONGES,  COKALS,  ETC. 569 

V.  ORDOVICIAN  GRAPTOLITES,  BRACHIOPODS,  ETC.        .        .        .  571 

VI.    ORDOVICIAN  MOLLUSCA 574 

VII.    ORDOVICIAN  TRILOBITES 576 

VIII.    SILURIAN  FOSSILS 585 

IX.    DEVONIAN  FOSSILS 601 

X.    CARBONIFEROUS  VEGETATION 626 

XI.     LOWER  CARBONIFEROUS  FOSSILS 630 

XII.  UPPER  CARBONIFEROUS  AND  PERMIAN  FOSSILS        .        .        .  633 

XIII.  TRIASSIC  INVERTEBRATE  FOSSILS 672 

XIV.  JURASSIC  INVERTEBRATE  FOSSILS 685 

XV.    CRETACEOUS  INVERTEBRATE  FOSSILS 715 

XVI.    AMERICAN  TERTIARY  FOSSILS   . 737 

XVII.    TERTIARY  FOSSILS  FROM  FLORIDA 765 


INTRODUCTION 

Geology  is  the  study  of  the  structure,  history,  and  development  oj 
the  earth  and  its  inhabitants,  as  revealed  in  the  rocks. 

From  this  definition  it  is  apparent  that  the  central  problem  in 
geology  is  the  deciphering  of  the  earth's  history,  and  that  the 
historical  standpoint  is  dominant  throughout.  For  this  purpose 
it  is  necessary  to  apply  the  results  and  principles  of  all  those  sci- 
ences which  can  aid  us  in  interpreting  the  record  contained  in 
the  rocks.  Astronomy,  physics,  chemistry,  mineralogy,  physical 
geography,  botany,  and  zoology  are  all  needed  in  the  task,  and 
geology  as  a  true  science  did  not  become  possible  until  the  other 
sciences  were  sufficiently  advanced  to  afford  a  solid  foundation 
for  it. 

The  history  of  the  earth  involves  vast  periods  of  time,  to  be 
measured  only  in  millions  of  years, — no  one  can  say  how  many, — 
so  that  all  our  familiar  conceptions  of  "  ancient  "and  "  modern," 
derived  from  the  history  of  our  own  race,  must  be  greatly  changed 
before  they  can  be  applied  to  geological  time.  In  reaching  its 
present  condition,  the  earth  has  passed  through  many  stages  of 
change  in  its  geographical,  climatic,  and  biological  relations,  most 
of  these  stages  leaving  behind  them  records  which  are  preserved 
in  the  successive  layers  of  rock. 

In  order  to  re.ad  the  record  contained  in  the  rocks,  it  is  first  of 
all  necessary  to  learn  the  language  in  which  it  is  written.  This 
can  be  done  only  through  an  intimate  acquaintance  with  all  the 
methods  in  which  rocks  are  made,  and  with  the  changes  which  the 
rocks  undergo.  This,  in  turn,  implies  a  knowledge  of  all  those 
processes  which  are  now  at  work  in  modifying  and  changing  the 
globe  internally  and  on  the  surface.  Just  because  our  knowledge 
of  these  methods  and  processes  is  often  incomplete  and  vague,  do 

B  I 


INTRODUCTION 

we  so  frequently  find  the  geological  record  ambiguous,  open  to 
several  interpretations,  or  even  quite  unintelligible.  Again,  many 
changes  go  on  under  conditions  which  render  direct  observation 
impossible,  either  because  they  are  confined  to  the  deep  interior 
of  the  earth  and  are  thus  beyond  our  reach,  or  because  their  opera- 
tion is  so  slow  that  a  lifetime  is  all  too  short  for  their  detection. 
In  such  cases  we  must  deduce  the  invisible  cause  from  the  visible 
effect,  but  it  is  often  extremely  difficult,  or  even  impracticable, 
from  many  possible  causes,  to  select  the  real  and  rightful  one. 
Hence  come  the  wide  differences  of  opinion  which  the  interpreta- 
tion so  often  calls  forth. 

As  a  living  and  growing  science,  geology  is  subject  to  continual 
change,  a  change  which  is  by  no  means  a  simple  advance  from  one 
point  to  another,  but  an  unending  revision  of  opinions,  a  perpetual 
tearing  down  and  rebuilding. 

To  many  intelligent  people  this  continual  modification  of  scien- 
tific opinion,  which  is  a  necessary  consequence  of  advancing  know- 
ledge, is  a  source  of  annoyance.  This  attitude  of  mind  comes  from 
a  failure  to  discriminate  between  fact  on  the  one  hand,  and  inference, 
or  hypothesis,  on  the  other.  Accurately  observed  facts  may  be 
added  to,  but  they  remain  trustworthy;  the  changeable  element  is 
the  inference  which  is  drawn  from  the  facts.  These  inferences  are 
of  very  different  degrees  of  certainty.  Some  such  deductions  which 
were  made  centuries  ago  remain  unshaken  to-day,  while  others  of 
far  more  recent  date  have  proved  illusory.  Thus,  when  we  find 
a  rock  composed  of  cemented  sand-grains,  arranged  in  regular 
beds  or  layers,  and  full  of  marine  shells,  we  infer  that  it  was  formed 
under  the  sea,  and  further  that  the  land  where  ihe  rock  is  now 
found  was  once  covered  by  the  sea.  Such  inferences  are  prac- 
tically certain,  because  they  explain  all  the  known  facts  and  are 
in  conflict  with  none.  On  the  other  hand,  the  hypotheses  of 
Cuvier  and  others  as  to  the  character  of  the  earth's  development, 
and  the  manner  in  which  the  successive  assemblages  of  animals 
and  plants  were  called  into  being,  have  been  long  abandoned. 

In  the  process  of  reasoning  from  the  known  to  the  unknown, 


INTRODUCTION  3 

the  inferences  become  the  more  uncertain,  the  farther  we  recede 
from  demonstrable  facts.  Hypotheses  are  assumptions  which  we 
make  to  explain  and  coordinate  large  numbers  of  facts,  and  so 
long  as  their  true  nature  is  understood,  they  are  useful,  indeed 
indispensable,  means  of  reaching  the  truth.  The  objection  is  that 
they  are  too  often  taught  as  though  they  were  established  beyond 
dispute.  A  true  hypothesis  will  prove  to  be  in  harmony  with 
newly  discovered  facts,  which  will  take  their  place  under  it  simply 
and  naturally.  A  false  hypothesis,  on  the  other  hand,  may  be  in 
accordance  with  all  the  facts  known  at  the  time  when  it  was  pro- 
posed, but  the  progress  of  discovery  will  bring  to  light  facts  which 
are  inconsistent  with  the  hypothesis,  until  it  is  plainly  seen  to  be 
inadequate  and  misleading.  Yet  even  a  false  hypothesis  may  serve 
a  useful  purpose,  for  it  puts  before  us  a  definite  problem,  instead 
of  a  mere  catalogue  of  uncorrelated  observations.  The  pathway 
of  every  science  is  strewn  with  wrecks  of  hypotheses  which  have 
been  used,  worn  out,  and  thrown  aside.  In  all  our  thinking 
and  reasoning  the  distinction  between  hypothesis  and  fact  must 
be  steadily  held  in  view. 

Geology  is  a  unit  and,  though  for  the  purpose  of  orderly  treat- 
ment, it  is  necessary  to  divide  the  subject  into  various  provinces, 
it  should  be  clearly  understood  that  these  provinces  are  rather 
the  various  aspects  and  phases  of  the  same  science  than  actual 
divisions.  Every  part  of  the  subject  is  so  intimately  related  to 
every  other  part,  that  any  possible  arrangement  involves  the  more 
or  less  violent  separation  of  things,  that  belong  together  and  requires 
much  anticipation  and  repetition.  The  past  is  meaningless  unless 
we  understand  the  present,  and  a  full  understanding  of  the  present 
can  only  be  gained  through  a  knowledge  of  the  past,  yet  it  is  ob- 
vious that  we  cannot  deal  with  both  past  and  present  simultane- 
ously. Although  it  is  an  undoubted  evil,  some  classification  is 
necessary,  if  we  would  avoid  losing  ourselves  in  bewildering  laby- 
rinths of  detail. 

The  departments  into  which  geology  is  usually  divided  are  as 
follows :  — 


4  INTRODUCTION 

1.  Dynamical  Geology,  or  the  study  of  the  forces  which  are 
now  at  work  in  modifying  the  surface  of  the  earth,  and  of  the 
chemical  and  mechanical  changes  which  they  effect.    This  is  the 
key  by  which  we  may  interpret  past  changes. 

2.  Structural  Geology,  or  the  study  of  the  materials  of  which 
the  earth  is  composed  and  of   the  manner   in  which  they  are 
arranged;   together  with  such  explanations  of  the  modes  in  which 
the  arrangement  was  produced  as   may  be  inferred   from  the 
structure. 

3.  Geomorphology  (also   called   Physiographical   Geology,  ^r 
Physiography)  is  an  examination  of  the  topographical  features 
of  the  earth  and  of  the  manner  in  which  they  were  produced. 
Primarily,  this  subject  is  a  province  of  physical  geography,  but  it 
is  a  valuable  adjunct  to  geology. 

The  three  foregoing  divisions  together  constitute  a  larger  division, 
which  is  called  Physical  Geology,  ind  which  is  contrasted  with  — 

4.  Historical  Geology. — This  is  the  study  of  the  earth's  history, 
the  changes  of  level  between  land   and   sea,   of   topography,  of 
climate,   and  of    tht\  successive   groups   of   animals  and  plants 
which  have  lived  upon  the  globe.     The  historical  is  the  dominant 
standpoint  in  geology,  the  main  problem  of  which  is  to  interpret 
the  records  of  the  earth's  history.    The  other  departments  are 
the  means,  to  this  great  end. 

While  the  geologist  needs  the  help  of  almost  all  the  other  physi- 
cal and  natural  sciences,  he  has  his  peculiar  province  in  the  rocks 
which  make  up  the  accessible  crust  of  the  earth.  These  rocks  are 
aggregates  of  a  comparatively  few  common  minerals,  called,  for 
that  reason,  the  rock-forming  minerals.  A  study  of  the  processes 
now  going  on  shows  that  rocks  are  formed  in  various  ways  and, 
in  accordance  with  these  modes  of  formation,  they  may  be  grouped 
in  three  great  classes:  I.  Igneous  Rocks,  or  those  which  have 
solidified  by  cooling  from  a  state  of  fusion  and  are  therefore  not 
divided  into  layers  or  beds,  are  either  glassy  or  crystalline,  and  are 
composed  of  complex  minerals.  The  igneous  rocks  have  forced 
their  way  upward  from  the  earth's  interior,  thus  penetrating  the 


INTRODUCTION  5 

overlying  rocks  in  various  ways.  A  familiar  example  of  this 
group  is  lava.  II.  Sedimentary,  or  Stratified  Rocks,  those 
which  were  accumulated  under  water  or  on  land  in  a  series  of 
successive  beds,  or  strata,  from  material  derived  from  the  dis- 
integration of  older  rocks,  and  are  generally  fragmentary,  or 
non-crystalline,  and  composed  of  simpler  minerals  than  those 
which  make  up  the  igneous  rocks.  Speaking  broadly,  the  beds 
of  the  sedimentary  rocks  were  originally  laid  down  in  a  horizontal 
position,  and  hence  when  they  are  found  to  be  tilted,  inclined,  or 
folded,  it  follows  that  they  have  been  disturbed  from  their 
original  attitudes.  III.  Metamorphic  Rocks,  or  those  igneous 
or  sedimentary  rocks  which  have  been  more  or  less  profoundly 
reconstructed  in  place,  often  with  the  generation  of  entirely  new 
minerals.  . 

In  the  accessible  part  of  the  earth's  crust,  rocks  of  all  kinds 
(other  than  loose  materials,  such  as  sand)  are  divided  into  pieces, 
by  vertical  and  horizontal  partings,  which  are  called  joints  (see  p. 
369).  In  addition,  the  rocks  are  divided  into  still  larger  masses, 
or  blocks,  by  a  profounder  system  of  fissures,  and  planes  of 
dislocation,  or  faults  (see  p.  353).  The  blocks  are  of  all 
sizes,  up  to  thousands  of  square  miles  and  down  to  areas  of  a 
few  square  feet,  and  thus  the  surface  of  the  earth  has  been  well 
compared  to  a  vast  mosaic  of  rock-pieces. 

The  crust  of  our  planet  is  called  the  lithosphere,  a  shell  of  rocks 
of  unknown  thickness.  Within  the  lithosphere  is  the  great  mass 
of  the  earth,  or  centrosphere,  concerning  which  we  know  only  that 
it  is  highly  heated,  of  great  density,  and  under  enormous  pressure. 
The  surface  of  the  globe  is  very  irregular  .and  covered  with 
elevations  and  depressions.  The  deeper  depressions  are  filled 
with  water  and  constitute  the  ocean  basins  which  in  area  bear 
to  the  land  the  proportion  of  2.54:1,  and  this  incomplete  en- 
velope of  water  is  the  hydrosphere.  If  the  surface  of  the  earth 
were  smooth,  the  ocean  would  cover  it  entirely  to  a  depth  of 
nearly  two  miles.  Finally,  the  atmosphere  is  a  gaseous  envelope, 
which  encloses  the  earth  completely. 


CHAPTER   A 
THE   ROCK-FORMING    MINERALS 

OF  the  simple  undecomposable  substances  which  chemists  call 
elements,  and  of  which  rather  more  than  seventy  have  been  dis- 
covered, only  sixteen  enter  at  all  largely  into  the  composition  of  the 
earth's  crust,  so  far  as  this  is  accessible  to  observation.  It  is 
estimated  that  98  %  of  the  crust  is  made  up  of  the  following 
eight  elements,  arranged  in  the  order  of  abundance,  with  the  per- 
centages as  calculated  by  F.  W.  Clarke. 

Oxygen 47-Q7  Calcium 3.44 

Silicon .  .  .  .|.  .  .  28.06  Magnesium 2.40 

Aluminium 7.90  Sodium 2.43 

Iron 4.43  Potassium 2.45 

The  remaining  eight  elements,  titanium,  carbon,  sulphur,  hydrogen, 
chlorine,  phosphorus,  manganese,  and  barium,  are  far  less  abun- 
dant, but  still  of  considerable  importance. 

Only  two  of  these  elements,  carbon  and  sulphur,  are  found  in  a 
more  or  less  impure  state  as  minerals  or  rock  masses;  the  others 
occur  as  compounds,  formed  by  the  union  of  two  or  more  of 
them. 

A  mineral  is  a  natural,  inorganic  substance,  which  has  a  homo- 
geneous structure,  definite  chemical  composition  and  physical 
properties,  and  usually  a  definite  crystal  form. 

Crystals  are  solids  of  more  or  less  regular  and  symmetrical  shape, 
bounded,  usually,  by  plane  surfaces.  The  number  of  known  crys- 
tal forms  is  very  great,  and  yet  they  may  be  all  grouped  in 
six  systems,  which  are  characterized  by  the  relations  of  their 

6 


SYSTEMS  OF  CRYSTAL  FORMS  7 

axes.  The  axes  of  a  crystal  are  imaginary  lines,  which  connect 
the  centres  of  opposite  faces,  or  opposite  edges,  or  opposite  solid 
angles,  and  which  intersect  one  another  at  a  point  in  the  interior 
of  the  crystal. 

The  Systems  of  Crystal  Forms  have  received  many  names,  the 
following  being  those  which  are  most  generally  used  in  this 
country:  — 

I.  Isometric  System  (monometric,  cubical,  regular). — In  this 
system  the  three  axes  are  of  equal  length  and  intersect  one  another 
at  right  angles. 

II.  Tetragonal  System  (dimetric,  pyramidal).  — The  axes  inter- 
sect at  right  angles,  but  while  the  lateral  axes  are  of  equal  length, 
the  vertical  axis  is  longer  or  shorter  than  the  laterals. 

HI.  Hexagonal  System.  —  Here  four  axes  are  employed,  three 
equal  lateral  axes  intersecting  at  angles  of  60°,  and  a  vertical 
axis,  which  is  perpendicular  to  and  longer  or  shorter  than  the 
laterals. 

IV.  Orthorhombic   System   (rhombic,   trimetric).  — The  three 
axes  intersect  at  right  angles  and  are  all  of  different  lengths. 

V.  Monoclinic  System  (monosymmetric,  oblique). — All  three 
axes  are  of  different  lengths;  the  two  laterals  are  at  right  angles  to 
each  other,  while  the  third  is  oblique  to  one  of  the  former. 

VI.  Triclinic  System  (anorthic,  asymmetric).  — Three  axes  of 
unequal  lengths  and  oblique  to  one  another. 

It  is  important  to  bear  in  mind  the  relations  which  the  forms 
sustain  toward  one  another.  For  example,  a  regular  octahedron 
may  be  derived  from  a  cube  by  evenly  paring  off  the  eight  solid 
angles,  until  the  planes  thus  produced  intersect  one  another, 
the  centres  of  the  faces  of  the  cube  becoming  the  apices  of  the 
solid  angles  of  the  octahedron.  Conversely,  a  cube  may  be 
formed  from  an  octahedron  by  symmetrically  truncating  the 
angles,  until  the  planes  thus  formed  intersect.  By  slicing  away  the 
twelve  edges  of  a  cube  or  an  octahedron  a  dodecahedron  will 
result.  These  ^crystal  forms  are,  therefore,  so  related  as  to  be 
all  derivable  one  from  another,  and  the  relations  of  their  axes 


8  THE   ROCK-FORMING   MINERALS 

remain  unchanged;  all  three  forms  may  be  assumed  by  the  same 
mineral,  and  they  thus  properly  belong  in  the  same  system.  Simi- 
lar relations  may  be  observed  between  the  crystal  forms  of  the 
other  systems. 

It  might  be  supposed  that  the  crystal  systems  and  the  rela- 
tions of  their  imaginary  axes  were  merely  mathematical  devices  to 
reach  a  convenient  classification  of  forms.  Such  a  conclusion 
would,  however,  be  a  very  erroneous  one.  Crystalline  form  is  an 
expression  of  molecular  structure,  and  the  physical  properties 
of  minerals  are  closely  related  to  their  mathematical  figure.  It 
is  clear  that  these  physical  properties  are  not  inherent  in  the 
molecules  of  the  mineral,  but  are  conditioned  by  the  way  in  which 
the  molecules  are  built  up  into  the  crystal.  Amorphous  substances 
refract  light  equally  in  all  directions,  and  are  thus  called  iso- 
tropic;  but  when  an  amorphous  substance  crystallizes,  it  assumes 
the  qualities  proper  to  its  crystal  form.  Thus  water  is  iso- 
tropic,  while  the  hexagonal  crystals  of  ice  are  singly  refractive 
in  only  one  direction,  doubly  refractive  in  all  others.  The  same 
substance  may,  unde^  different  circumstances,  crystallize  in  differ- 
ent systems,  and  will  then  display  the  properties  appropriate  to 
each  system. 

Not  only  the  refractive  powers  of  a  crystal,  but  also  its  mode  of 
expansion  when  heated,  and  its  conductivity  of  electricity  and  heat 
are  controlled  by  the  molecular  structure  which  determines  its 
shape. 

The  crystals  of  the  isometric  system,  which  have  their  three  axes 
of  equal  length,  are  singly  refractive  in  all  directions,  expand 
equally  when  heated,  and  conduct  heat  and  electricity  equally  in 
all  directions.  Those  of  the  tetragonal  and  hexagonal  systems, 
which  have  one  axis  longer  or  shorter  than  the  others,  are  doubly 
refractive  along  the  lateral  axes,  expand  equally  when  heated,  and 
show  equal  conductivity  along  these  axes.  Along  the  principal 
axis  they  are  singly  refractive,  expand  to  a  different  degree  when 
heated,  and  display  a  different  conductivity  along  this  axis  than 
along  the  others.  In  the  orthorhombic,  monoclinic,  and  triclinic 


FORMS   AND  COMBINATIONS  9 

systems,  which  have  all  the  axes  of  unequal  lengths,  the  crystals 
are  singly  refractive  in  two  directions;  they  expand  unequally  and 
conduct  differently  along  all  their  axes. 

The  optical  properties  of  minerals  are  of  great  value  in  the 
study  of  rocks,  and  by  the  aid  of  the  polarizing  microscope  very 
minute  crystals  may  be  identified. 

Cleavage  (see  p.  n)  is  still  another  physical  property,  the 
dependence  of  which  upon  crystal  form  is  very  clear. 

Most  inorganic  substances  which  are  solid  under  any  circum- 
stances are  capable  of  assuming  a  crystal  form,  so  that  solidi- 
fication and  crystallization  are  usually  identical.  For  the  forma- 
tion of  large  and  regular  crystals,  it  is  necessary  that  the  process 
be  gradual  and  that  space  be  given  for  the  individual  crystals  to 
grow.  Usually  crystallization  begins  at  many  points  simultane- 
ously, and  the  crystals  crowd  upon  one  another,  resulting  in  a 
mass  of  more  or  less  irregular  crystalline  grains.  The  same  sub- 
stance which,  when  very  rapidly  solidified,  forms  an  amorphous 
glass,  will  give  rise  to  distinct  crystals,  if  slowly  solidified. 

Crystallization  requires  that  the  molecules  be  free  to  move  upon 
each  other,  and  thus  to  arrange  themselves  in  a  definite  fashion. 
It  may  take  place  either  by  the  deposition  of  a  solid  from  solu- 
tion, by  cooling  from  a  state  of  fusion,  or  by  solidification  from 
the  condition  of  vapour.  In  all  cases  the  size  and  regularity 
of  the  crystals  depend  upon  the  time  and  space  allowed  for 
their  growth.  In  a  manner  not  yet  understood,  amorphous 
solids  may  be  converted  into  crystalline  aggregates.  This  has 
been  observed  in  the  case  of  certain  glassy  volcanic  rocks,  which, 
though  amorphous  when  first  solidified,  have  gradually  become 
crystalline,  without  losing  their  solidity,  and  a  similar  change  has 
been  observed  in  certain  artificial  glasses.  This  process  is  called 
devitrification. 

The  actual  steps  of  crystallization  may  be  observed  by  slowly 
evaporating  a  solution  of  some  crystalline  salt  under  the  micro- 
scope. The  first  visible  step  in  the  process  is  the  appearance  of 
innumerable  dark  points  in  the  fluid,  which  rapidly  grow,  until 


10  THE   ROCK-FORMING   MINERALS 

their  spherical  shape  is  made  apparent.  The  globules  then  begin 
to  move  about  rapidly  and  arrange  themselves  in  straight  lines, 
like  strings  of  beads,  and  next  suddenly  coalesce  into  straight  rods. 
The  rods  arrange  themselves  into  layers,  and  thus  build  up  the 
crystals  so  rapidly,  that  it  is  hardly  possible  to  follow  the  steps  of 
change.  In  certain  glassy  rocks,  which  solidified  too  quickly  to 
allow  crystallization  to  take  place,  the  incipient  stages  of  crystals, 
in  the  form  of  globules  and  hair-like  rods,  may  be  detected  with 
the  microscope. 

Forms  and  Combinations.  —  A  form  is  an  assemblage  of  faces, 
all  of  which  have  similar  relations  to  the  axes.  Two  or  more 
forms  occurring  as  a  single  crystal  constitute  a  combination. 
Only  forms  belonging  to  the  same  system  can  occur  in  combina- 
tion, but,  even  with  this  limitation,  the  variety  and  complexity 
of  crystals  are  very  great.  Certain  forms  occur  which  may  be 
regarded  as  developed  from  other  forms  by  the  suppression  of 
one-half  or  three-quarters  of  the  faces  of  the  latter. 

Irregularities  of  growth  (distortion)  are  very  common,  some 
faces  of  a  form  being  larger  than  others,  while  certain  faces  may 
even  be  obliterated;'  but  however  great  the  variation,  the  angle 
at  which  corresponding  faces  meet  invariably  remains  constant 
for  each  mineral. 

Massive  and  imperfectly  crystallized  minerals  may  consist  of 
grains,  fibres,  or  thin  layers  (lamina). 

Hardness.  — The  hardness  of  minerals  is  a  useful  means  of  iden- 
tifying them.  For  this  purpose  they  are  referred  to  a  scale  of 
hardness,  ranging  from  such  soft  substances  as  may  be  readily 
scratched  with  the  finger-nail,  to  the  hardest  known  substance, 
diamond.  The  degree  of  hardness  is  expressed  by  the  numerical 
place  of  the  mineral  in  the  scale,  and  intermediate  grades  are 
indicated  by  fractions.  Thus  a  mineral  which  is  scratched  by 
quartz,  and  scratches  orthoclase  with  equal  ease,  has  a  hardness  of 
6.5.  The  scale  is  as  follows:  — 


THE   ROCK-FORMING   MINERALS  II 

1.  Talc.  6.  Orthoclase. 

2.  Gypsum.  7.  Quartz. 

3.  Calcite.  8.  Topaz. 

4.  Fluorite.  9.  Sapphire. 

5.  Apatite.  10.  Diamond. 

Cleavage.  —Many  minerals  split  more  or  less  readily  in  certain 
fixed  directions,  while  in  other  directions  they  break  irregularly. 
This  property  is  called  cleavage.  Cleavage  is  uniform  in  different 
crystals  of  the  same  mineral,  and  is  parallel  to  actual  or  possible 
crystal  faces. 

Pseudomorphs  occur  when  one  mineral  assumes  the  crystal 
form  proper  to  another.  This  may  take  place  either  by  the  addi- 
tion or  the  removal  of  certain  constituents,  or  some  constituents 
may  be  removed  and  others  substituted  for  them.  The  entire 
substance  of  a  mineral  may  be  removed  and  its  place  taken, 
molecule  by  molecule,  by  another,  retaining  the  form,  sometimes 
even  the  cleavage,  of  the  first.  The  study  of  pseudomorphs  is 
often  of  the  greatest  service,  as  throwing  light  upon  the  history 
of  the  rock  in  which  they  occur. 

Compound  crystals  are  formed  by  the  joining  of  simple  crystals. 
When  two  half-crystals  are  united  along  a  plane  in  such  a  way 
that  their  faces  and  axes  do  not  correspond,  they  are  said  to  be 
twinned.  When  the  twinning  is  repeated  along  numerous  parallel 
planes,  the  crystal  is  a  polysynthetic  twin.  Two  crystals  united  at 
the  ends  to  form  a  right  angle  are  called  geniculate,  while  two 
geniculate  crystals  may  be  so  combined  as  to  form  a  cross,  and 
then  are  said  to  be  cruciform. 

Rock-forming  Minerals.  — The  number  of  known  minerals  is 
large  and  constantly  increasing,  but  only  a  few  enter  in  any 
important  way  into  the  constitution  of  the  earth's  crust.  We 
now  proceed  to  a  consideration  of  these  constituent  minerals, 
which  are  called  rock-forming  minerals,  because  the  rocks  are 
aggregations  of  them.  It  must  be  emphasized  that  the  student 
can  gain  no  real  knowledge  of  minerals  or  rocks  by  merely  reading 


12  THE   ROCK-FORMING   MINERALS 

about  them;    it  is  necessary  that  he  should  familiarize  himseli 
with  actual  specimens. 

A.  MINERALS  COMPOSED  OF  SILICA 

Next  to  oxygen,  silicon  is  by  far  the  most  abundant  constituent 
of  the  earth's  crust,  though  never  occurring  alone.  It  is  united 
with  oxygen  to  form  silica  (SiO2)  or  enters  into  the  formation  of 
more  complex  compounds. 

1.  Quartz  (SiO2)  is  anhydrous  silica  in  a  crystalline  state  and  is 
one  of  the  most  abundant  of  minerals.     It  belongs  in  the  hexag- 
onal system,  and   crystallizes  in    hexagonal    prisms  capped   by 
six-sided  pyramids,  or  in  double  six-sided  pyramids,  or  in  modi- 
fications of  these  forms.     It  is  insoluble  in  acids,  except  hydro- 
fluoric, and  only. very  slowly  soluble  in  boiling  caustic  alkalies. 

Quartz  has  no  cleavage  and  is  very  hard  (H=y),  scratching 
glass  readily,  while  it  cannot  be  scratched  with  a  knife;  the  spe- 
cific gravity  (sp.  gr.)  is  2.6. 

When  pure  and  symmetrically  crystallized,  quartz  is  transparent, 
colourless,  and"  lustrous  (rock  crystal),  but  it  more  commonly  is 
found  in  dull  masses.  Many  different  colours  are  produced  by 
minute  quantities  of  foreign  substances  in  the  crystals. 

2.  Chalcedony  occurs  in  spheroidal  or  stalactitic  masses,  com- 
posed of  more  or  less  concentric  shells.    The  chemical  composition 
and  behaviour  of  this  mineral  are  the  same  as  in  quartz,  but  the 
specific  gravity  is  somewhat   lower,    and   the  optical  properties 
are  different.    Chalcedony  has  a  waxy  appearance,  and  is  trans- 
lucent or  semi-opaque,  and  of  various  pale  colours. 

3.  Flint  and  Chert  are  mixtures  of  hydrated  and  anhydrous 
silica.    They  occur  in  amorphous  masses  of  neutral  or  dark  colours, 
and  are  opaque,  or  somewhat  translucent  in  thin  pieces. 

B.  MINERALS  COMPOSED  OF  SILICATES 

There  are  several  silicic  acids,  which  form  a  very  extensive 
series  of  compounds  with  various  metallic  bases.  As  rock-form- 
ing minerals  the  silicates  are  of  the  first  importance. 


THE  FELSPAR  GROUP  13 


I.  THE  FELSPAR  GROUP 

The  felspars  are  essentially  silicates  of  alumina  (A12O3)  to- 
gether with  potash,  soda,  or  lime.  Orthoclase  and  microcline 
are  potash  felspars  (K2O,  A12O3,  6  SiO2);  albite  is  a  soda  felspar 
(Na2O,  A12O3,  6  SiO2);  and  anorthite,  a  lime  felspar  (CaO, 
A12O3,  2  SiO2).  From  the  combination  of  these  two  series  are 
formed:  the  lime-soda  series,  oligodase,  andesine,  and  labradorite, 
the  plagioclases,  and  the  potash-soda  series,  anorthoclase. 

The  felspars  crystallize  in  either  the  monoclinic  or  triclinic  sys- 
tems, but  the  forms  of  the  crystals  are  very  much  alike.  With 
few  exceptions,  these  minerals  are  of  pale  colours  and,  except 
when  decomposing,  are  very  hard. 

i.    Monoclinic  Felspars 

Orthoclase  is  a  potash  felspar  (K2O,  A12O3,  6  SiO2=K,Al, 
Si3O8),  though  soda  may  replace  part  of  the  potash.  Hardness 
=  6,  sp.  gr.  =  2.54-2.57.  Orthoclase  crystallizes  in  oblique  rhombic 
prisms  and  is  very  generally  twinned;  there  are  two  sets  of  cleav- 
age planes,  which  intersect  at  a  right  angle  and  have  thus  given 
its  name  to  the  mineral.  Orthoclase  is  usually  dull  and  turbid, 
which  is  due  to  the  presence  of  various  alteration  products,  and 
even  thin  sections  under  the  microscope  are  commonly  hazy. 
Sanidine  is  a  glassy,  transparent  variety  of  orthoclase,  which  is 
found  in  lavas  of  late  geological  date.  Its  clearness  is  due  to  the 
absence  of  the  decomposition  products,  which  render  ordinary 
orthoclase  turbid. 

2.    Triclinic  Felspars 

The  minerals  of  this  series  are  grouped  together  under  the  com- 
prehensive term  of  Plagioclase,  because  of  the  difficulty  of  distin- 
guishing them  from  each  other  under  the  microscope;  they  are 
very  generally  characterized  by  polysynthetic  twinning,  which 
makes  fine  parallel  lines  on  the  basal  cleavage  planes.  Chemi- 


14  THE   ROCK-FORMING   MINERALS 

cally,  they  are  isomorphous  mixtures  of  albite  and  anorthite.  The 
following  table  (from  Levy  and  Lacroix)  gives  the  composition  of 
the  various  members  of  this  series,  representing  the  soda-felspar 
constituent,  or  albite,  by  Ab,  and  the  lime-felspar  constituent,  or 
anorthite,  by  An:  — 

NAME  COMPOSITION  SPECIFIC  GRAVITY 

Albite Ab 2.62 

Oligoclase  ....     AbioAna 2.65 

Andesine     ....     Ab2Ani 2.67 

Labradorite     .     .     .     Ab2An3 2.70 

Anorthite    ....     An 2.75 

It  will  be  observed  that  the  specific  gravity  increases  with  the 
lime  constituent,  and  the  fusibility  diminishes  in  the  same  propor- 
tion. Anorthite  is  decomposed  by  hydrochloric  acid,  labradorite 
is  slightly  attacked  by  it,  while  the  other  members  of  the  series 
are  not  affected. 

Anorthoclase  is  a  triclinic  potash-soda  felspar  (Ab2Ori),  but  is 
less  common  than  the  plagioclases  as  a  constituent  of  rocks. 

Microcline  has  the  composition  of  orthoclase  and  plays  a  similar 
role  in  rocks,  but  crystallizes  in  the  triclinic  instead  of  the  mono- 
clinic  system. 

II.  THE  FELSPATHOID  GROUP 

These  minerals  are  very  closely  allied  to  the  felspars  in  chemi- 
cal composition  and  geological  relations,  but  differ  from  them  in 
crystal  form  and  physical  properties.  They  have  a  much  more 
restricted  distribution  than  the  felspars,  but  have,  nevertheless, 
an  important  bearing  upon  the  classification  of  certain  groups  of 
rocks  in  which  they  occur. 

1.  Nepheline  is  a  silicate  of  potash,  soda,  and  alumina  (Na,  K)2 
O(A12O3,  2  SiO2).     It  crystallizes  in  transparent  and   colourless 
six-sided  prisms,  of  the  hexagonal  system.    11=5.5-6;  sp.  gr.  =  2.6. 
It  is  an  important  constituent  of  certain  lavas. 

2.  Leucite  is  composed  as  follows:   K2O,  A12O3,  4  SiO2,  with 
some  of  the  potash  replaced  bv  soda.     It  crvstallizes  in  twenty- 


AMPHIBOLE  AND   PYROXENE   GROUPS  15 

four-sided  figures  (trapezohedrons) ,  which  belong  to  the  tetrag- 
onal system,  but  can  be  distinguished  from  the  isometric  only  by 
very  careful  measurement.  11=5.5-5.6;  sp.  gr.  =  2.44~2.56. 

III.  THE  MICA  GROUP 

These  minerals  have  a  complex  chemical  composition,  and  are 
so  variable  that  it  is  difficult  to  give  formulae  for  them;  they  are 
silicates  of  alumina,  together  with  potash,  lithia,  magnesia,  iron,  or 
manganese.  When  crystallized,  the  micas  all  form  six-sided 
prisms,  which,  though  of  hexagonal  habit,  are  in  reality  mono- 
clinic.  All  varieties  have  a  remarkably  perfect  cleavage,  and  split 
into  thin,  elastic,  and  flexible  leaves,  by  which  they  may  be  readily 
recognized.  They  are  quite  soft,  and  most  of  them  may  be 
scratched  with  the  finger-nail. 

1.  Muscovite   may   be   selected   as   the   most   important   and 
wide-spread  of  the  numerous  alkaline  micas,  with  the  general 
formula,  K2O,  3  A12O3,  6  SiO2,  2  H2O.     It  is  a  lustrous,  silvery- 
white  mineral,  usually  transparent  and  colourless  in  thin  leaves; 
it  has  a  specific  gravity  of  2.76-3.1,  and  a  hardness  of  2.1-3. 

Sericite  is  a  silvery  or  pale  green  form  of  muscovite,  which  is 
an  alteration  product  and  often  is  derived  from  a  felspar. 

2.  Biotite  is  the  most  important  and  widely  disseminated  of  the 
numerous  dark-coloured,  ferromagnesian  micas.    This  mineral  is 
black  or  dark  green  in  mass,  and  smoky  even  in  thin  leaves; 
chemically  it   is  a  silicate  of  potash,  alumina,  iron,  and    mag- 
nesia.    In  hardness  and   specific  gravity   it  differs  little  from 
muscovite. 

IV.  THE  AMPHIBOLE  AND  PYROXENE  GROUPS 

These  two  groups  contain  parallel  series  of  minerals  of  similar 
chemical  composition,  but  differing  in  their  crystallization  and 
physical  properties.  In  composition  they  range  from  silicates  of 
magnesia  to  those  of  lime  and  lime-alumina,  while  iron  is 


1 6  THE  ROCK-FORMING   MINERALS 

present  in  most  of  them.  They  belong  to  the  orthorhombic  and 
monoclinic  systems,  and  can  be  distinguished  by  their  cleavage. 
The  pyroxenes  have  a  prismatic  cleavage  of  nearly  90°,  while  in 
the  amphiboles  the  angles  are  124°  30'  and  55°  30'.  The  ortho- 
rhombic  amphiboles  are  rare  and  unimportant  as  rock-forming 
minerals,  but  the  pyroxenes  of  this  form  are  widely  distributed, 
though  less  so  than  the  monoclinic. 

a.  Orthorhombic  Pyroxenes  are  silicates  of  magnesia  and  iron 
(Mg,  Fe)O,  SiO2. 

1.  Enstatite  has  less  than  5  %  of  FeO. 

2.  Bronzite  has  5-14  %  of  FeO. 

3.  Hypersthene  has  more  than  14  %  of  FeO. 

The  colour  becomes  darker  and  the  optical  properties  change 
with  the  increase  in  the  percentage  of  iron. 

b.  Monodinic  Pyroxenes. 

1.  Augite.  — This  very  abundant  and  important  mineral  is  a 
silicate  of  kme,  magnesia,   iron,  and  alumina  (Ca,  Mg,  Fe)O, 
(Al,  Fe)2O3,  4SiO2.     Sp.  gr.  =3.3-3. 5;    H  =  5~6.     It  crystallizes 
in  oblique  rhombic  .prisms,  and  in  colour  is  green  to  black  and 
opaque. 

2.  Diallage  is  a  variety  of  augite,  usually  of  a  green  colour, 
which  is  distinguished  by  its  laminated  structure,  with  lustrous 
faces. 

c.  Monoclinic  Amphiboles. 

1.  Hornblende,  like  augite,  which  it  closely  resembles  in  chemi- 
cal composition,  is  among  the  most  important  of  rock-forming 
minerals.     In  colour  it  is  usually  green,  brown,  or  black,  and  it 
crystallizes  in  modified  oblique  rhombic  prisms.     Sp.  gr.  =  2.9~ 
3.5;   H.  =  5-6. 

2.  Tremolite  is  a   silicate  of  magnesia  and   lime  (Ca,  Mg)O, 
SiO2.    This  mineral  is  pale  green  or  white  and  occurs  in  laminae  or 
long,  blade-like  crystals. 

3.  Actinolite  resembles  tremolite  in  composition,  with  the  addi- 
tion   of   iron  (Ca,  Mg,   Fe)O,  SiO2.       Colour,  green;    sp.  gr.  = 
3-3.2;  usually  occurs  in  long  and  thin  crystals.     A  fibrous  variety 


GARNET  GROUPS  17 

of  tremolite  or  actinolite,  in  which  the  fibres  are  often  like  flexible 
threads  and  may  be  woven  into  cloth,  is  called  asbestus. 

V.  THE  OLIVINE  GROUP 

Olivine  is  the  only  mineral  of  this  group  of  sufficient  impor- 
tance to  require  mention;  it  is  a  silicate  of  magnesia  and  iron, 
2(Mg,  Fe)O  SiO2,  though  the  percentage  of  iron  varies  greatly. 
Sp.  gr.  =3.2-3.5;  11  =  6.5-7.  Olivine  crystallizes  in  the  ortho- 
rhombic  system,  and  occurs  in  prisms,  flat  tables,  or  irregular 
grains.  The  colour  varies  from  olive-green  to  yellow,  or  it  may 
be  colourless,  and  usually  the  irregular  grains  look  like  fragments 
of  bottle  glass. 

VI.  THE  EPIDOTE  GROUP 

Epidote  is  a  silicate  of  alumina,  with  iron  and  lime,  the  different 
varieties  being  named  according  to  the  preponderance  of  one  or 
other  base.  Iron-epidote  (Pistazite)  forms  monoclinic  crystals, 
with  a  specific  gravity  of  3.2-3.5,  and  a  hardness  of  6-7.  Lime- 
epidote  (Zoisite),  which  has  little  or  no  iron,  is  orthorhombic. 

VII.  THE  GARNET  GROUP 

The  Garnets  are  highly  complex  silicates  of  alumina,  iron,  lime, 
magnesia,  chromium,  and  manganese,  though  in  most  cases  only 
two  or  three  of  the  bases  are  present  in  any  considerable  quantity, 
and  the  different  varieties  have  received  names  in  accordance 
with  the  predominating  bases.  They  usually  crystallize  as  dodeca- 
hedrons, or  twelve-sided  figures.  Sp.  gr.=3.4~4.3;  £[=6.^-7.5. 
Clear  and  brilliantly  coloured  garnets  are  considerably  used  in 
jewellery. 

The  commonest  variety  (Almandine)  is  a  silicate  of  alumina  and 
iron,  and  is  usually  red. 

C.  OTHER  SILICATES,  CHIEFLY  DECOMPOSITION  PRODUCTS 

Many  of  the  complex  silicates,  when  long  exposed  to  the  action 
of  the  weather  and  of  percolating  waters,  become  more  or  less 
c 


1 8  THE  ROCK-FORMING   MINERALS 

profoundly  changed  chemically,  a  change  which  is  known  as 
alteration  and  forms  an  early  stage  of  decay.  One  of  the  common- 
est of  these  changes  is  hydration,  or  the  taking  up  of  water  into 
chemical  union,  and  this  may  be  accompanied  by  the  loss  of  soluble 
ingredients,  or  the  replacement  of  some  constituents  by  others. 

I.   ZEOLITES 

In  this  group  are  included  a  large  number  of  minerals,  which 
are  hydrated  silicates  of  alumina,  potash,  soda,  lime,  etc.  They 
all  contain  water  and  hence  boil  and  effervesce  when  heated 
before  the  blowpipe.  All  these  minerals  are  products  of  decom- 
position and  do  not  occur  as  original  constituents  of  rocks. 

II.  TALC  AND  CHLORITE  GROUPS 

1.  Chlorite.  —  Under  this  name  are  grouped  a  number  of  closely 
allied  minerals,  which  are  hydrated  silicates  of  alumina,  magnesia, 
and  iron.    They  are  soft  minerals,  with  a  hardness  of  1-1.5  an^  a 
specific  gravity  of  2.6-2.96,  and  are  usually  of  a  green  colour. 
They  crystallize  in  the  monoclinic  system,  with  a  pseudo-hexagonal 
symmetry.    These  minerals  are  laminated  and  split  readily  into 
thin  leaves,  as  do  the  micas,  from  which  they  may  be  distinguished 
by  the  fact  that  the  leaves  are  not  elastic. 

The  chlorites  result  from  the  decomposition  of  hornblende, 
augite,  or  the  magnesian  micas. 

2.  Talc  is  a  hydrated  silicate  of  magnesia,  3MgO,  4  SiO2,  H2O; 
the  water  varies  in  amount  to  as  much  as  7  per  cent.     Sp.  gr.  =  2.56- 
2.8;  H= i.     It  is  of  a  white  or  pale  green  colour,  with  a  pearly 
lustre  and  a  greasy,  soapy  feeling  to  the  touch.    Talc  is  rarely  found 
crystallized;  the  crystals  have  a  false  hexagonal  symmetry,  and  it 
is  doubtful  whether  they  should  be  referred  to  the  orthorhombic 
or  monoclinic  systems.     Usually  it  occurs  in  flakes  or  foliated 
masses,  which  split  into  thin,  non-elastic  leaves.    Talc  results  from 
the  alteration  of  magnesian  minerals. 


CALCAREOUS  MINERALS  19 

3.  Steatite,  or  Soapstone,  has  the  same  composition  as  talc,  but 
is  not  foliated,  and  may  be  much  harder,  as  much  as  2.5. 

4.  Serpentine   is  a  hydrated  silicate  of  magnesia  and  iron: 
3(Mg,  Fe)O,  2  SiO2,  2  H2O.      It  does  not  crystallize,  but  is  rather 
common    in   pseudomorphs.     Sp.  gr.  =  2.5~2.65;  11=2.5-4.      Its 
proper  colour  is  green,  but  it  is  usually  mottled  with  red  or  yellow 
by  iron  stains.     Serpentine  is  generally  formed  from  the  decay  of 
olivine,  less  commonly  from  augite,  or  hornblende. 

Kaolinite  is  the  hydrated  silicate  of  alumina,  A12O3,  2  SiO2, 
2  H2O.  It  is  usually  soft  and  plastic,  but  orthorhombic  crystals  of 
pseudo-hexagonal  symmetry  may  be  sometimes  detected  with  the 
microscope.  Kaolinite  arises  from  the  decomposition  of  the  fel- 
spars and  especially  of  orthoclase. 

Glauconite  is  a  hydrated  silicate  of  iron  and  potassium,  with 
small  quantities  of  alumina,  lime,  magnesia,  and  soda.  It  is  of  a 
green  colour,  soft  and  friable. 

D.  CALCAREOUS  MINERALS 

1.  Calcite,  carbonate  of  lime,  CaCO3.     Sp.  gr.  =  2.72;    H=3. 
This  mineral  crystallizes  in  the  hexagonal  system,  in  a  great  vari- 
ety of  forms;    rhombohedrons  and  scalenohedrons  are  common; 
hexagonal  prisms  and  pyramids  less  so.    Cleavage  is  very  perfect, 
parallel  to  the  faces  of  a  rhombohedron,  and  the  mineral  breaks  up 
into  rhombohedrons  when  struck  a  sharp  blow.    Calcite  is  rapidly 
attacked,  even  by  cold  and  weak  acids,  CO2  escaping  with  effer- 
vescence.    When  pure,  as  in  Iceland  spar,  the  mineral  is  colour- 
less, very  transparent,  and  lustrous,  and  displays  the  phenomenon 
of  double  refraction  strongly;   but  more  commonly  it  is  cloudy  or 
white,  or  stained  red  or  yellow  by  iron.     It  is  soluble  in  water 
holding  CO2,  affording  calcium  bicarbonate  which  is  found  in 
nearly  all  natural  waters.     It  is  widely  diffused  among  the  rocks, 
and  in  a  state  of  varying  purity  forms  great  masses  of  limestone. 

2.  Aragonite  (CaCO3)  is  somewhat  harder  and  heavier  than 


2O  THE   ROCK-FORMING   MINERALS 

calcite,  with  a  specific  gravity  of  2.93  and  a  hardness  of  3.5-4, 
and  crystallizes  in  compound  prismatic  forms  which  belong  to  the 
orthorhombic  system.  It  has  not  the  marked  cleavage  of  calcite 
and  is  less  stable  as  a  rule;  when  heated  it  is  converted  into  calcite 
and  falls  into  tiny  rhombohedrons  of  that  mineral. 

3.  Dolomite  is  a  carbonate  of  lime  and  magnesia  (Ca,Mg)  CO3; 
it  resembles  calcite  in  appearance,  and  crystallizes  in  rhombohe- 
drons which  of  ten  have  curved  faces.     Sp.  gr.  =  2.8-2.9;  H  =  3.5-4. 
Dolomite  may  be  readily  distinguished  from  calcite  by  the  fact 
that  cold  acids  affect  it  but  little. 

4.  Gypsum,  hvdrated  sulphate  of  lime,  CaSO4,  2  H2O.     Sp.  gr. 
=  2.31-2.33;     H  =  1.5-2.     It    crystallizes   in    right   rhomboidal 
prisms,  belonging  to  the  monoclinic  system,  and  cleaves  into  thin, 
non-elastic  leaves.     When  pure,  gypsum  is  transparent  and  colour- 
less, but  is  often  stained  by  iron.    This  mineral  occurs  largely  in 
granular  masses,  from  which  plaster  of  Paris  is  made  by  calcining 
the   gypsum   and    so   driving   off    the  water   of    crystallization. 
Alabaster  is  a  gypsum  of  especially  fine  grain,  mottled  in  pale 
colours,  or  white.     Selenite  is  a  transparent  variety. 

5.  Anhydrite,  Ca^O4,  is  sulphate  of  lime  without  water;    it  is 
harder  and  heavier  than  gypsum  (Sp.  gr.  =  2.9-2.98;  11  =  3-3.5), 
and  crystallizes  in  a  different  system,  the  orthorhombic.    The 
crystals  have  three  sets  of  cleavage  planes,  which  intersect  each 
other  at  right  angles. 

6.  Apatite  is  a  phosphate  and  chloride  or  fluoride  of  calcium, 
3(Ca3P2O8),  2(Ca,Cl,  F).     Sp.  gr.  =  2.92-3.25;  H  =  5.     It  crys- 
tallizes in  hexagonal  prisms,  terminated  by  hexagonal  pyramids, 
and  also  occurs  in  masses.     It  is  sometimes  transparent  and  colour- 
less, but  more  commonly  opaque  brown  or  green.     Apatite  is  solu- 
ble in  acids,  and  in  water  containing  carbon  dioxide,  or  ammonia; 
and  gives  rise  to  a  valuable  plant  food. 

7.  Fluorite,   fluoride   of   calcium,  CaF2.     Sp.  gr.  =3.01-3.25; 
H  =  4.    Crystallizes  in  the  isometric  system,  usually  in  cubes,  and 
has  a  perfect  octahedral  cleavage.     When  pure,  fluorite  is  either 
clear  and  colourless,  or  blue,  green,  yellow,  or  brown. 


IRON   MINERALS  21 

E.  IRON  MINERALS 

1.  Haematite,  or  Specular  Iron,  is  ferric  oxide,  Fe2O3.     Sp.  gr.  = 
4.5-5.3;     H  =  6.5-    Crystallizes     in    rhombohedrons,     or    more 
commonly,  in  nodular  masses.    The  colour  is  black,  steel-grey, 
or  red,  and  always  is  red  when  the  mineral  is  finely  powdered. 
Haematite  frequently  contains  earthy  and  other  impurities  and  is 
one  of  the  most  important  ores  of  iron. 

2.  Limonite,   or   Brown   Haematite,   is   hydrated   ferric  oxide 
(2  Fe2O3,  3  H2O)  containing  more  than  14%  of  water.     It  is  softer 
than  haematite  and  of  a  yellow  or  brown  colour.     Sp.  gr.  =3.6-4; 

H  =  S-5.5. 

3.  Magnetite  is  the  black  oxide  of  iron,  Fe3O4  (or  FeO,  Fe2O3). 
Sp.    gr.  =4.9-5.2;    H  =  5.5~6.5.    Crystallizes    in   the    isometric 
system,   usually   in    octahedrons,   sometimes   in   dodecahedrons. 
This  mineral  is  strongly  magnetic  and  is  black  in  colour,  with  a 
bluish-black  metallic  lustre,  when  viewed  in  reflected  light.    Mag- 
netite is  widely  diffused  in  certain  classes  of  rocks,  and  also  occurs 
in  veins  and  beds,  which  form  an  important  source  of  supply  of 
the  metal. 

4.  Hmenite  is  an  oxide  of  iron  and  titanium  (Ti,  Fe)2O3.     Sp.  gr. 
=  4.5-5.2;    H=5~6.     When  crystallized,  this  mineral  is  rhom- 
bohedral,  but  is  generally  massive. 

5.  Siderite   is   ferrous  carbonate,    FeCO3.     Sp.    gr.  =3.7-3.9; 
H  =  3.5-4.5.    Crystallizes  in  rhombohedrons,  the  faces  of  the  crys- 
tals frequently  much  curved,  and  often  the  crystals  are  very  much 
flattened.     When  fresh,  the  mineral  is  grey  or  brown.     It  is  but 
slightly  acted  on  by  cold  acids;    hot  acids  dissolve  it  with  effer- 
vescence.    Mixed    with   clay,   siderite    forms  clay  iron-stone,  a 
valuable  ore. 

6.  Pyrite,  or  Iron  Pyrites,  bisulphide  of  iron,  FeS2.     Sp.  gr. 
=  4.9-5.2;  H  =  6-6.5.     Crystallizes  in  the  isometric  system,  usu- 
ally in  cubes,  sometimes  in  dodecahedrons,  and  has  a  very  char- 
acteristic brassy  lustre  and  colour,  to  which  it  owes  the  popular 
name  of  "  fools'  gold."     It  is   very  hard,  cannot  be   scratched 


22  THE   ROCK-FORMING   MINERALS 

with  a  knife,  and  strikes  fire,  like  flint,  when  struck  with  steel. 
The  mineral  is  soluble  in  nitric  acid:  it  is  widely  disseminated 
in  the  rocks. 

7.  Marcasite,  or  White  Iron  Pyrites,  has  the  same  composition 
as  pyrite,  but  crystallizes  in  the  orthorhombic  system,  in  modified 
prisms,  but  more  commonly  occurs  in  nodular  masses,  with  a  radial 
structure.  It  has  the  same  hardness  as  pyrite,  but  is  not  quite 
so  heavy.  Sp.  gr.  =  4.68-4.85.  In  colour  it  is  paler  than  pyrite, 
with  a  tendency  to  grey,  green,  or  even  black.  It  decomposes 
very  readily,  and  after  a  few  months'  exposure,  even  to  dry  air, 
often  crumbles  to  a  whitish  powder. 

The  iron  minerals  are  seldom  largely  represented  in  any  given 
rock,  with  the  exception  of  the  ore  beds;  but  iron  is  one  of  the 
most  widely  diffused  of  substances,  few  rocks  being  altogether  free 
from  it,  and  its  various  compounds  play  a  very  important  role  as 
colouring-matter  in  the  rocks. 


PART   I 

DYNAMICAL  GEOLOGY 

WE  have  already  seen  that  the  chief  task  of  geology  is  to  con- 
struct a  history  of  the  earth,  to  determine  how  and  in  what  order 
the  rocks  were  formed,  through  what  changes  they  have  passed, 
and  how  they  reached  their  present  position.  The  logical  order 
of  treatment  might  seem  to  require  that  we  should  first  learn  what 
the  rocks  are,  of  what  they  are  composed,  and  how  they  are 
arranged,  before  attempting  to  explain  these  facts.  In  such  a 
study,  however,  we  should  meet  with  so  much  that  would  be  quite 
unintelligible,  that  a  more  convenient  way  will  be  to  begin  with 
a  study  of  the  agencies  which  are  at  work  upon  and  within  the 
earth,  and  which  tend  to  modify  it  in  one  or  other  particular.  In 
other  words,  we  must  employ  the  present  order  of  things  as  a  key 
by  means  of  which  to  decipher  the  hieroglyphics  of  the  past,  and 
proceed  from  what  may  be  directly  observed  to  past  changes 
which  can  only  be  inferred. 

We  might  assume  that  the  present  was  so  radically  different 
from  the  far-distant  past,  that  the  one  could  throw  no  light  upon 
the  other.  Such  an  assumption,  however,  would  be  most  illogical, 
for  there  is  nothing  to  support  it.  There  is  no  reason  to  imagine 
that  physical  and  chemical  laws  are  different  now  from  what  they 
have  always  been,  and  the  more  we  study  the  earth,  the  more 
clearly  we  perceive  that  its  history  is  a  continuous  whole,  deter- 
mined by  factors  of  the  same  sort  as  are  now  continuing  to 
modify  it.  Some  geologists  have  assumed  that  these  agencies  have 
always  acted  with  just  the  same  intensity  as  they  do  to-day;  but 

23 


24  IYNAMICAL  GEOLOGY 

this  assumption  is  neither  necessary,  nor  in  itself  probable. 
There  is,  on  the  contrary,  much  reason  to  believe  that  while  cer- 
tain forces  act  with  greater  efficiency  at  the  present  time  than 
they  did  in  the  past,  others  act  with  less. 

An  attentive  examination  of  the  changes  which  are  now  in  pro- 
gress on  the  surface  of  the  earth,  will  show  us  that  nothing  terres- 
trial is  quite  stable  or  unchangeable,  but  that  there  is  a  slow, 
ceaseless  circulation  of  matter  taking  place  on  the  surface  and 
within  the  crust  of  the  globe.  Matter,  chemistry  teaches  us,  is 
indestructible,  and,  disregarding  the  relatively  insignificant  amount 
of  material  which  reaches  us  from  outer  space  in  the  form  of 
meteorites,  the  sum  total  of  matter  composing  the  globe  remains 
constant.  But  while  practically  nothing  is  added  to  or  taken  away 
from  the  materials  which  make  up  the  earth's  crust,  ceaseless 
cycles  of  change  continually  alter  the  position,  physical  relations, 
and  chemical  combinations  of  those  materials.  This  circulation 
of  matter  may  be  aptly  compared  to  the  changes  which  take  place 
in  the  body  of  a  living  animal,  only,  of  course,  they  are  of  a  differ- 
ent kind  and  are  effected  at  an  infinitely  slower  rate.  In  the  ani- 
mal body,  so  long  as  life  lasts,  old  tissues  break  down  into  simpler 
compounds  and  are  ejected,  while  new  tissues  are  built  up  out 
of  fresh  material.  So,  on  the  earth  rock-masses  decay,  their 
particles  are  swept  away,  accumulate  in  a  new  place,  perhaps  far 
distant  from  their  source,  and  are  consolidated  into  new  rocks, 
which  in  their  turn  are  attacked  *hd  yield  materials  for  further 
combinations.  The  studv  of  the  physical  and  chemical  changes 
in  the  bodies  of  animals  and  plants  constitutes  the  science  of 
physiology,  and  by  analogy  we  may  call  dynamical  geology  the 
physiology  of  the  earth's  crust.  Analogies,  however,  must  not 
be  pushed  too  far,  or  they  land  us  in  absurdity.  One  essential 
difference  between  the  earth  and  a  living  organism  suggests  itself 
at  once;  namely,  that  the  former  is  self-contained,  and  neither 
ejects  old  material  nor  receives  new,  but  employs  the  same  matter 
over  and  over  again  in  ever-varying  combinations.  The  animal  or 
plant,  on  the  contrary,  continually  takes  in  new  material  from 


DYNAMICAL  GEOLOGY  25 

without,  in  the  shape  of  food,  and  ejects  the  waste  resulting  from 
the  breaking  down  of  tissue. 

Although  the  earth  needs  no  fresh  supplies  of  matter,  its  dynami- 
cal operations  are,  to  a  very  large  extent,  maintained  by  energy 
from  without;  namely,  from  the  sun.  The  circulation  of  the  winds 
and  waters,  the  changes  of  temperature,  and  the  activities  of  living 
beings,  all  depend  upon  the  sun's  energy,  and  were  that  with- 
drawn, only  such  changes  as  are  brought  about  by  the  earth's 
internal  heat  could  continue  in  operation. 

The  study  of  dynamical  agencies,  subterranean  and  surface, 
necessarily  gathers  together  an  enormous  mass  of  detail.  But  we 
need  concern  ourselves  with  only  so  much  of  this  as  throws  light 
upon  the  earth's  history,  so  that  the  sciences  of  dynamical  geology 
and  physical  geography,  though  having  much  in  common,  are 
not  coextensive.  In  order  to  make  clear  the  operations  of  the 
forces  which  tend  to  modify  the  surface  of  the  earth,  it  is  neces- 
sary that  we  should  classify  and  arrange  them,  so  that  they  may 
be  treated  in  a  more  or  less  logical  order.  However,  in  making 
such  a  classification",  it  is  impossible  to  avoid  entirely  a  certain 
arbitrariness  of  arrangement,  since  we  must  consider  separately 
agencies  that  act  together.  Natural  phenomena  are  not  due  to 
single  causes,  but  to  combinations  and  series  of  causes,  and  yet, 
to  make  them  intelligible,  they  must  be  treated  singly  or  in  simple 
groups,  else  we  shall  be  confronted  by  a  chaos  of  uncorrelated 
facts.  The  career  of  a  raindrop,  from  its  first  condensation  to  its 
entrance  into  the  sea  through  the  moutfy  of  some  river,  is  a  con- 
tinuous one,  yet  rain  and  rivers  are  distinct  geological  agencies  and 
do  different  kinds  of  work.  Again,  the  very  important  way  in 
vvhich  the  various  dynamical  agents  modify,  check,  or  augment 
one  another,  must  not  be  overlooked  in  a  systematic  arrangement 
of  these  agents. 

Some  of  the  agencies  that  we  shall  consider  may  seem,  at  first 
sight,  to  be  very  trivial  in  their  effects,  but  it  must  be  remembered 
that  they  appear  so  only  because  of  the  short  time  during  which 
we  observe  them.  For  enormously  long  periods  of  time  they 


26  DYNAMICAL  GEOLOGY 

have  been  steadily  at  work,  and  their  cumulative  effects  must 
not  be  left  out  of  account  in  estimating  the  forces  which  have 
made  the  earth  what  we  find  it. 

Much  as  may  be  learned  by  the  study  of  the  operation  of  the 
forces  which  are  still  at  work  in  modifying  the  earth,  this  method 
of  study  is  yet  insufficient  to  solve  all  geological  problems.  Many 
of  the  changes  which  have  indisputably  taken  place  are  such  as 
no  man  has  evei  observed,  because  they  are  brought  about  so 
slowly  or  so  deep  down  within  the  crust  that  no  direct  observation 
is  possible,  and  we  can  only  infer  the  mode  of  procedure  by 
examining  the  result.  No  human  eye  has  ever  witnessed  the  birth 
of  a  mountain  range,  or  has  seen  the  beds  of  solid  rock  folded 
and  crumpled  like  so  many  sheets  of  paper,  or  observed  the  pro- 
cesses by  which  a  rock  is  changed  in  all  its  essential  characteristics; 
"  metamorphosed,"  as  it  is  technically  called.  All  such  problems 
must  be  discussed  in  connection  with  structural  geology. 

The  dynamical  agencies  may,  primarily,  be  divided  into  two 
classes:  I,  the  Subterranean  Agencies,  which  act,  or  at  least  origi- 
nate, at  considerable  depths  within  the  earth;  and  II,  the  Surface 
or  Superficial  Agencies,  whose  action  takes  place  at  or  near  the 
surface  of  the  earth.  The  former  are  due  to  the  inherent  energy 
of  the  earth,  and  their  seat  is  primarily  subterranean,  though  their 
effects  are  very  frequently  apparent  at  the  surface.  These  agen- 
cies are  also  called  igneous  (from  ignis,  fire),  which  is  a  misnomer; 
but  the  term  is  nevertheless  in  common  use.  The  surface  agents 
are  those  which  are  derived  from  the  energy  of  the  sun. 

The  logical  order  of  treatment  of  these  subjects  is  to  begin  with 
the  subterranean  agencies,  because  the  most  ancient  rocks  of  the 
earth's  crust  were  doubtless  formed  by  these  forces,  and  the  circu- 
lation of  matter  upon  and  through  the  crust  started  originally 
from  igneous  rocks. 

The  subterranean  agents  originate,  primarily  at  least,  either  be- 
low or  deep  within  the  lithosphere,  while  the  surface  agents  origi- 
nate at  the  surface  and  penetrate  to  varying  depths.  Thus  it 
happens  that  at  certain  levels  and  along  certain  lines  the  two  classes 


DYNAMICAL  GEOLOGY  2? 

of  agents  have  a  common  place  of  activity  and  cooperate  to  pro- 
duce effects  which  neither  could  produce  alone.  We  may  regard 
the  lithosphere  as  composed  of  a  series  of  concentric  shells,  in  each 
of  which  the  conditions  of  pressure  and  temperature  differ  from 
those  of  the  other  shells  and,  in  consequence,  the  characteristic 
chemical  and  mechanical  processes  are  different  in  each.  Depth 
is  thus  a  controlling  factor  of  great  importance  in  the  operation 
of  the  dynamical  agencies.  "  Under  any  given  set  of  conditions, 
minerals  tend  to  form  which  remain  permanent  under  those 
conditions  "  (Van  Hise),  but  when  new  conditions  arise  a  readjust- 
ment begins  and  new  combinations  are  formed. 

The  outermost  of  the  concentric  lithosphere-shells  (the  thick- 
ness of  which  cannot  be  definitely  stated,  but  may  be  as  much  as 
20,000-25,000  feet)  is  characterized  by  a  tendency  of  the  minerals 
to  change  from  more  complex  to  simpler  compounds.  At  greater 
depths,  this  tendency  is  reversed  and  the  change  is  from  simpler  to 
more  complex  compounds.  Changes  of  this  class  constitute  the 
process  of  metamorphism ,  to  which  the  metamorphic  rocks  owe 
their  origin,  but,  locally,  metamorphism  may  occur  at  much 
higher  levels  and  even  very  near  the  surface.  It  may  be  doubted 
whether  any  chemical  recombinations  occur  at  very  great  depths. 

The  boundaries  of  the  concentric  shells  are  not  always  definite, 
one  shell  gradually  passing  into  another,  nor  are  they  always 
fixed,  but  may  fluctuate  from  time  to  time  within  certain  limits. 


SECTION    I 

SUBTERRANEAN   OR  IGNEOUS  AGENCIES 

CHAPTER   I 
DIASTROPHISM.     EARTHQUAKES 

THE  subterranean  agencies  are  those  which  are  due  to  the  earth's 
own  inherent  energy  and  arise  deep  within  the  earth's  interior, 
though  they  are  often  displayed  at  the  surface  in  a  most  striking 
manner.  No  problems  of  geology  are  more  difficult  and  obscure 
than  those  connected  with  the  internal  constitution  of  the  earth, 
and  satisfactory  explanations  of  the  subterranean  processes  have 
not  yet  been  devised. 

These  agencies  fall  naturally  into  two  great  groups:  I,  Dias- 
trophism,  or  the  movements  of  the  earth's  crust;  and  II,  Vulcan- 
ism,  or  the  phenomena  of  volcanoes,  geysers,  thermal  springs,  etc., 
while  a  third  set  of  phenomena,  Earthquakes,  is  intimately  asso- 
ciated with  each  of  the  others,  but,  on  account  of  its  great  im- 
portance, will  require  separate  treatment.  It  is  extremely  prob- 
able that  all  of  these  so-called  igneous  processes  are  but  different 
manifestations  of  the  same  forces,  in  ways  that  we  cannot  yet  clearly 
understand,  but,  until  the  physical  constitution  of  the  earth's 
interior  shall  have  been  determined,  such  unity  of  origin  cannot 
be  definitely  proved. 

Diastrophism  is  the  general  term  for  differential  movements  of 
the  lithosphere,  whether  upward,  downward,  or  horizontal,  and 
whether  slow  and  imperceptible,  or  sudden  and  violent.  These 
movements  are  of  different  kinds  and  may  be  classified  as  follows  : 
I,  Orogenic  (Greek  Oros,  a  mountain"),  the  upheaval  of  long  and 
{/  ,8 


DIASTROPHISM.     EARTHQUAKES  2Q 

relatively  narrow  belts  of  land  by  compression  and  crumpling  of 
the  rocks.  As  will  be  shown  in  a  later  chapter,  the  orogenic  pro- 
cesses take  place  at  considerable  depths  below  the  surface  and 
hence  cannot  be  directly  observed;  they  are  included  here  merely 
for  the  sake  of  completeness,  for  the  study  of  them  belongs  properly 
to  structural  geology.  II,  Epeirogenic  (Greek  Epeiros,  a  conti- 
nent) ,  the  broad  uplift  or  depression  of  areas  of  the  land  or  of  the 
sea-bottom,  in  which  the  strata  are  not  folded  or  crumpled,  but 
may  be  tilted  or  may  retain  their  original  horizontal  attitude. 
Movements  of  this  class  may  be  distinguished  as  (i)  Warping,  or 
Bradyseism  (Greek  Brady  s,  slow,  and  Seismos,  earthqua'ke) ,  which 
is  a  broad  gentle  curving  of  the  surface  upward,  upwarp,  or  down- 
ward, downwarp;  (2)  direct  upheaval  or  depression,  with  frac- 
turing and  dislocation  of  the  rocks,  which  may  be  accompanied 
by  a  tilting  of  the  strata.  Diastrophic  movements  of  this  class 
are  almost,  if  not  quite,  invariably  associated  with  earthquakes 
and  can  be  most  conveniently  studied  in  connection  with  the  latter. 
Warpings  or  bradyseisms  manifest  themselves  most  clearly  as 
changes  of  relative  level  between  land  and  sea,  because  even  slight 
changes  of  that  character  are  often  easily  detected,  while  in  the 
interior  of  the  continents  they  can  be  demonstrated  only  under 
exceptionally  favourable  circumstances. 

CHANGES  OF  LEVEL 

Permanent  changes  of  level  frequently  accompany  earthquakes, 
bat  these  are  sudden  and  appear  to  be  nearly  always  the  result 
of  dislocation  or  faulting.  By  change  of  level,  in  the  general  sense, 
is  meant  the  gradual  elevation  or  subsidence  of  land,  with  reference 
<o  the  sea,  over  considerable  areas.  Such  movements  are  very 
slow  and  hence  are  apt  to  escape  observation,  so  that  there  is 
much  dispute  as  to  the  facts  and  still  more  as  to  their  interpretation. 

The  change  may  be  in  the  land  or  in  the  sea;  any  important  and 
permanent  change  in  the  bed  of  the  sea  must  affect  its  surface, 
but  then  such  changes  must  be  widespread.  On  the  other  hanc^ 


3O  DIASTROPHISM 

movements  of  the  land  may  be  either  locally  restricted  or  of  great 
extent.  The  absolute  direction  of  the  movements  we  have  no 
means  of  determining;  that  is,  whether  at  a  given  point  the  earth's 
radius  is  shortened  or  lengthened.  The  movement  may  be 
always  downward,  but  at  different  rates  in  adjoining  areas,  or 
may  be  sometimes  in  one  direction  and  sometimes  in  the  other. 
In  view  of  these  uncertainties,  it  has  been  proposed  to  avoid  the 
use  of  the  terms,  "  elevation  and  depression  of  land,"  and  to 
substitute  for  them  "  negative  and  positive  displacements  of  the 
coast-line,"  respectively.  For  the  sake  of  convenience,  it  will  be 
best  to  retain  the  older  and  more  current  terms,  without  insisting 
that  in  all  cases  the  land  moves  rather  than  the  sea. 

It  is  by  no  means  true  that  all  displacements  of  the  coast-line 
are  diastrophic  in  origin;  other  processes  that  produce  similar 
results  must  be  carefully  distinguished  from  actual  changes  of 
level.  Thus,  in  many  places  the  sea  is  advancing  upon  the  land 
by  cutting  back  its  shore,  and  areas  which  once  were  covered  with 
farms  and  villages  are  now  permanently  underwater;  but  this  is 
not  due  to  any  sinking  of  the  land.  Another  process  which 
simulates  depression  is  the  settling  of  loose  masses  of  sediment, 
which  sometimes  allows  the  sea  to  cover  a  flat  coast.  In  other 
places  the  sea  is  building  up  the  coast  by  depositing  sand  upon 
it,  extending  it  seaward,  and  rivers  build  their  deltas  out  into 
the  sea,  but  such  changes  are  not  diastrophic. 

Along  coast-lines  the  evidences  of  elevation  are  n^ore  obvious 
than  those  of  depression,  because  the  traces  of  marine  action  are 
always  present  on  land  which  has  recently  risen  from  the  sea, 
while  a  submerged  land-surface  is  soon  changed  and  buried  out 
of  sight. 

Evidences  of  Elevation.  —  On  certain  coasts  long  inhabited  by 
civilized  man,  ancient  structures  like  quays  and  bridges,  which 
were  built  in  the  water,  may  now  be  found  high  above  it.  Such 
changes  have  been  noted  in  the  Mediterranean  lands,  especially 
in  southern  Italy  and  the  island  of  Crete.  The  so-called  Sera- 
peum  at  Pozzuoli,  near  Naples,  is  a  famous  and  much  discussed 


EVIDENCES  OF  ELEVATION  3! 

example  of  repeated  oscillations  upward  and  downward.  This 
structure  was  built  in  Roman  times  and  probably  began  to  sink 
while  still  in  use,  as  appears  from  the  two  ancient  pavements,  one 
above  the  other.  Three  large  monolithic  columns  of  marble, 
about  40  feet  high,  are  still  standing  erect,  and  on  each  of  them 
is  a  belt  about  10  feet  above  the  ground  and  9  feet  wide,  honey- 
combed by  the  boring  mollusc,  Lithodomus,  which  still  abounds 
in  the  neighbouring  bay,  and  many  of  the  shells  were  actually 


FIG.  i.  —  Columns  of  the  "  Serapeum  " ;  Pozzuoli,  Italy 

found  in  the  columns.  Evidently,  the  building  was  once  sub- 
merged to  a  depth  of  nearly  20  feet,  and  when  under  water,  the 
columns  were  attacked  and  perforated  by  the  mollusc.  Just 
when  the  reelevation  began  is  not  definitely  known,  but  there  is 
some  documentary  evidence  to  show  that  it  was  in  progress  in 
the  early  years  of  the  sixteenth  century  and  was  probably  com- 
pleted in  1538,  when  a  volcanic  eruption  in  the  neighbourhood 
resulted  in  the  formation  of  Monte  Nuovo  '(see  p.  66).  For 


32  DIASTROPHISM 

nearly  a  century  past  a  slow  movement  of  subsidence  has  been 
going  on. 

Rocks  and  cliffs  long  exposed  to  the  action  of  the  surf  are  worn 
and  marked  in  a  characteristic  fashion  and  cut  into  terraces,  and 
when  found  above  the  level  at  which  the  sea  can  now  reach  them, 
are  evidences  of  upheaval  at  that  point.  Such  well-defined  sea- 
marks high  above  the  present  sea-level  are  common  in  the  high 
latitudes  of  the  northern  hemisphere  and,  in  many  places,  the 
change  is  still  in  progress.  The  Scandinavian  peninsula  shows 
slow  changes  of  level,  which  constitute  an  up  warp;  the  south  coast 
of  Sweden  is  stationary  or  sinking  slightly,  but  elsewhere  the  move- 
ment is  upward  and  increases  in  amount  towards  the  interior,  the 
successive  terraces  rising  toward  the  heads  of  the  numerous  deep 
fjords  which  indent  the  coast.  These  facts  have  been  strongly 
disputed,  but  have  recently  been  emphatically  reaffirmed  by  the 
Swedish  geologist  De  Geers,  who  shows  that  the  isobasic  curves, 
connecting  points  of  equal  elevation,  form  ellipses,  the  long  axis 
of  which  coincides  with  the  water-shed  between  Sweden  and 
Norway.  Along  this  line,  the  elevation  is  at  a  maximum,  reaching 
nearly  1000  feet,  and  diminishing  from  the  axis  toward  the  periph- 
ery. Such  a  result  can  be  explained  only  by  an  elevation  of  the  land, 
not  by  a  withdrawal  of  the  sea,  which  could  not  have  changed 
the  level  of  the  terraces. 

"  Raised  beaches,"  filled  with  the  remains  of  marine  animals, 
are  a  decisive  proof  of  a  rise  of  the  land,  or  a  fall  in  the  sea,  and 
evidence  of  a  similar  kind  is  given  by  raised  coral  reefs.  Such 
raised  beaches  now  far  above  the  sea  occur  in  Scandinavia,  Great 
Britain,  the  West  Indies,  the  west  coast  of  South  America,  the 
Red  Sea,  and  elsewhere.  The  eastern  coast  of  North  America 
shows  marks  of  relatively  late  elevation,  increasing  in  amount 
northward.  At  the  mouth  of  the  Connecticut,  the  highest  beach 
is  40  to  50  feet  above  sea-level,  at  Boston  it  is  75  to  100  feet,  on 
the  coast  of  Maine  it  is  200,  and  on  that  of  Labrador  500  feet. 
On  the  eastern  shore  of  Hudson's  Bay  the  marine  terraces  and 
beaches  extend  up  to  700  feet  above  sea-level. 


EVIDENCES  OF  DEPRESSION  33 

Still  another  kind  of  evidence  of  recent  elevation  may  often  be 
gained  from  the  form  and  character  of  the  coast  itself,  as  will  be 
explained  in  Part  III. 

In  the  geological  period  (Pleistocene)  immediately  preceding 
the  recent  one,  in  which  we  are  living,  several  immense  lakes  existed 
in  the  interior  of  North  America,  some  around  the  basins  of  the 
present  Great  Lakes,  others  in  Utah  and  Nevada.  The  ancient 
shore-lines  of  these  vanished  lakes  may  still  be  seen,  for  the  most 
part,  in  admirable  preservation;  when  first  formed  by  the  action 
of  the  waters,  these  beaches  must  have  been  level,  but  accurate 
surveys  show  that  they  are  no  longer  so,  but  have  undergone  ex- 
tensive warpings. 

Wherever  rocks  of  marine  origin  occur  on  land,  they  prove 
the  elevation  of  the  area  where  they  are  found.  The  great 
importance  of  the  process  is  shown  by  the  fact  that  the  larger 
part  of  all  the  continents  is  composed  of  rocks  which  were  laid 
down  in  the  sea  and  are  of  all  geological  dates. 

Evidences  of  Depression.  —  As  ancient  structures  on  long-in- 
habited coasts  sometimes  show  elevation,  they  likewise  sometimes 
show  depression.  On  the  north  coast  of  Egypt  ancient  rock-cut 
tombs  are  now  visible  beneath  the  waters  of  the  Mediterranean. 
The  testimony  of  old  buildings  shows  that  the  eastern  end  of  the 
island  of  Crete  is  sinking,  while  the  west  and  south  coasts  are 
rising.  The  Roman  mole  at  Pozzuoli  has  sunk,  as  is  shown  by 
the  mooring-rings  for  ships,  now  permanently  below  sea-level. 
South  of  Stockholm,  in  Sweden,  the  remains  of  an  ancient  hut 
were  found  in  place,  65  feet  below  the  surface,  buried  in  marine 
deposits  which  contain  shells  of  the  same  species  now  living  in  the 
Baltic.  On  the  west  coast  of  Greenland  the  sinking  is  so  rapid  as 
to  have  attracted  the  attention  of  the  natives. 

Buried  forests  found  below  sea-level  indicate  subsidence.  Such 
forests  occur  in  the  delta  of  the  Mississippi,  on  the  shores  of 
Chesapeake  Bay  and  at  many  points  on  the  sea-coast  of  the  southern 
and  middle  Atlantic  States,  notably  in  New  Jersey,  where  the  coast 
is  sinking  at  a  rate  estimated  at  2  feet  per  century.  Submerged 


34  DIASTROPHISM 

forests  are  also  found  on  the  coast  of  Holland  and  along  the  whole 
north  coast  of  Germany,  both  on  the  North  and  Baltic  Seas. 

A  river  channel  invaded  and  covered  by  the  sea  is  still  another 
proof  of  depression,  because  a  river  flowing  into  the  sea  cannot 
excavate  the  sea-bottom  below  the  level  of  its  mouth.  Very  many 
such  instances  are  known,  of  which  it  will  suffice  to  mention  two 
or  three.  The  ancient  channel  of  the  Hudson  has  been  traced 
by  soundings  out  to  the  edge  of  the  continental  platform,  more  than 
100  miles  southeast  of  Sandy  Hook.  In  the  same  manner  the 
channel  of  the  St.  Lawrence  may  be  followed  out  through  the  Straits 
of  Belle  Isle,  and  that  of  the  Congo  extends  out  70  miles  from  the 
west  coast  of  Africa,  with  a  depth  of  nearly  1000  fathoms. 

The  apparently  contradictory  evidence  in  the  case  of  the  St. 
Lawrence  channel,  which  indicates  depression,  and  that  of  the 
Labrador  coast,  which  is  rising,  is  not  so  in  reality,  for  the  move- 
ments are  successive,  not  simultaneous. 

Coral  reefs  often  give  proof  of  depression,  for,  as  most  of  the 
reef-building  corals  cannot  live  in  water  more  than  20  fathoms 
deep,  a  greater  thickness  of  the  reef  than  this  indicates  a  slow 
sinking,  at  a  rate  not  exceeding  the  upward  growth  of  the  coral. 
Borings  made  in  the  South  Pacific  island  of  Funafuti  show  that 
that  reef  exceeds  noo  feet  in  thickness,  and  must  therefore  have 
been  gradually  depressed.  Another  obvious  proof  of  subsidence 
is  a  great  thickness  of  shallow  water  deposits;  for,  if  the  sea-bot- 
tom did  not  sink,  the  shallow  water  would  soon  be  filled  up  and 
the  coast-line  advanced.  The  study  of  the  materials  now  accu- 
mulating on  the  ocean-floor  enables  us  to  determine  the  depth 
of  water  in  which  ancient  deposits  were  formed,  and  applying 
this  knowledge,  we  learn  (to  give  only  one  example)  that  from 
the  Hudson  River  southward,  the  coastal  plain  of  the  Atlantic 
States  is  covered  by  very  thick,  shallow- water,  marine  beds,  as  is 
revealed  by  the  numerous  artesian  wells  which  have  been  driven 
through  them.  The  fact  that  these  beds  are  now  part  of  a 
land-surface  indicates,  of  course,  that  they  have  been  elevated 
subsequently  to  their  formation. 


EVIDENCES  OF   DEPRESSION  35 

Finally,  the  form  and  topography  of  a  coast  may  betray  its 
recent  subsidence,  as  will  be  more  fully  explained  in  Part  III. 

As  regards  the  oscillations  of  level  which  are  now  going  on,  it 
is  not  definitely  known  whether  they  proceed  continuously  at  a 
uniform  rate,  or  spasmodically  with  intervals  of  complete  rest. 
In  the  case  of  a  succession  of  marine  terraces,  one  above  another, 
the  movement  cannot  have  been  uniform,  or  else  a  continuous 
slope  would  have  been  produced;  each  terrace  and  beach  indicates 
a  pause,  during  which  the  waves  cut  the  rocky  shelf,  or  accumu- 
lated the  beach,  while  the  steep  slope  between  two  successive 
terraces  points  to  a  relatively  rapid  rise. 

The  following  table,  which  exhibits  the  data  gathered  chiefly 
by  Kayser,  will  be  serviceable  as  showing  the  extent  and  char- 
acter of  the  diastrophic  movements  which,  it  is  inferred,  are  still, 
or  have  recently  been  in  progress  along  the  principal  coast-lines 
of  the  world.  In  this  table  no  account  is  taken  of  the  movements 
which  have  here  and  there  been  detected  in  the  interior  of  the 
different  continents,  Such  as  those  already  mentioned  for  North 
America,  and  others  which  have  been  observed  in  northern  Switz- 
erland. 

RISING  SINKING 

North  America 


East  coast  of  Greenland. 
East  coast  down  to  45°  N.  lat. 
Gulf  of  Mexico  and  Antilles. 
Pacific  coast. 


West  coast  of  Greenland. 

East  coast  from  45°  N.  lat.  to  end 

of  Florida. 
East  coast  of  Central  America. 


South  America 


Pacific  coast,  except  that  of  Peru. 
Atlantic  coast  from  mouth  of  La 


Plata  to  20°  S.  lat. 


Coast  of  Peru. 

Atlantic    coast,    except     Uruguay 


and  South  Brazil. 


Asia 


Entire  north  coast  and  east  coast 


South  coast  and  Malay  Archipel- 
ago. 
Asia  Minor. 


East  coast  of  southern  China  and 

Tonkin. 
Laccadive  and  Maldive  Islands. 


EARTHQUAKES 


Australasia 


Australia,  south  coast,  and  Tas- 
mania. 

East  coast  of  New  Zealand. 

Pacific  coast  of  New  Guinea. 

Solomon,  New  Hebrides,  Samoan, 
Sandwich  Islands,  and  many 
others. 


Australia,  northeast  coast. 
West  coast  of  New  Zealand. 
South  coast  of  New  Guinea. 
Caroline,       Marshall,        Gilbert, 
Tonga,  Society  Islands,  etc. 


Ajrica 


Coast  of  Red  Sea ;  east  coast,  west 
coast  up  to  Gulf  of  Guinea. 


Atlantic    coast  of  Morocco,   east 

coast  of  Tripoli. 
North   coast  of  Egypt:    Gulf  of 

Guinea. 


Europe 


Peloponnesus,  Sicily,  Sardinia,  Li- 
gurian  coast,  Balearic  Islands, 
south  coast  of  Spain ;  west  coast 
of  France,  Ireland  and  Scotland ; 
Scandinavia. 


England ;  north  coast  of  France, 
the  Netherlands  and  Germany. 


From  this  table  it  is  apparent  that  few  coasts  are  stationary, 
but  that  almost  all  are,  or  have  lately  been,  in  movement,  and 
further  that  upheaval  greatly  preponderates  over  subsidence. 
Still  another  significant  result  of  these  observations  is  that  areas 
of  opposite  movement  may  be  in  close  juxtaposition,  as  on  the  two 
sides  of  the  Baltic,  the  east  and  west  coasts  of  Greenland,  the 
eastern  coast  of  North  America  and  Asia,  and  many  other  regions. 
In  such  cases  the  movement  must  be  in  the  land  rather  than  in 
the  sea. 

EARTHQUAKES 

An  earthquake  is  caused  by  a  series  of  elastic  waves  due  to  a 
sudden  shock  in  the  earth's  interior;  the  visible  phenomena  at 
the  surface  are  produced  by  the  outcropping  of  these  waves  and 
by  the  movements  of  the  soft  and  inelastic  soil,  which  is  set  in 


EARTHQUAKES  37 

motion  by  the  outcropping  waves.  While  the  elastic  waves, 
which  in  mode  of  transmission  resemble  those  of  sound,  are  very 
regular  in  hard  and  homogeneous  rocks,  the  actual  movements 
of  a  given  particle  at  the  surface  are  highly  irregular  and  confused, 
as  is  well  shown  in  the  wire  model,  Fig.  2.  This  model,  con- 
structed from  the  records  of  seismographs,  gives  in  magnified 
form  the  movements  of  an  earth-particle  from  the  2oth  .to  the  4oth 
second  of  the  shock,  the  numbers  indicating  the  position  of 
the  particle  at  each  successive  second.  The  seismographs 
referred  to  are  recording  instruments,  commonly  horizontal 
pendulums,  which  register  on  paper  strips  the  various  components, 
horizontal  and  vertical,  of  the  movements.  So  delicate  are  these 


FIG.  2. —  Magnified  model,  showing  the  movements  of  a  surface  particle  of  earth 
from  the  2Oth  to  the  4Oth  second  of  a  shock.     (Omori) 

instruments,  that  they  register  even  those  violent  shocks  which 
originate  at  the  very  antipodes  of  the  observatory  where  the  instru- 
ment is  installed. 

The  study  of  seismographic  records  has  brought  to  light  many 
highly  significant  facts,  among  others  that  minute  and  insensible        >• 
tremors  of  the  earth  are  almost  incessant,  but  some,  at  least,  of^' 
these  tremors  are  due  to  atmospheric  changes  and  it  is  not  known 
how  large  a  proportion  of  them  are  of  subterranean  origin.    The 
term   "  earthquake  "   is   usually   restricted   to   those   movements 
of  the  ground  which  can  be  felt,  though  the  distinction  is  a  some- 
what arbitrary  one.     Another  very  important  result  of  the  seismo- 


38  EARTHQUAKES 

graphic  observations  is  that  when  a  very  distant  earthquake  is 
registered,  three  series  of  waves  are  indicated,  viz.,  the  ist  and  2d 
phases  of  the  preliminary  tremors,  and  the  larger  waves  of  the 
main  shock.  Those  first  to  arrive,  called  the  preliminary  tremors, 
are  believed  to  be  transmitted  through  the  mass  of  the  earth  along 
the  chord  of  the  arc  included  between  the  point  of  origin  and  the 
point  of  observation.  The  preliminary  tremors  include  two  of 
the  three  series  of  waves,  known  as  phases.  The  heterogeneous 
mass  of  rocks  which  forms  the  outermost  crust  of  the  earth  does  not 


FlG.  3.  —  Seismographic  record  of  the  San  Francisco  earthquake  of  1906,  U.  S.  Coast 
Survey  observatory,  Cheltenham,  Md.  A,  Preliminary  tremors,  ist  phase;  B,  Pre- 
liminary tremors,  2d  phase;  C,  Main  shock.  The  upper  record  shows  the  north- 
south  component,  and  the  lower  gives  the  east-west  component.  (Bauer) 


permit  the  transmission  of  any  simple  form  of  wave-motion,  and 
it  is  only  at  a  distance  of  about  10°  of  arc  of  the  earth's  surface 
(about  700  miles)  that  the  three  different  kinds  of  waves  begin  to 
appear  upon  the  instrumental  records.  The  preliminary  tremors, 
which  pass  through  the  subcrustal  region  of  the  earth  and  travel 
at  a  higher  rate  of  speed  than  the  waves  which  follow  the  surface, 
appear,  as  already  mentioned,  in  two  phases.  The  waves  of  the 
first  phase  are  believed  to  be  the  normal,  or  compressional  waves, 
and  those  of  the  second  phase  to  be  the  transverse  or  distortional 
waves,  the  two  known  kinds  of  wave  motion  which  can  be  trans- 


DISTRIBUTION   OF   EARTHQUAKES 


39 


mitted  through  a  homogeneous  solid.  The  waves  of  the  third 
series  are  longer  and  slower  (i.e.  of  greater  amplitude  and  period) 
and  constitute  the  "  main  shock  ";  they  are  believed  to  follow  the 
curvature  of  the  earth's  surface. 


FIG.   4.  —  Earthquake  regions  of  the  Eastern  Hemisphere,     (de  Montessus   de 

Ballore ) 

Distribution  of  Earthquakes.  —  Sensible  earthquakes  are  very 
numerous,  not  less  than  30,000  is  the  estimated  number  per  annum; 
of  course,  the  great  majority  of  these  are  very  light.  While  any 
part  of  the  earth's  surface  may  be  visited  by  earthquakes,  there  is 
a  very  great  difference  between  different  regions  in  regard  to  their 


4O  EARTHQUAKES 

seismictty,  i.e.  the  frequency  and  violence  of  the  shocks  which 
affect  them.  The  main  seismic  regions,  when  platted  upon  a 
map,  are  found  to  be  arranged  in  two  great-circle  belts,  one  of 
which  encloses  the  Pacific  Ocean  and  the  other  girdles  the  whole 


FIG.  5.  —  Earthquake  regions  of  the  Western  Hemisphere,     (de  Montessus  de 

Ballore) 

earth.  The  latter  includes  the  Mediterranean  region,  the  Azores, 
the  bed  of  the  Atlantic  westward  from  the  Azores  to  the  West 
Indies,  those  islands  themselves,  Central  America,  Hawaii,  Japan, 
China,  India,  Afghanistan,  Persia,  and  Asia  Minor. 

It  must  not  be  supposed  that  these  belts  are  uninterrupted 


DISTRIBUTION  OF  EARTHQUAKES  41 

zones  of  seismic  activity;  they  are  rather  seismic  tracts  separated 
by  other  tracts  of  low  seismicity.  For  example,  the  eastern  Aleu- 
tian Islands  and  the  Alaskan  coast  form  a  region  of  frequent  and 
sometimes  very  violent  quakes,  while  the  coast  between  Alaska 
and  California  is  not  often  shaken.  California  is  an  earthquake 
region,  as  is  also  southern  Mexico,  and  Central  America  has  a  very 
high  degree  of  seismicity,  but  there  is  a  long  interval  before  the 
earthquake  region  of  Ecuador  is  reached.  Though  the  belts 
are  thus  discontinuous,  it  is  nevertheless  significant  that  the 
separate  seismic  tracts  are  arranged  in  belts. 

The  regions  most  subject  to  earthquakes  are  those  which  have 
the  steepest  slopes  and  are  associated  with  the  great  lines  of 
corrugation  of  the  earth's  surface.  A  sea-bottom  steeply  de- 
scending from  the  shore  is  apt  to  be  unstable,  especially  if  high 
mountains  arise  near  the  coast,  while  a  low-lying  coastal  plain 
and  adjoining  gently  sloping  sea-bottom  are  usually  stable. 

Beside  the  main  seismic  regions  above  enumerated,  there  are 
many  others  where  the  shocks,  though  not  infrequent,  are  seldom 
violent.  Examples  of  such  regions  are  New  England,  Switzer- 
land, Austria,  and  South  Germany. 

Although  earthquakes  are  commonly  perceptible  upon  the  land, 
the  most  frequent  seats  of  disturbance  are  in  the  bed  of  the  sea. 
These  submarine  quakes  occur  at  all  depths  of  water,  and  their 
frequency  and  violence  are  independent  of  the  distance  from 
volcanoes.  In  the  sea  there  are  regions  quite  free  from  quakes 
and  others  of  a  high  degree  of  seismicity,  but  quakes  also  occur 
in  an  isolated  and  scattered  manner. 

In  the  Atlantic  there  are  two  remarkable  seismic  belts,  one, 
already  mentioned  as  part  of  the  great  earthquake  zone,  extending 
westward  from  the  mouth  of  the  Tagus  in  Portugal,  the  other 
nearly  equatorial  and  reaching  from  the  north  shore  of  the  Gulf 
of  Guinea  toward  Brazil.  In  this  second  belt  the  sea-floor  has 
precipitous  slopes. 

Submarine  cables  are  frequently  interrupted  at  the  same  points. 
Thus,  the  cable  from  the  Lipari  Islands  to  Sicily  has  been  broken 


42  EARTHQUAKES 

five  times  at  the  same  point;  on  October  4,  1884,  three  parallel 
cables,  about  10  miles  apart,  were  simultaneously  broken  at  the 
base  of  the  steep  continental  slope,  330  miles  east  of  St.  John,  N.B. 
Many  similar  instances  might  be  given. 

Classification  of  Earthquakes.  —  Earthquakes  may  be  classified 
in  several  ways,  according  to  the  purpose  in  view.  With  regard 
to  the  manner  of  production,  they  may  be  grouped  into  volcanic 
and  tectonic  quakes,  which  will  be  explained  when  we  take  up  the 
causes  of  these  phenomena.  For  our  present  purpose,  which  is 
chiefly  descriptive,  it  will  be  most  convenient  to  divide  earth- 
quakes into:  (i)  Macroseismic,  or  large  earthquakes  which 
"  disturb  continental  areas  and  frequently  disturb  the  world  as 
a  whole"  (Milne);  and  (2)  Micro  seismic  ^  or  local  earthquakes 
which,  as  a  rule,  affect  areas  of  only  a  few  miles'  radius,  rarely  as 
much  as  100  or  200  miles.  While  the  macroseismic  quakes  are 
due  to  a  disturbance  both  of  the  earth's  crust  and  of  the  homo- 
geneous interior,  those  of  the  microseismic  class  "  appear  to  be 
the  shiverings  within  the  crust  "  (Milne). 

It  should  be  observed  that  these  terms  have  been  frequently 
employed  in  senses  very  different  from  those  here  used,  which 
are  taken  from  Professor  Milne.  That  there  should  be  no  very 
distinct  line  of  demarcation  between  the  two  classes,  is  not  sur- 
prising, for  they  represent  different  degrees  of  intensity  or  violence 
in  similar  phenomena. 

Phenomena  of  Earthquakes.  — The  phenomena  of  earthquakes 
differ  greatly  in  accordance  with  the  number,  duration,  and 
intensity  of  the  shocks,  and  with  the  distance  of  the  place  of  ob- 
servation from  that  of  the  origin  of  the  disturbance.  One  of  the 
greatest  of  modern  earthquakes  is  that  of  northern  India  of  1897, 
which  is  well  summed  up  in  the  official  report. 

"  On  the  afternoon  of  June  12, 1897,  there  burst  upon  the  western 
portion  of  Assam  an  earthquake  which,  for  violence  and  extent, 
has  not  been  surpassed  by  any  of  which  we  have  historic  record. 
Lasting  about  iwo  and  one-half  minutes,  it  had  not  ceased  at 
Shillong  before  an  area  of  150,000  square  miles  had  been  laid  in 


PHENOMENA  OF  EARTHQUAKES 


43 


mins,  all  means  of  communication  interrupted,  the  hills  rent  and 
cast  down  in  landslips,  and  the  plains  fissured  and  riddled  with 


FIG.  6.  —  Earthquake  fissure  in  limestone,  Arizona.     (U.  S.  G.  S.) 

vents,  from  which  sand  and  water  poured  out  in  most  astounding 
quantities;  and  ten  minutes  had  not  elapsed  from  the  time  when 


44  EARTHQUAKES 

Shillong  was  laid  in  ruins  before  about  one  and  three-quarter 
million  square  miles  had  felt  a  shock,  which  was  everywhere 
recognized  as  one  quite  out  of  the  common."  (R.  D.  Oldham.) 

A  great  earthquake  usually  begins  suddenly  and  without 
warning.  A  rumbling  sound,  quickly  becoming  a  loud  roar, 
accompanies  or  slightly  precedes  the  movement  of  the  ground, 
which  is  at  first  a  trembling,  then  a  shaking,  and  finally  a  rapid 
swaying,  wriggling  motion,  describing  a  figure  8,  which  is 
extremely  destructive  and  overthrows  the  buildings  affected,  and 
even  in  the  open  country  it  is  impossible  to  keep  one's  feet.  The 
surface  of  the  ground  has  been  repeatedly  observed  to  rise  in  low, 
very  swiftly  moving  waves,  somewhat  like  those  on  the  surface  of 
water,  upon  the  crests  of  which  the  soil  opens  in  cracks,  closing 
again  in  the  wave-troughs.  When  the  earth-waves  traverse  a 
forested  region,  the  trees  sway  violently  from  side  to  side,  like  a 
field  of  ripe  grain  in  the  breeze.  In  the  details  of  movement 
earthquakes  differ  greatly  from  one  another;  sudden  and  ex- 
tremely violent  vertical  shocks  may  come  from  below,  or  the  surface 
may  writhe  and  twist  in  every  direction,  instead  of  rolling  in 
waves;  there  may  be  only  a  single  shock,  or  many  successive  ones. 

Violent  earthquakes,  which  affect  extensive  areas,  are  almost 
always  followed  by  a  succession  of  after-shocks,  which  may  continue 
for  weeks,  months,  or  even  years.  These  may  be  very  violent, 
though  never  equalling  the  primary  shock  in  this  respect,  but 
gradually  die  away,  until  the  region  once  more  comes  to  rest. 

In  the  sea  the  elastic  waves  producing  shock  soon  die  away  in 
the  water.  Observations  made  on  the  several  ships  affected  by  the 
same  quake  frequently  show  a  lineal  arrangement  of  the  dis- 
turbances. A  special  manifestation  of  earthquakes  in  the  bed 
of  the  sea  is  the  great  sea-wave  (sometimes  erroneously  called  the 
tidal  wave),  which  is  a  gravity  wave  produced  by  disturbances  of 
the  sea-floor  or  by  a  submarine  volcanic  eruption.  The  great 
sea-wave,  though  not  strikingly  displayed  in  the  open  sea,  piles 
up  on  the  coast  into  enormous  breakers,  which  often  are  more 
terribly  destructive  than  the  earth-waves  themselves. 


EFFECTS  OF   EARTHQUAKES  45 

Effects  of  Earthquakes.  —  Strictly  speaking,  the  geological 
effects  of  earthquakes  are  of  less  importance  than  is  usually  sup- 
posed. The  violent  shaking  of  the  surface  often  brings  about 
great  land-slips  in  mountain  regions,  which  precipitate  enormous 
masses  of  earth  and  rock  from  the  heights  down  into  the  valleys. 
A  striking  example  of  this  was  given  by  the  earthquakes  of  north- 
western Greece  in  1870,  in  which  the  rockslides  were  on  a  gigantic 
scale.  The  falling  masses  may  temporarily  or  permanently  block 
the  valleys,  converting  their  streams  into  lakes. 

On  the  other  hand,  the  diastrophic  forces  which  produce  the 
earthquake  often  have  other  effects  of  the  greatest  importance. 
In  all  of  the  more  violent  quakes  cracks  and  fissures  of  the  ground 
are  formed,  which  may  close  again  or  remain  open,  and  may  show 
a  lineal,  curved,  zigzag,  or  radiating  arrangement.  Through  these 
fissures  great  quantities  of  water  and  sand  are  often  forced  up 
from  below  and  form  little  sand  craters,  or  water-filled  funnels  on 
the  surface.  Frequently  the  fissures  assume  the  character  of 
/faults,  or  dislocations,  one  side  being  raised,  the  other  depressed, 
that  long  scarps,  or  low  cliffs,  are  left  standing.  A  long  list  of 
such  faults  formed  in  modern  earthquakes  might  be  given,  though 
the  limitations  of  space  forbid  the  mention  of  more  than  a  few. 

In  1811-1812,  near  New  Madrid,  Mo.,  hundreds  of  faults  re- 
sulted from  the  violent  earthquakes  which  shook  that  region,  and 
a  depressed  area,  70  X  30  miles  in  extent,  known  as  the  "  sunk 
country,"  was  formed.  The  earthquake  of  Owen's  Valley,  Cal, 
in  1872,  was  accompanied  by  the  formation  of  a  fault  40  miles  in 
length  and  with  a  vertical  displacement,  or  throw,  of  5-20  feet, 
along  the  eastern  base  of  the  Sierra  Nevada.  In  the  Sonora  earth- 
quake of  1887,  in  Arizona  and  Mexico,  a  zigzag  fault,  35  miles 
long  and  with  a  maximum  throw  of  20  feet,  was  produced.  The 
Sierra  Teras,  in  Mexico,  appears  to  have  been  raised  in  this  move- 
ment, for  a  second  fault,  with  opposite  inclination,  was  formed  on 
the  eastern  side  of  that  range.  The  Japanese  earthquake  of  1891 
was  accompanied  by  a  fault  of  40  miles  in  length,  with  throws 
exceeding  33  feet  in  height. 


46  EARTHQUAKES 

Mention  has  already  been  made  of  the  great  Indian,  or  Assam, 
earthquake  of  1897;  it  was  accompanied  by  several  large  faults, 
from  which  lesser  ones  branch  out.  As  a  result  of  the  earthquake 
of  1899,  the  region  of  Yakutat  Bay,  Alaska,  was  much  disturbed, 
with  an  elevation  of  the  coast  at  one  point  of  47  feet.  "  The 
change  of  level  was  differential,  indicating  a  complex  system 
of  faulting  on  a  large  scale;  and  shattering  of  the  rocks  proves 


PIG.  7.  —  Fault-scarp  in  the   Neo  Valley,  Japan,  earthquake  of  1891.      The  road 
shows  a  slight  horizontal  as  well  as  vertical  displacement.     (Milne) 

much  differential  movement  on  a  smaller  scale."  (Tarr  and  Martin.) 
In  the  San  Francisco  earthquake  of  1906  two  long  lines  of  parallel 
faults  were  formed,  with  varying  throw  up  to  20  feet. 

Earthquake  displacements  may  be  horizontal  as  well  as  vertical, 
though  the  former  are  less  obvious  and  have  not  therefore  been  so 
frequently  observed.  Lateral  displacements  amounting,  in  some 
localities,  to  as  much  as  20  feet,  in  a  direction  parallel  with  the 


EFFECTS   OF  EARTHQUAKES 


47 


fault-scarps,  were  a  very  marked  feature  of  the  San  Francisco 
earthquake.  Horizontal  displacements  were  also  observed  after 
the  Indian  earthquake  of  1897,  the  Sumatran  of  1892,  the  Japanese 
of  1891,  the  Owen's  Valley  of  1872,  and  others. 

The  most  remarkable  of  modern  faults  are  those  which  have  been 
detected  by  soundings  in  the  floor  of  the  eastern  Mediterranean. 
The  earthquake  of  October,  1873,  off  the  western  coast  of  Greece 
resulted  in  a  scarp  on  the  sea-floor  with  a  depth  of  2000  feet,  where 


FIG.  8.  — Fence  broken  and  shifted  horizontally  15  feet,  San  Francisco  earthquake 

of  1906 


formerly  the  depth  had  been  1400  feet.  In  1878  the  cable  to  the 
island  of  Crete  was  broken  in  two  places  by  a  violent  earthquake 
and  the  sea-floor  had  become  so  irregular  that,  in  relaying  the 
cable,  it  was  found  necessary  to  make  a  long  detour.  The  cable 
from  Zante  to  Crete  was  broken  by  an  earthquake  in  August,  1886, 
and  at  the  break  soundings  revealed  an  increased  depth  of  1300 
feet.  Some  of  the  Mediterranean  scarps  are  3000-5000  feet  higl} 


48 


EARTHQUAKES 


but  the  time  of  formation  of  these  enormous  dislocations  is  not 
definitely  known. 


FlG.  9.  —  Horizontal  shifting  of  the  ground,  San  Francisco  earthquake,  1906.     Before 
the  earthquake  the  road  was  straight.     (Photograph  by  Sinclair) 

A  very  common  result  of  earthquakes  is  a  change  in  the  circu- 
lation of  underground  waters.     Wells  and  springs  go  dry,  while 


EFFECTS   OF  EARTHQUAKES  49 

other  springs  are  formed  in  new  places,  or  old  ones  may  be  increased 
in  volume.  The  changes  in  the  form  of  the  land  surface  produce 
corresponding  changes  in  surface  drainage;  rivers  are  diverted 
into  new  channels  or  dammed  into  lakes,  while  streams  intersected 
by  fault-scarps  form  new  cascades.  Many  new  lakes  resulted 
from  the  Indian  earthquake  of  1897. 

The  general  results  obtained  from  the  study  of  the  diastrophic 
movements  which  accompany  earthquakes  are  thus  summed  up 
by  Professor  Hobbs :  — 

"  i.  Appreciable  surface  dislocations  appear  to  be  formed  only 
at  the  time  of  macroseisms,  and  the  throws  upon  these  planes  stand 
in  some  relation  to  the  magnitude  of  the  disturbance. 

"  2.  The  evident  dislocations  produced  are  generally  of  two 
orders  of  magnitude,  those  of  the  higher  order  being  generally 
very  limited  in  number,  while  those  of  the  lower  order  are  often 
quite  numerous. 

"3.  Earthquake  dislocations  are  normal  faults  with  hades 
approaching  the  vertical."  That  is  to  say,  the  hade,  or  inclina- 
tion, of  the  fault-plane  slopes  downward  toward  the  depressed  side, 
as  though  the  latter  had  merely  slipped  down  the  inclined  plane. 

"  4.  The  crustal  movements  indicated  at  the  surface  at  the  time 
of  earthquakes  appear  to  be  due  to  an  adjustment  in  position  of 
individual  blocks." 

The  formation  of  normal  faults  and  open  fissures  during  an 
earthquake  shows  that  the  stresses  to  which  the  outer  portion  of 
the  crust  yields  are  tensional,  or  stretching.  Compression  may  also 
occur  locally,  as  appears  from  the  upward  or  lateral  bending  of 
railway  rails,  so  frequent  a  phenomenon  of  great  disturbances,  but 
such  compression  is  frequently  due  to  the  slipping  of  deep  masses 
of  soil  and  to  be  compensated  by  stretching  at  other  points.  The 
measurements  of  the  railway  lines  after  the  great  Indian  earth- 
quake of  1897  proved  that  whenever  the  rails  had  been  bent  at 
one  point,  they  had  been  dragged  apart  by  an  equivalent  amount 
at  another.  This  refers  to  the  movements  of  soil;  horizontal 
faulting,  on  the  other  hand,  implies  a  true  compression. 
E 


5O  EARTHQUAKES 

Causes  of  Earthquakes.  —  In  respect  to  their  mode  of  causation, 
earthquakes  are  usually  divided  into  two  classes,  volcanic  and 
tectonic,  though  it  is  often  impossible  to  determine  to  which  of  the 
two  classes  a  given  earthquake  should  be  referred.  The  volcanic 
earthquakes,  which  are  closely  associated  in  time  and  space  with 
volcanic  eruptions,  are  due  to  steam  explosions  and  to  the  struggles 
of  the  rising  lava  within  the  earth  to  escape.  In  their  typical 
manifestation,  volcanic  quakes  have  a  definite  centre  of  origin, 
which  is  in  or  near  a  volcano,  and  are  rarely  felt  over  any  very 
extensive  area  of  country.  The  earthquake  of  1883  in  the  island 
of  Ischia  was  of  terrible  violence  and  completely  destroyed  the 
town  of  Casamicciola,  with  great  loss  of  life,  yet  the  shock  was 
hardly  perceptible  at  Naples,  a  distance  of  twenty-two  miles.  The 
great  eruption  of  Mauna  Loa  in  the  Hawaiian  Islands  in  1868  was 
preceded  for  six  days  by  earthquakes  of  gradually  increasing  force, 
until  they  became  frightfully  destructive.  When  the  lava  burst 
out  from  the  volcano,  the  earthquakes  rapidly  died  away.  Ex- 
treme as  was  the  violence  of  these  shocks,  they  were  almost  con- 
fined to  the  southern  side  of  the  island ;  elsewhere  they  did  little 
damage,  and  at  a  distance  of  1 50  miles  were  barely  sensible.  The 
earthquakes  of  Central  America  and  those  at  the  base  of  ^Etna  and 
Vesuvius  and  other  volcanoes  are  of  this  class. 

Tectonic  earthquakes  are  believed  to  be  due  to  stresses  in  the 
interior  of  the  earth  which,  when  suddenly  yielded  to  by  the  rocks, 
cause  the  jar  and  shock  which  generates  the  earthquake.  Macro- 
seisms  are  probably  caused  by  the  formation  of  new  and  great 
fractures  and  microseisms  by  settling  along  existing  lines  of  frac- 
ture. Tectonic  earthquakes  are  linear,  the  maximum  destruc- 
tiveness  being  along  the  line  of  fracture  and  rapidly  diminishing 
transversely  to  this  line;  owing  to  the  deeper  position  of  their 
origins  and  the  greater  masses  of  rock  involved  in  the  movement, 
their  effects  are  far  more  widely  spread  than  those  of  the  volcanic 
class.  Another  distinction  from  the  latter  is  the  succession  of 
after-shocks  which  follow  the  great  tectonic  quakes  and  which 
last  longest  away  from  the  primary  line  of  fracture.  These  are 


CAUSES  OF   EARTHQUAKES  51 

due  to  the  gradual  readjustment  of  the  mosaic  of  blocks,  which, 
as  we  have  already  seen,  makes  up  the  outermost  part  of  the  earth's 
crust. 

Just  how  the  internal  stresses  above  referred  to  are  generated, 
is  by  no  means  clear.  The  explanation  usually  accepted  is  that  the 
earth  is  slowly  contracting  on  account  of  the  loss  of  heat,  and  that 
the  crust,  which  must  follow  the  shrinking  interior,  is  being 
crowded  into  a  smaller  space,  with  resultant  ruptures  and  shocks. 
However,  this  contractional  hypothesis  is  altogether  rejected  by 
several  high  authorities,  and  no  very  satisfactory  substitute  for  it 
has  been  proposed. 

On  the  other  hand,  it  is  contended  by  some  geologists  that  all 
earthquakes  are  essentially  volcanic  in  origin.  These  observers 
call  attention  to  the  "  marked  synchronism  or  close  following  of 
the  major  disturbances,  whether  volcanic  or  seismic,  at  distantly 
removed  points  of  the  earth's  surface,  at  certain  periods."  (Heil- 
prin.)  It  is  undoubtedly  true  that  such  an  association  is  indi- 
cated by  many  facts,  but  much  remains  to  be  learned  before  thr 
full  significance  of  these  facts  can  be  determined. 


CHAPTER   II 
VOLCANOES 

A  VOLCANO  is  usually  a  conical  mountain  or  hill,  with  an  opening, 
or  crater,  through  which  various  solid,  molten,  or  gaseous  materials 
are  ejected.  The  essential  part  of  the  volcano  is  the  opening,  or 
vent,  and  some  volcanoes  consist  of  almost  nothing  else.  The 
mountain,  when  present,  is  secondary  and  is  formed  by  the 
materials  which  the  volcano  itself  has  piled  up;  it  is  thus  the 
effect  and  in  no  sense  the  cause  of  the  phenomena. 

Present  Distribution  of  Volcanoes.  — The  geographical  dis- 
tribution of  volcanic  vents  has  greatly  changed  at  different  periods 
of  the  earth's  history.  While  there  are  some  large  land  areas, 
like  most  of  the  Mississippi  Valley,  which  appear  never  to  have 
been  visited  by  volcanic  activity,  such  areas  are  comparatively 
few  in  number.  In  most  regions  we  find  distinct  traces  of  such 
action,  though  it  may  have  died  out  ages  ago  and  though  at  present 
no  active  vent  may  occur  for  very  great  distances  in  any  direc- 
tion. Such  is  the  case  with  the  valley  of  the  Connecticut  and 
northern  New  Jersey,  Ireland,  Great  Britain,  and  very  many  other 
countries. 

We  cannot  definitely  determine  the  number  of  vents  which  are 
at  present  in  activity  in  various  regions  of  the  earth,  because  a 
volcano  may  remain  dormant  for  centuries,  and  then  break  out 
again.  Almost  all  tradition  of  the  volcanic  nature  of  Vesuvius  had 
died  away  among  the  inhabitants  of  Italy,  until  the  dreadful  erup- 
tion of  the  year  79  A.D.  showed  that  it  had  only  been  slumbering. 
Many  volcanic  regions,  such  as  the  western  part  of  North  and 
South  America,  and  the  East  Indian  islands,  have  been  known  to 
civilized  man  for  only  a  few  centuries,  and  in  such  regions  the 


PRESENT   DISTRIBUTION   OF  VOLCANOES  53 

distinction  between  dormant  and  extinct  vents  cannot  always  be 
made. 

Furthermore,  the  number  of  vents  is  constantly  changing,  new 
openings  forming,  and  old  ones  closing  up,  while  some  that  had 
escaped  observation  are  not  infrequently  discovered.  Another 
distinction  which  is  often  arbitrary,  is  that  between  independent 
volcanoes  and  mere  subsidiary  vents  connected  with  larger  ones. 
Several  submarine  volcanoes  have  been  observed,  but  it  is  alto- 
gether probable  that  many  more  exist  which  have  escaped  detec- 
tion. Making  these  allowances,  the  number  of  volcanoes  now  active 
may  be  estimated  at  about  328,  of  which  rather  more  than  one- 
third  are  situated  in  the  continents,  and  the  remainder  on  islands. 

The  active  volcanoes  are  not  scattered  haphazard  over  the  sur- 
face of  the  globe,  but  are  arranged  in  belts  or  lines,  which  bear  a 
definite  relation  to  the  great  topographical  features  of  the  earth 
as  well  as  to  the  seismic  belts  described  in  the  preceding  chapter. 
Two  of  these  belts  together  encircle  the  Pacific  Ocean;  one  on 
the  west  coast  of  the  Americas  runs  from  Alaska  to  Cape  Horn, 
the  other,  a  very  long  and  sinuous  band,  running  from  Kamschatka 
through  the  islands  parallel  to  the  east  coast  of  Asia,  the  East 
Indian  and  south  Pacific  islands,  to  the  Antarctic  circle,  where  it 
joins  the  American  band. 

As  in  the  case  of  the  seismic  belts,  the  volcanic  bands  are  not 
continuous,  but  consist  of  a  series  of  volcanic  tracts '  separated 
by  others  which  have  no  volcanoes.  The  coincidence  of  the  vol- 
canic and  earthquake  belts  which  encircle  the  Pacific  Ocean  is 
very  close  and  striking. 

A  third  band  occupies  a  ridge  in  the  eastern  bed  of  the  Atlantic, 
from  Iceland  to  beyond  St.  Helena,  from  which  arise  numerous 
volcanic  islands  and  submarine  vents.  Included  in  this  Atlantic 
band  are  Jan  Mayen,  Iceland,  the  Azores,  Canary  and  Cape  Verde 
Islands,  Ascension,  St.  Helena,  and,  in  the  far  south  (38°  S.  lat), 
Tristan  d'Acunha.  South  of  Iceland  there  are  no  known  volcanoes 
for  a  great  distance,  until  the  Azores  are  reached,  and  on  the  east 
coast  of  the  Americas  are  none  at  all. 


54  VOLCANOES 

A  subsidiary  belt,  parallel  to  the  Atlantic  band,  includes  the 
volcanoes  of  east  Africa,  the  Mascarene  and  Comores  Islands,  to- 
gether with  the  extinct  vents  of  Madagascar  in  the  south  and  those 
of  Armenia,  Syria  and  Arabia  in  the  north. 

Other  subsidiary  volcanic  belts  are  scattered  along  the  great 
equatorial  seismic  zone  mentioned  in  Chapter  I.  Thus,  the  vol- 
canoes of  southern  Mexico  and  Central  America  have  a  general 
east-west  arrangement  and  are  in  line  with  those  of  the  West 
Indies.  On  another  portion  of  the  same  zone  are  placed  the 
Mediterranean  vents.  At  the  crossing  of  the  equatorial  and  west 
Pacific  zones  are  the  volcanoes  of  the  Philippines  and  Japan,  and 
those  of  the  former  continue  westward  through  Java,  Sumatra, 
the  Nicobar  and  Andaman  Islands,  to  Burmah. 

A  very  striking  fact  is  the  nearness  of  most  active  volcanoes  to 
the  sea;  by  far  the  greater  number  of  vents  are  upon  islands, 
and  those  of  the  continents  are,  with  a  few  exceptions,  not  far  from 
the  coasts.  Some  of  the  volcanoes  in  Mexico  and  Ecuador  are 
150  miles  from  the  ocean,  and  Kirunga  in  east  Africa  is  more  than 
600  miles  from  the  sea.  Another  relation  which  should  be  noted, 
is  that  between  the  volcanic  bands  and  the  mountain-chains,  the 
bands  running  parallel  to  or  coinciding  with  the  mountains,  as  in 
the  great  volcanoes  of  the  Andes.  Not  all  coast-lines  or  all  moun- 
tain chains  have  volcanoes  associated  with  them,  but  where  the 
mountains  are  near  the  seashore,  volcanoes  are  usually,  though 
not  invariably,  found.  The  seat  of  volcanic  activity  is  frequently 
shifted,  as  we  have  learned,  and  it  has  been  observed  that  this 
activity  tends  to  die  out  of  the  older  rocks  and  to  make  its  appear- 
ance in  those  of  a  later  date.  * 

The  relations  of  volcanoes  to  lines  of  fracture  and  faulting  are 
much  disputed.  As  we  shall  see,  lava  may  force  its  way  to  the 
surface  independently  of  such  lines;  nevertheless,  "the  great 
majority  of  recent  and  earlier  eruptions  are  connected  with  fissures 
and  zones  of  fracture  in  the  earth's  crust."  (Kayser.) 

Volcanic  Eruptions.  — The  phenomena  displayed  by  different 
volcanoes,  or  even  by  the  same  volcano  at  different  times,  vary 


VOLCANIC   ERUPTIONS  55 

greatly.  It  often  seems  difficult  to  believe  that  similar  forces  are 
involved,  and  that  the  divergences  are  due  merely  to  different 
circumstances  attending  the  outbreak.  A  careful  comparison,  how- 
ever, of  the  varying  phenomena  brings  to  light  a  fundamental  like- 
ness in  them  all.  Some  vents,  like  Stromboli  in  the  Mediterranean, 
are  in  an  almost  continual  state  of  eruption  of  a  quiet  kind;  others, 
like  Vesuvius,  have  long  periods  of  dormancy,  broken  by  eruptions 
of  terrible  violence.  In  a  general  way,  it  may  be  said  that  the 
longer  the  period  of  quiet,  the  more  violent  and  long-continued 
will  the  subsequent  eruption  be,  while  weak  eruptions  and  those 
of  short  duration  recur  at  brief  intervals. 

As  one  extreme  of  the  various  forms  of  volcanic  activity  should 
be  regarded  the  explosive  type,  in  which  little  or  no  lava  is  produced  , 
and,  in  some  cases,  even  the  finely  shattered  fragments  of  lava, 
the  so-called  volcanic  ash,  are  absent.  An  instructive  example  of 
this  kind  is  afforded  by  the  eruption  of  the  Japanese  volcano 
Shirane  in  1882,  which  consisted  of  a  single  tremendous  explosion 
of  steam,  hurling  a  vast  column  of  rock  into  the  air,  but  without 
the  emission  of  lava  or  ashes.  Another  eruption  of  the  same  kind 
was  that  of  Bandai  San,  also  a  Japanese  volcano,  in  1888;  a  single 
terrific  steam  explosion  blew  away  the  greater  part  of  the  mountain, 
which  was  more  than  2000  feet  high,  likewise  without  the  forma- 
tion of  ash  or  lava. 

The  first  recorded  eruption  of  Vesuvius,  which  occurred  in 
79  A.D.  and  is  described  in  two  letters  written  to  Tacitus  by  the 
younger  Pliny,  was  of  the  explosive  type,  but  was  much  more  pro- 
longed than  those  of  the  Japanese  vents  above-mentioned  and  was 
accompanied  by  the  production  of  immense  volumes  of  ashes  and 
larger  fragments.  In  this  frightful  paroxysm  little  or  no  molten 
lava  was  ejected,  and  so  enormous  was  the  quantity  of  ashes  that 
at  Misenum,  across  the  bay  of  Naples,  the  sun  was  darkened,  as 
Pliny  reports,  "  not  as  on  a  moonless  cloudy  night,  but  as  when  the 
light  is  extinguished  in  a  closed  room  ...  In  order  not  to  be 
covered  by  the  falling  ashes  and  crushed  by  their  weight,  it  was 
often  necessary  to  rise  and  shake  them  off."  Herculaneum  was 


56  VOLCANOES 

overwhelmed  with  floods  of  ashes  mixed  with  water,  while  Pom- 
peii was  completely  buried  in  dry  ashes  and  small  fragments. 

The  explosive  type  of  eruption  is  exhibited  in  its  extreme  form 
by  several  of  the  East  Indian  volcanoes,  and  preeminently  by  Kra- 
katoa,  the  eruption  of  which  in  1883  was  the  most  frightful  ever 
recorded.  This  volcanic  island,  situated  in  the  Strait  of  Sunda, 
was  little  known,  except  that  it  had  been  in  eruption  in  1680.  As 


FIG.  10.  —  Pompeii,  showing  depth  of  volcanic  accumulations.     (Photograph  by 

McAllister) 

the  island  was  uninhabited,  the  earliest  stages  of  the  outburst  were 
not  observed,  but  on  May  20  a  great  cloud  of  steam  was  seen  over 
the  vent.  The  catastrophe  occurred  in  August,  when,  besides  the 
fearful  devastation  caused  by  the  disturbances  of  the  sea  on  the 
coasts  of  Sumatra  and  Java,  the  island  itself  was  almost  annihilated. 
Hardly  one-third  of  its  original  surface  was  left  above  water,  and 
where  formerly  was  land  are  now  depths  of  100  to  150  fathoms  of 
water.  The  force  of  the  explosion  produced  waves  in  the  atmos- 


VOLCANIC  ERUPTIONS  57 

phere  which  were  propagated  around  the  whole  earth,  and  the 
first  one  was  observed  in  Berlin  ten  hours  after  the  explosion. 
The  ejected  materials  were  all  fragmentary  and  of  an  incredible 
volume;  ashes  were  distributed  over  an  area  of  300,000  square 
miles,  the  greater  part  falling  within  a  radius  of  eight  miles  around 
the  island;  stretches  of  water  that  had  had  an  average  depth  of 
117  feet  were  so  filled  up  as  to  be  no  longer  navigable.  Enor- 
mous masses  of  pumice  floated  upon  the  sea  and  stopped  naviga- 
tion except  for  the  most  powerful  steamers.  Even  more  remark- 


FlG.  ii.  —  Profiles  of  Krakatoa.  The  full  curved  line  is  the  present  condition,  the 
dotted  line  the  condition  before  the  eruption  of  1883,  while  the  horizontal  line 
is  that  of  sea-level.  (Judd) 


able  is  the  fact  that  the  finest  dust,  which  was  hurled  into  the 
upper  atmosphere,  remained  suspended  in  the  air  for  many 
months  and  was  gradually  diffused  over  the  world.  The  wonder- 
ful, flaming-red  sunsets  which  characterized  the  autumn  and 
winter  of  1883-1884,  have  been  very  generally  ascribed  to  the 
refractive  effects  of  the  impalpably  fine  Krakatoa  dust. 

These  tremendous  explosions,  even  when  they  do  not  tear  out 
one  whole  side  of  the  mountain  as  in  the  case  of  Krakatoa,  may 
blow  off  1,he  top  and  thus  leave  a  great  crater  ring  many  miles  in 
circumference,  within  which  subsequent  eruptions  may  build  up 
a  new  cone.  When  the  volcanic  activity  dies  out,  the  ring  may  be 
filled  with  water,  forming  a  circular  lake.  Crater  rings  may  also 
be  formed  in  another  way,  illustrated  by  Crater  Lake  in  Oregon. 
The  glacial  markings  on  the  outer  side  of  the  mountain  prove  that 
the  latter  must  once  have  been  much  higher  than  at  present. 
On  the  other  hand,  the  surrounding  country  displays  no  such 
quantity  of  fragments  as  would  have  been  formed,  had  the  top 


58  VOLCANOES 

been  blown  off  by  an  explosion.  In  this  case,  the  upper  portion 
of  the  cone  was  probably  engulfed  in  the  crater  and  perhaps 
remelted. 

The  year  1902  was  made  memorable  by  a  series  of  excessively 
violent  explosive  eruptions,  in  some  instances  accompanied  by 
frightful  destruction  and  loss  of  life,  in  Central  America  and  the 


FIG.  12.  —  Crater  Lake,  Oregon.    The  small  island  is  a  cone  of  eruption,  built  up 
after  the  formation  of  the  crater  ring 


Lesser  Antilles.  In  Nicaragua  there  was  an  unimportant  eruption 
of  Masaya  (June  25),  and  Isalco  in  Salvador,  after  a  pause  of  more 
than  a  year,  began  erupting  on  May  10,  but  this  eruption  was  not 
of  the  explosive  type  and  produced  streams  of  lava.  Far  more 
violent  was  the  outbreak  of  Santa  Maria  in  Guatemala,  a  volcano 
which  had  been  regarded  as  extinct.  The  eruptions  began  October 


VOLCANIC   ERUPTIONS 


59 


24  and  were  repeated,  with  diminishing  energy,  for  more  than  a  year, 
and  were  accompanied  by  an  incredible  quantity  of  ashes,  which 
covered  several  hundred  thousand  square  miles  and,  for  a  long 
distance  around  the  vent,  destroyed  a  great  amount  of  property. 


FIG.  13.  —  Gorge  200  feet  deep  filled  by  ash  from  La  Soufriere,  St.  Vincent,  eruption 
of  1902.     (E.  O.  Hovey,  courtesy  of  the  American  Museum  of  Natural  History) 


The  islands  of  St.  Vincent  and  Martinique  in  the  Lesser  Antilles 
were  devastated  by  a  series  of  fearful  and  nearly  simultaneous 
eruptions,  which  in  certain  important  respects  differ  from  those 
of  any  other  known  volcanoes.  The  volcano  of  St.  Vincent,  known 
as  La  Soufriere  (the  last  violent  eruption  of  which  had  been  in 
1812),  began  to  show  signs  of  activity  in  February,  1901,  by  a 


6o 


VOLCANOES 


succession  of  earthquakes,  which  were  repeated,  with  longer  or 
shorter  intervals,  until  April,  1902,  in  the  latter  part  of  which  they 
increased  in  number  and  violence.  The  actual  outbreak  began 
on  May  6,  1902,  in  a  series  of  tremendous  steam  explosions;  May  7 


FIG.  14.  — Spine  of  Mt.  Pelee.     (Photograph  by  Heilprin) 


the  eruption  became  continuous  and  on  the  same  day  occurred  the 
dreadful,  descending  "  hot  blast,"  a  cloud  of  superheated  steam 
and  other  gases,  mingled  with  red-hot  particles  of  ash,  which  rushed 
down  the  mountain  and  destroyed  1400  human  lives.  The  erup- 


VOLCANIC   ERUPTIONS  6 1 

tions,  which  were  repeated  at  varying  intervals  and  with  different 
degrees  of  violence  for  considerably  more  than  a  year,  were 
characterized  by  the  absence  of  lava  and  by  the  vast  quantity  of 
finely  divided  ash  ejected  by  the  explosions. 

The  eruptions  of  Mont  Pelee  in  Martinique  were  actually  less 
violent,  but  far  more  destructive  to  life  than  those  of  St,  Vincent. 
The  previous  outbreaks  of  Mont  Pelee  within  historic  times  had  been 
those  of  1792  and  1851,  both  of  which  occurred  with  the  same 
suddenness  as  the  awful  catastrophe  of  1902.  In  the  latter  year 
slight  earthquakes  were  noted  on  April  23,  and  on  the  25th  a  heavy 
cloud  of  "  smoke  "  appeared  over  the  volcano.  On  May  2  the 
ejections  of  ash  became  heavier  and  more  frequent,  increasing 
until  the  8th,  when  a  descending  cloud  of  hot  vapours  and  glowing 
ash  swept  with  terrible  velocity  down  the  ravine  of  the  Riviere 
Blanche  upon  the  city  of  St.  Pierre,  which,  together  with  its  30,000 
inhabitants,  was  instantly  annihilated.  The  velocity  of  the  air 
set  in  motion  by  the  descending  cloud  was  sufficiently  great  to 
hurl  from  its  pedestal  the  great  iron  statue  of  Notre  Dame  de  la 
Garde,  weighing  several  tons,  to  a  distance  of  more  than  40  feet. 

Mont  Pelee  had  a  long  succession  of  subsequent  eruptions  of 
varying  violence,  especially  on  May  19,  20,  and  25,  June  6,  July  9 
and  13,  August  25,  28,  and  30,  September  3,  1902;  January  25, 
March  26,  September  12  and  16,  1903;  the  last  hardly  less  violent 
than  the  first  terrible  outbreak  of  May  8,  1902. 

It  is  the  descending  clouds  which  lend  such  an  exceptional 
character  to  the  eruptions  of  St.  Vincent  and  Martinique,  but  Mont 
Pelee  also  displayed  certain  other  peculiar  phenomena.  While 
no  lava  streams  were  produced,  very  stiff  and  viscous  lava  ap- 
peared at  the  summit,  filling  up  the  old  crater  and  forming  a  steep 
cone,  through  which  protruded  a  lofty  obelisk  or  spine,  which, 
thrust  up  from  below,  grew  irregularly  in  height,  as  it  continually 
lost  material  by  scaling  off  of  the  top  and  sides;  eventually  it  fell 
altogether. 

In  all  eruptions  of  the  explosive  kind,  a  few  typical  examples 
of  which  are  described  above,  the  active  agency  is  obviously 


62 


VOLCANOES 


exploding  masses  of  intensely  heated  and  compressed  steam,  and 
all  such  eruptions  are  accompanied  by  gigantic  steam-clouds, 
which,  condensing  in  the  atmosphere,  fall  in  rains  of  torrential 
volume  and  violence.  The  hot  water  thus  produced  mingles  with 
the  volcanic  ash  in  the  air  and  on  the  ground,  forming  streams  of 
hot  mud,  which  are  often  more  destructive  than  the  lava  flows 
themselves.  When  cold,  the  mud  sets  into  quite  a  firm  rock, 
called  tuff. 


FIG.  15.  —  Crater-floor  of  Kilauea,  showing  the  lava  lake,  Hale-mau-mau. 
tograph  by  Libbey) 


(Pho- 


The opposite  extreme  of  volcanic  activity  from  the  explosive 
type  is  to  be  found  in  the  volcanoes  of  the  Sandwich  Islands, 
such  as  Mauna  Loa  and  Kilauea.  Here  the  eruptions  are  usually 
not  heralded  by  earthquakes;  the  lava  is  remarkably  fluid  and 
simply  wells  up  over  the  sides  of  the  crater,  pouring  down  the  sides 
of  the  mountain  in  streams  which  flow  for  many  miles.  More 
commonly  the  walls  of  the  crater  are  unable  to  withstand  the  enor- 


VOLCANIC   ERUPTIONS  63 

mous  pressure  of  the  lava  column,  and  the  molten  mass  breaks 
through  at  some  level  below  the  crater,  rising  through  the  fissure 
in  giant  fountains,  sometimes  1000  feet  high.  Even  in  the  ordi- 
nary activity  of  Kilauea  jets  of  30  and  40  feet  in  height  are  thrown 
up.  Hardly  any  ashes  or  other  fragmental  products  are  formed; 
and  though  the  clouds  of  steam,  the  invariable  accompaniments 
of  volcanic  outbursts,  are  present,  yet  the  quantity  of  steam  is  rela- 
tively less  than  in  those  volcanoes  in  which  explosions  occur. 

Between  such  extremes  as  the  Hawaiian  volcanoes  on  the  one 
hand,  and  the  explosive  East  Indian  type  (Krakatoa),  on  the 
other,  we  may  find  every  intermediate  gradation.  The  com- 
paratively gentle  operations  of  Stromboli,  one  of  the  Lipari  Islands, 
northwest  of  Sicily,  give  an  opportunity  to  observe  directly  the 
essential  phenomena  of  a  volcanic  eruption.  Though  occasionally 
breaking  out  with  violence,  Stromboli  has  been  in  a  state  of  almost 
continuous  activity  for  more  than  2000  years,  and  is,  for  long 
periods,  in  such  exact  equilibrium,  that  barometric  changes  have 
a  marked  effect  upon  its  activity  and  the  Mediterranean  sailors 
make  use  of  it  as  a  weather  signal. 

The  crater-floor  is  formed  by  hardened  lava,  the  cracks  in 
which  glow  at  night  from  the  heat  of  the  molten  mass  below,  and 
which  is  perforated  by  various  openings.  From  some  of  these 
steam  is  given  out,  from  others  molten  lava  wells  up  occasionally. 
In  openings  of  a  third  class  the  lava  may  be  seen  rising  and  sink- 
ing, until  a  great  bubble  forms  on  its  surface  and  bursts  with  a 
loud  roar,  scattering  the  hardened  lava  scum  about  the  crater  in 
fragments  of  various  sizes,  some  very  fine,  others  coarse.  The 
bubble  is  of  steam,  and  when  set  free,  the  steam  globule  rises  to 
join  the  cloud  which  always  overhangs  the  mountain.  The 
bursting  of  the  bubble  is  followed  by  a  rush  of  steam  through 
the  mass  of  the  lava,  the  pressure  is  relieved,  and  the  lava  column 
sinks  down  out  of  sight,  until  the  steam  pressure  again  accumu- 
lates and  the  performance  is  repeated. 

Evidently,  one  active  agent  in  these  phenomena  is  imprisoned 
steam  in  its  struggles  to  escape.  Different  as  are  the  manifesta- 


64 


VOLCANOES 


tions  at  other  volcanoes,  steam  is  an  important  cause  of  the  erup- 
tion in  all  cases,  though  the  conditions  under  which  it  acts  vary 
widely.  Little  or  no  combustion  is  involved,  and  that  not  as  a 
cause,  but  as  an  effect  of  the  activity. 

In  the  modern  eruptions  of  Vesuvius  essentially  the  same  phe- 


FlG.  16.  —  Crater  of  Vesuvius  in  moderate  eruption 

nomena  may  be  observed,  but  on  a  far  grander  and  more  terrible 
scale.  Earthquakes  usually  announce  the  coming  eruption,  in- 
creasing in  force  until  the  outbreak  occurs.  Terrific  explosions 
blow  out  fragments  of  all  sizes,  from  great  blocks  to  the  finest  and 
most  impalpable  dust.  The  finer  fragments  arise  chiefly  from  the 
scattering  of  the  partly  hardened  lava  by  the  force  of  the  explo- 
sion, but  in  part  also  from  the  crashing  together  of  the  blocks  as 


VOLCANIC  ERUPTIONS  6$ 

they  rise  and  fall  through  the  air.  Inconceivable  quantities  of 
steam  are  given  off  with  a  loud  roar,  which  is  awe-inspiring  in  its 
great  and  steady  volume.  The  condensation  of  such  masses  of 
vapour  produces  torrents  of  rain,  which,  mingling  with  the 
"ashes  "  and  dust,  gives  rise  to  streams  of  hot  mud  that  flow  for 
long  distances.  Great  floods  of  molten  rock,  or  lava,  issue  from  the 
crater,  or  burst  their  way  through  the  walls  of  the  cone,  and  pour 
down  the  mountain  side,  until  they  gradually  stiffen  by  cooling. 

During  historic  times  Vesuvius  has  had  long  periods  of  dormancy 
and  the  violence  of  the  subsequent  outbreak  has  been,  in  a  general 
way,  proportionate  to  the  length  of  the  dormant  period,  though 
one  of  the  most  notable  eruptions,  that  of  1906,  occurred  after 
quite  a  short  period  of  rest.  With  the  exception  of  a  moderate 
outbreak  in  1500,  the  mountain  was  quiet  between  1139  and  1631; 
one  of  the  three  most  violent  recorded  eruptions  took  place  in  the 
latter  year. 

Radically  different  as  the  various  types  of  volcanic  activity 
appear  to  be,  they  are  all  connected  together  in  one  continuous 
series.  In  all  cases,  steam  of  very  high  temperature  and  under 
enormous  pressures  is  an  important  agent,  while  the  differing 
results  are  due  to  varying  degrees  of  pressure,  quantity  of  impris- 
oned steam,  amount  of  resistance  to  be  overcome,  the  character  of 
the  lava,  and  similar  factors.  The  intermediate,  or  Vesuvian,  type 
of  eruption  is  the  most  frequent. 

Submarine  Volcanoes.  —  Several  instances  of  submarine  erup- 
tions have  been  actually  observed,  and  there  is  much  reason  to 
believe  that  the  number  of  vents  on  the  ocean-floor  is  very  large. 
Volcanic  islands  are  merely  submarine  volcanoes  which  have  built 
their  cones  above  sea-level  and  these  represent  a  great  proportion 
of  the  vents  now  active.  The  durability  of  volcanic  islands  de- 
pends upon  the  materials  of  which  they  are  constructed.  Cones 
built  of  loose  masses,  or  of  ash  and  tuff,  are  speedily  destroyed 
by  the  sea  when  the  activity  ceases,  and  cut  down  into  reefs  and 
shoals,  while  masses  of  solidified  lava  resist  destruction  for  very 
long  periods. 


66 


VOLCANOES 


New  Volcanoes.  —  During  historic  times  a  considerable  number 
of  new  volcanoes  have  been  formed,  both  on  land  and  in  the  bed  of 
the  sea,  the  latter  resulting  in  the  birth  of  new  islands.  Aside 
from  certain  newly  formed  volcanoes,  the  origin  of  which  has  been 
recorded  by  ancient  writers  of  Greece,  Rome,  and  Japan,  a  few 
more  modern  instances  may  be  cited. 

Near  Pozzuoli  is  a  hill  called  Monte  Nuovo,  440  feet  high, 
which  is  hardly  distinguishable  from  the  other  low  volcanic  cones 


FIG.  17.  —  Monte  Nuovo,  near  Pozzuoli,  formed  in  1538 

among  which  it  stands,  and  which  are  mentioned  by  several 
classical  writers.  Monte  Nuovo  was  formed  by  an  eruption  which 
broke  out  September  29,  1538,  the  ground  swelling  up  and  bursting, 
leaving  a  fissure,  which  disclosed  glowing  lava,  and  which  ejected 
great  masses  of  blocks,  sand,  and  ash.  The  activity  lasted  for  a 
week  and  has  not  since  been  repeated. 

Jorullo,  in  Mexico,  was  formed  in  1759,  the  eruptions  continuing 
for  several  years  and  then  dying  out.     Immense  quantities  oi 


NEW  VOLCANOES 


67 


lava  flowed  forth  and  several  "  cinder  cones  "  were  built  up,  one 
of  which'is  1300  feet  high.  Isalco,  a  volcano  on  the  west  coast  of 
Central  America,  north  of  the  city  of  San  Salvador,  was  first  formed 
in  1770,  and  has  been  in  almost  uninterrupted  activity  ever  since, 
sometimes  with  great  violence.  A  cone  of  more  than  2000  feet 
in  height  has  been  built  up.  In  1831  a  new  island  appeared  off 
the  southwest  coast  of  Sicily,  where  previous  soundings  had 


FIG.  18.  — Another  view  of  the  crater-floor  and  walls  of  Kilauea.     (U.  S.  G.  S.) 

shown  a  depth  of  100  fathoms,  and  in  the  course  of  a  few  weeks 
grew  to  a  height  of  200  feet  above  the  sea  and  a  diameter  of  a  mile 
at  sea-level.  The  activity  soon  ceased  and  the  island,  composed 
of  loose  materials,  was  swept  away  by  the  sea. 

In  the  Greek  archipelago  the  group  of  islands  known  as  Santorin 
has  been  the  scene  of  repeated  operations  for  more  than  2000  years. 
The  outer  islands  are  evidently  fragments  of  an  old  crater  ring, 


68  VOLCANOES 

within  which  are  several  small  islands,  which  were  formed  in 
186  B.C.,  1573,  1707,  and  1866,  respectively. 

Finally  may  be  mentioned  an  especially  interesting  group  of 
three  new  islands,  the  Bogoslofs,  north  of  the  line  of  the  Aleutian 
islands,  Alaska.  The  first  of  the  islands,  Old  Bogoslof ,  was  formed 
by  a  submarine  eruption  in  1796,  which  was  observed  by  a  Russian 
trader.  New  Bogoslof  was  formed  in  1883  and  was  first  seen  in 
September  of  that  year,  but  the  exact  date  of  origin  is  not  known. 
A  third  and  very  large  island,  composed  of  jagged  lava,  was  seen 
between  the  older  ones  on  May  28,  1906,  "  giving  off  clouds  of 
steam  and  smoke  from  any  number  of  little  craters  scattered  all 
over  it."  (C.  H.  Gilbert.) 


CHAPTER   III 

VOLCANOES  (cont.)— INTERNAL  CONSTITUTION  OF   THE 

EARTH 

Volcanic  Products.  — These  form  the  most  important  part  of  the 
subject  from  the  geological  point  of  view,  because  they  contribute 


FIG.  19.  —  Edge  of  Hale-mau-mau,  showing  the  ropy  forms  of  the  highly  fluid  lava,' 
when  cooling.     (Photograph  by  Libbey) 

largely  to  the  permanent  materials  of  the  earth's  crust.  We  meet 
with  such  materials  of  all  geological  ages,  sometimes  developed  on 
a  vast  scale.  The  study  of  volcanic  products  is  the  key  which 

69 


7O  VOLCANOES 

enables  us  to  comprehend  the  great  group  of  rocks  which  are 
called  igneous,  though,  as  we  shall  see  later,  by  no  means  all  of 
these  were  poured  out  on  the  surface  of  the  ground. 

Volcanic  products  are   of  three  kinds:    (i)   lava,  or  molten 
rock;    (2)  fragmental  material,  including  blocks,  lapilli,  bombs, 


FIG.  20.  —  Ropy  lava,  Vesuvius 


the  so-called  volcanic  ashes,  cinders,  and  the  like;    (3)  gases 
.vapours. 

(i)  Lava.  —  A  lava  is  a  more  or  less  completely  melted  rock; 
the  degree  of  fluidity  varies  greatly  in  different  lavas,  but  is  rarely, 
if  ever,  perfect.  Instead  of  being  a  true  liquid,  a  lava  ordinarily 
consists  of  larger  and  smaller  crystals,  embedded  in  a  pasty  mass, 
which  is  saturated  with  steam,  and  gases.  The  degree  of  fluidity 


VOLCANIC  PRODUCTS  7 1 

depends  upon  several  factors,  the  most  obvious  of  which  is  tempera- 
ture; the  more  highly  heated  the  mass  is,  the  more  perfectly  will 
it  be  melted.  The  quantity  of  imprisoned  gases  and  vapours 
present  has  also  an  important  effect,  and  some  lavas  appear  to  owe 
nearly  all  their  mobility  to  these  vapours.  A  third  and  most  sig- 
nificant factor  is  the  chemical  composition.  Those  lavas  which 
contain  high  percentages  of  silica  (SiO2),  the  acid  lavas,  are  much 


FIG.  21.  —  Sunset  Butte,  Arizona.    An  extinct  volcano,  with  scoriaceous  block-lava  in 
foreground.     (Photograph  by  A.  E.  Hackett,  Flagstaff,  Ariz.) 

less  readily  fusible  than  the  basic  lavas,  in  which  the  percentage  of 
silica  is  lower.  The  difference  in  the  proportion  of  silica  present 
is  associated  with  other  chemical  differences  which  have  a  similar 
effect  upon  fusibility,  the  basic  kinds  having  much  more  lime, 
magnesia  and  iron  in  them,  which  act  as  fluxes. 

The  experiments  of  Barus  on  the  fusibility  of  lavas,  which  he 
divides  into  three  groups,  resulted  as  follows:  (i)  Certain  lavas 
fuse  readily  (2250°  F.);  these  are  of  basic  composition  and  are 


72  VOLCANOES 

made  up  of  lime-soda  felspars,  the  augitic  and  allied  ferro-magnesian 
minerals,  and  iron  oxide,  but  rarely  have  quartz.  (2)  A  second 
group  is  of  medium  fusibility  (2520°  F.),  and  is  made  up  of  lime- 
soda  felspars,  augitic  or  hornblende  minerals,  and  frequently 
quartz.  (3)  The  third  series  melts  with  difficulty  (2700°  F.),and 
remains  pasty  at  even  3100°  F.  These  are  acid  lavas,  and  are  com- 
posed of  potash  felspars,  with  quartz,  hornblende,  or  mica.  Lavas 


FIG.  22.  —  Lava-tunnel,  and  "  Spatter-cone"  formed  by  escaping  steam,  Kilauea. 
(Photograph  by  Libbey) 

which,  like  those  of  the  Sandwich  Islands,  are  notably  fluid,  are 
always  of  basic  composition. 

When  a  lava  stream  reaches  the  surface  of  the  ground,  the  im- 
prisoned vapours  immediately  begin  to  escape  and  the  surface 
of  the  molten-  mass  to  cool  and  harden.  The  surface  layers  are 
blown  by  the  steam  bubbles  into  a  light,  frothy  or  slaggy  consist- 
ency, forming  "  scoriae  "  or  cindery  masses.  The  motion  of  the 
lava  breaks  up  this  thin  crust  into  loose  slabs  and  blocks,  and  on 


VOLCANIC  PRODUCTS 


73 


the  advancing  front  of  the  stream  these  loose  masses  rattle  down 
over  one  another  in  the  wildest  confusion.  The  less  perfectly 
fused  lavas  are  soon  covered  with  heaped-up  cindery  blocks,  while 
the  more  completely  fluid  lavas  are  characterized  by  curiously 
twisted,  ropy  surfaces,  such  as  may  be  observed  in  the  slag  from  an 
iron  furnace. 
The  front  of  a  lava  stream  advances,  not  by  gliding  over  the 


FIG.  23.  —  Lava  stalactites  and  stalagmites  in  lava-tunnel,  Kilauea. 

by  Libbey) 


(Photograph 


ground,  but  by  rolling,  the  bottom  being  retarded  by  the  friction 
of  the  ground  and  the  top  moving  faster,  so  that  it  is  continually 
rolling  down  at  the  curved  front  end  and  forming  the  bottom. 
Thus,  the  scoriae,  though  formed  mostly  on  the  top  of  the  stream, 
are  rolled  beneath  it,  and  the  whole  is  enclosed  in  a  cindery 
envelope.  Or  the  flow  may  be  checked  by  the  mass  of  cinders, 
until  the  fluid  lava  bursts  through  them  in  a  fresh  stream.  The 


74  VOLCANOES 

scoriaceous  mass  is  a  non-conductor  of  heat,  and  greatly  retards 
the  cooling  of  the  interior  mass,  which  may  remain  hot  for  many 
years.  The  arched  surface  of  cindery  blocks  may  become  self- 
supporting,  and  then  the  still  fluid  mass  will  flow  away  from 
beneath  it,  leaving  long  tunnels  or  caverns.  These  tunnels  are 
especially  well  shown  in  Iceland  and  the  Sandwich  Islands. 
The  distance  to  which  lava  streams  extend  and  the  rapidity 

.with  which  they  move  are  determined  by  the  abundance  and 
fluidity  of  the  lava  and  the  slope  over  which  it  flows.  Some  lavas 
are  so  liquid  that  they  flow  for  many  miles,  even  down  moderate 
slopes,  while  others  are  so  pasty  that  they  stiffen  and  set  within  a 
short  distance  of  the  vent,  even  on  steep  grades.  Ordinarily  the 
motion  soon  becomes  very  slow,  though  thoroughly  melted  masses 
pouring  down  steep  slopes  may,  for  a  short  time,  move  very  swiftly. 
One  of  the  lava  floods  from  Mauna  Loa  moved  fifteen  miles  in 
two  hours,  and  for  shorter  distances  much  higher  rates  of  speed 
have  been  observed;  but  this  is  very  exceptional. 

The  cooling  of  the  surfaces  of  the  lava  stream  takes  place  rap- 
idly, while  the  interior  cools  but  slowly,  and  great  thicknesses 
require  very  long  periods  of  time  to  become  entirely  cold.  The 
differences  in  the  rate  of  cooling  produce  very  strongly  marked 
varieties  in  the  appearance  and  texture  of  the  resulting  rock.  The 

/  portions  which  have  chilled  and  solidified  very  quickly  are  glassy 

I  and  form  the  volcanic  glass,  obsidian.  If  the  swiftly  cooling  por- 
tions have  been  much  disturbed  by  the  bubbles  of  steam  and 

1  vapours,  they  are  made  light  and  frothy;  in  some  cases,  as  in 
pumice,  they  will  float  upon  water.  Otherwise,  the  glass  is  solid 
and  is  usually  very  dark  in  colour,  resembling  an  inferior  bottle 
glass  in  appearance.  Microscopic  examination  shows  minute, 
hair-like  bodies  in  the  glass,  which  are  called  crystallites,  and 
represent  the  incipient  stages  of  crystallization. 

Passing  inward  from  the  surface  of  the  lava  stream,  we  find 
the  steam  bubbles  becoming  rarer,  until  they  cease  altogether,  the 
vapours  having  escaped  while  the  lava  was  still  so  soft  that  the 
bubble  holes  soon  collapsed.  At  the  same  time  the  glassy  texture 


VOLCANIC   PRODUCTS 


75 


of  the  rock  is  replaced  by  a  stony  character,  which  the  microscope 
shows  to  be  due  to  the  formation  of  crystals  too  minute  to  be 
recognized  by  the  unaided  eye.  Still  deeper  in  the  rock  the  stony 
texture  passes  gradually  into  an  obviously  crystalline  one;  and  the 
slower  the  cooling,  the  larger  will  these  crystals  be,  though  in  lava 
streams  which  have  cooled  on  the  surface  of  the  ground,  the  whole 
mass,  even  of  the  deeper  parts,  is  never  coarsely  crystalline. 


FIG.  24.  — A  hand-specimen  of  obsidian,  showing  the  glassy  lustre  and  fracture 


Large  crystals  are,  it  is  true,  very  often  found  in  lavas,  but  these 
were  formed  before  the  ejection  of  the  mass  from  the  volcano. 
Such  crystals  frequently  contain  enclosures  of  glass,  which  indi- 
cate that  the  crystallization  went  on  while  the  surrounding  mass 
was  still  fluid.  The  edges  and  angles  of  these  crystals  are  often 
corroded  by  the  action  of  the  melted  portion  of  the  lava,  and  the 
motion  of*  the  stream  often  cracks  them.  These  facts  go  to  prove 
that  the  large  crystals  were  complete  when  the  lava,  as  a  whole, 
was  still  fluid  and  in  motion.  Stromboli  ejects  great  numbers  of 


76  VOLCANOES 

perfect  crystals  of  augite,  which  must  have  existed  in  the  molten 
lava  of  the  vent.  The  lavas  which  contain  large  crystals  embedded 
in  a  fine  stony  or  glassy  base  are  said  to  be  of  a  porphyritic  texture. 

It  is  important  to  remember  that  all  these  various  textures  may 
be  found  in  one  continuous  rock  mass,  and  bear  witness  to  the 
circumstances  under  which  each  part  cooled  and  solidified.  These 
textures  also  recur  again  and  again  in  ancient  rocks  and  enable  us 
to  determine  their  volcanic  origin.  The  processes  of  rock  destruc- 
tion and  removal  have  in  many  cases  laid  bare  deep-seated  masses 
which  were  plainly  once  melted  like  true  lavas,  but  which  have 
cooled  very  slowly  and  under  great  pressures.  In  such  rocks  the 
texture  is  usually  coarsely  crystalline  and  shows  no  traces  of  glass 
or  scoriae.  Between  the  surface  lava  flows  and  such  deep-seated 
reservoirs  every  form  of  transition  may  be  traced,  often  in  con- 
tinuous rock  masses. 

Where  several  successive  lava  flows  issue  from  one  vent,  at  in- 
tervals which  allow  one  stream  to  be  consolidated  before  the  next 
is  poured  out  over  it,  a  rough  bedding  or  stratification  results, 
each  flow  being  perfectly  distinguishable  when  seen  in  section. 
Deceptive  resemblances  to  the  true  stratification  of  sedimentary 
rocks  may  thus  arise,  especially  when  the  exposed  section  is  short. 
But  the  wedge-like  form  of  the  sheets,  the  absence  of  bedding 
within  the  limits  of  each  flow,  and  the  nature  of  the  rock  itself, 
always  enable  us  to  distinguish  these  masses  from  the  sediments 
which  have  been  stratified  by  the  sorting  power  of  water. 

A  mass  of  lava,  when  it  cools  and  solidifies,  necessarily  contracts, 
and  since  the  cohesion  of  the  mass  is  insufficient  to  allow  it  to 
contract  as  a  whole,  it  must  crack  into  blocks,  separated  by 
fine  crevices,  which  are  called  joints.  The  mutual  relations  of 
the  jointing  planes,  and  the  consequent  shape  of  the  blocks,  are 
determined  largely  by  the  grain  of  the  lava  and  its  degree  of 
homogeneity.  In  fine-grained  (and  some  coarse-grained) 
homogeneous  lavas  the  jointing  is  apt  to  be  very  regular,  and  to 
give  rise  to  prismatic  or  columnar  blocks,  which  are  usually  hex- 
agonal. This  shape  is  due  to  the  fact  that  the  formation  of  hexa- 


VOLCANIC   PRODUCTS 


77 


gons  requires  less  expenditure  of  work  than  other  figures,  and  is 
produced  by  the  intersection  of  systems  of  three  cracks,  radiating 
from  equidistant  points  at  angles  of  120°.  The  long  axes  of  the 
prisms  are  at  right  angles  to  the  cooling  surface.  Starch  and 
fire-clay,  which  shrink  on  drying,  joint  in  the  same  way.  The 
coarser  and  more  heterogeneous  lavas  usually  break  up  into  blocks 
of  irregular  size  and  shape. 


FIG.  25.  —  Stream  gorge,  island  of  Hawaii;  displaying   modern  columnar  lava. 
(Photograph  by  Libbey) 

It  must  not  be  inferred  that  the  joints  of  all  rocks  are  due  to 
shrinkage  on  cooling.  It  will  be  shown  in  a  subsequent  chapter 
that  such  is  very  far  from  being  the  case. 

Not  all  the  lava  produced  in  and  around  a  volcanic  vent  can 
reach  th°  surface.  Some  of  it  may  be  forced  horizontally  be- 
tween the  beds  of  the  surrounding  rocks,  thus  forming  intrusive 
sheets,  which?  when  exposed  in  section,  may  be  readily  distin- 


VOLCANOES 


guished  from  surface  flows  by  the  fact  that  they  have  consolidated 
under  pressure,  and  hence  have  no  slag  or  scoriae  associated  with 


c'lG.  26.  —  Obsidian  Cliff,  Yellowstone  Park.     Hexagonal  jointing;.     (U.,  S.  G.  S.) 


VOLCANIC   PRODUCTS  79 

them.    Other  portions  of  the  lava  will   fill  up  vertical  fissures? 
in  the  volcanic  cone  or  in  the  underlying  rocks,  and,  solidifying? 
in  these  fissures,   form  dykes.     Such  a  fissure,   twelve  miles  in 
length  and  filled  with  molten  lava,  was  observed  by  Sir  Charles 
Lyell  in  the  neighbourhood  of  ^Etna.     In  the  great  eruption  of 
Skaptar  Jokul  (Iceland)  in  1783  lava  was  poured  out  at  several 
points  along  a  line  two  hundred  miles  long,  and  doubtless  this 
was  a  great  lava-filled  fissure  which  consolidated  into  a  dyke. 

We  thus  see  that  the  molten  masses  may  not  all  well  up  through 
the  crater  of  a  volcano,  but  will  seek  egress  along  the  line  of 
least  resistance,  wherever  that  happens  to  be,  breaching  the  walls 
of  the  volcanic  cone,  rising  up  through  vertical  fissures,  or  forc- 
ing their  way  as  intrusive  sheets  between  the  beds  of  preexisting 
rocks.  In  these  various  situations  the  different  rates  of  cooling 
produce  many  varieties  of  rocks,  though  the  original  molten 
mass  may  have  been  nearly  or  quite  identical  in  all  of  them. 

Lavas  which  flow  into  the  sea  from  a  terrestrial  vent,  or  are 
poured  out  from  a  submarine  one,  show,  as  a  rule,  but  little  differ- 
ence from  those  which  solidified  on  land,  because  the  rapid  forma- 
tion of  a  cindery  crust  will  protect  the  hot  lava  from  contact  with 
the  water.  Sometimes,  however,  the  sudden  chill  will  cause  the 
lava  to  disintegrate  into  a  mass  like  black  sand. 

The  lavas  which  flow  from  a  given  vent  do  not  always  remain 
constant  in  character  and  composition,  but  change  at  successive 
periods  of  activity.  It  has  frequently  been  observed,  for  example, 
that  a  series  of  lavas,  at  first  intermediate  in  chemical  composition, 
then  acid  and  finally  basic,  have  been  successively  ejected  from 
the  same  volcano.  It  does  not  appear,  however,  that  there  is  any 
definite  law  of  succession  in  the  kinds  of  lava  emitted. 

It  should  also  be  noted  that  neighbouring  vents  may  simultane- 
ously produce  lavas  of  different  composition.  Thus,  in  the  Lipari 
Islands,  the  lava  of  Stromboli  is  basic,  while  that  of  Vulcano  is 
highly  acid. 

(2)   Fragmental  Products.  — This  division  includes  all  the  mate- 
rials which  are  ejected  from  the  volcano  in  a  solid  state.    These 
G 


8o 


VOLCANOES 


are  of  all  sizes  and  shapes,  from  huge  blocks  weighing  many  tons, 
down  to  the  most  impalpable  dust,  which  the  wind  will  carry  foi 
thousands  of  miles.  The  very  large  blocks  are  commonly  frag- 
ments of  the  older  rocks  through  which  the  volcanic  vent  has 
burst  its  way,  tearing  a  great  hole  and  scattering  the  fragments 
widely.  For  fifteen  miles  around  the  lofty  volcano  of  Cotopaxi 
in  Ecuador  lie  great  blocks  of  this  nature,  some  of  them  measur- 
ing nine  feet  in  diameter. 


FlG.  27. — Volcanic  bomb,  showing  scoriaceous  texture;  about  f  natural  size 

More  important  and  much  more  extensively  formed  and  widely 
spread  are  those  fragmental  products  which  are  derived  from  the 
lava  itself  and  are  due  to  the  sudden  and  explosive  expansion  of 
the  vapours  and  gases  with  which  the  molten  mass  is  intimately 
commingled  and  saturated.  The  more  violently  explosive  the 
eruption,  the  greater  the  proportion  of  the  lava  that  will  be  blown 
into  fragments.  In  such  eruptions  as  that  of  Krakatoa,  all  of  it  is 
thus  dispersed  and  none  remains  to  form  lava  flows.  Cindery 


VOLCANIC  PRODUCTS  8 1 

fragments  thrown  out  of  the  vent  are  called  scoria,  while  portions 
of  still  liquid  lava  thus  ejected  will,  on  account  of  their  rapid 
rotation,  'take  on  a  spheroidal  form  and  are  called  volcanic  bombs. 
Lapilli  are  smaller,  rounded  fragments,  and  volcanic  ash  and  dust 
are  very  fine  particles,  though  with  a  wide  range  of  variation  in 
size.  The  term  ash  is  so  far  unfortunate  that  it  implies  com- 
bustion, but  nevertheless  it  accurately  describes  the  appearance 
of  these  masses. 

In  the  immediate  neighbourhood  of  the  vent  fragments  of  all 
sizes  accumulate,  but  the  farther  we  get  from  the  volcano,  the 
smaller  do  the  fragments  become.  The  coarser  masses  around  the 
vent  form  a  volcanic  agglomerate,  in  which  the  fragments  are  of  all 
shapes  and  sizes,  heaped  together  without  any  arrangement.  More 
regular  sheets  of  large  angular  fragments  form  volcanic  breccia, 
and  these  may  be  seen  on  a  grand  scale  in  the  Yellowstone  Na- 
tional Park,  and  in  many  other  parts  of  the  Rocky  Mountain 
region.  The  finer  accumulations  of  ash,  formed  at  a  greater  dis- 
tance from  the  vent,  are  roughly  sorted  by  the  air  and  often  quite 
distinctly  divided  into  layers,  while,  as  already  explained,  the 
muds  on  drying  set  into  quite  a  firm  rock,  called  tuff  or  tufa. 

As  volcanoes  so  generally  stand  in  or  near  the  sea,  and  as  the 
lighter  fragments,  such  as  pumice,  often  drift  for  months  upon  the 
water  before  they  sink,  while  the  finer  dust  is  carried  vast  dis- 
tances by  the  wind,  it  would  naturally  be  expected  that  volcanic 
materials  should  have  a  very  wide  distribution  upon  the  sea- 
bottom.  Such,  indeed,  proves  to  be  the  case,  and  this  kind  of 
material,  laid  down  in  the  sea,  has  formed  important  rock  masses 
in  nearly  all  the  recorded  ages  of  the  earth's  history.  The  exact 
character  of  the  rock  formed  in  this  fashion  will  be  governed  by 
various  circumstances,  such  as  the  fineness  and  abundance  of  the 
material,  whether  it  is  showered  into  quiet  waters  or  along  a  wave- 
beaten  coast,  whether  and  in  what  proportion  it  is  mingled  with 
sand  or  mud.  When  the  volcanic  ash  preponderates,  a  tuff  is 
formed,  very  much  like  those  which  accumulate  on  land,  but  more 
regularly  stratified. 


82  VOLCANOES 

The  fragmental  volcanic  products,  whether  coarse  or  fine,  retain 
their  characteristic  texture  and  appearance,  so  as  to  be  readily 
recognizable,  though  perhaps  only  with  the  microscope.  The 
great  bulk  of  these  materials  consists  of  lava  shattered  by  the 
steam  explosions  and  quickly  chilled.  The  coarser  fragments 
display  the  frothy  and  vesicular  nature  of  scoriae,  while  the  finer 
particles  are  glassy  or  crystalline.  Mere  comminution  of  the  mass 
does  not  change  its  essential  texture. 

It  will  be  readily  imagined  that  lavas  very  rarely  contain  fossils. 
Though  the  flows  often  overwhelm  living  beings,  the  intense  heat 
at  once  destroys  them,  seldom  leaving  a  trace  behind,  though 
charred  tree-trunks  are  sometimes  recognizably  preserved.  In 
tuffs,  on  the  other  hand,  fossils,  especially  those  of  plants,  are 
frequently  well  preserved,  and  tuffs  formed  under  water  have 
fossils  as  abundantly  as  any  other  aqueous  rocks. 

(3)  The  Gaseous  Products  are  important  as  agents  of  the  erup- 
tions, in  promoting  the  crystallizing  of  the  lavas,  and  in  alter- 
ing the  rocks  with  which  they  come  in  contact.  The  most  abun- 
dant is  steam.  Carbon  dioxide  is  common,  especially  when  the 
action  is  failing,  and  often  continues  after  all  other  signs  of  activity 
have  died  out.  Sulphur  dioxide  (SO2)  is  very  characteristic  and 
is  the  source  of  many  other  compounds.  Sulphuretted  hydrogen 
(H2S)  is  a  common  volcanic  gas,  as  is  also  hydrochloric  acid 
(HC1).  Several  solids  are  vapourized,  such  as  the  chlorides  of 
ammonium,  iron,  calcium,  etc.,  but  these  are  of  little  s'gnificance. 

It  is  important  to  emphasize  the  vast  quantity  of  material 
which,  in  many  volcanic  eruptions,  is  brought  from  the  interior 
of  the  earth  and  deposited  on  the  surface.  Thus,  the  eruption  of 
Skaptar  Jokul  in  Iceland,  in  1783,  produced  an  amount  of  lava 
which  is  calculated  as  exceeding  six  cubic  miles  in  volume.  The 
fragmental  materials  derived  from  the  great  explosion  of  Kraka- 
toa,  in  1883,  are  estimated  at  4.3  cubic  miles,  while  for  that  of  Tem- 
boro,  in  1815,  Verbeek  gives  the  almost  incredible  figures  of  28.6 
cubic  miles. 


VOLCANIC  CONES  83 

Volcanic  Cones  are  built  up  by  the  material  which  the  volcanoes 
eject,  and  vary  in  shape  according  to  the  character  of  those  ma- 
terials and  to  the  violence  of  the  eruptions.  Those  vents  which 
yield  only  lavas  build  up  cones  of  solid  rock,  the  steepness  of 
which  corresponds  to  the  degree  of  fluidity  of  the  flows.  The  re- 
markably liquid  lavas  of  the  Sandwich  Islands  have  formed  cones 
of  exceedingly  gentle  slope,  3°  to  10°  (see  Fig.  28,  the  cone  of  Mauna 
Loa).  Very  stiff  lavas  which  consolidate  rapidly  form  very  steep- 
sided  cones.  The  cones  which  are  constructed  principally  out  of 
fragmental  materials  are  steep  (30°);  the  more  so,  the  coarser 


FlG.  28.  —  Mauna  Loa,  seen  from  a  distance  of  40  miles.     (Photograph  by  Libbey) 

the  fragments  which  compose  them,  and  often  beautifully  sym- 
metrical, as  in  the  noble  mountains  of  our  Pacific  States,  such  as 
Mt.  Shasta,  Mt.  Hood,  and  Mt.  Rainier.  Most  cones  are  built  up 
of  scoriae,  ashes,  and  lava  flows,  while  the  fissures  that  radiate 
from  the  crater  are  filled  by  dykes,  greatly  strengthening  the  moun- 
tain, as  in  the  case  of  Vesuvius.  The  latter  is  noted  for  its  double 
head,  Monte  Somma  being  part  of  an  ancient  crater  ring,  one  side 
of  which  was  destroyed  by  an  explosion  before  the  cone  of  Vesu- 
vius was  built  up.  It  is  usually  stated  that  the  explosion  which 
destroyed  part  of  Monte  Somma  was  that  of  79  A.D.,  but  it  is  not 


84 


VOLCANOES 


improbable  that  the  destruction  took  place  at  a  much  earlier 
date. 

Violent  explosions  occurring  within  a  volcano  blow  off  more 
or  less  of  the  top,  thus  producing  the  truncated  cones  and  crater 
rings  so  often  seen  among  volcanic  mountains. 

Volcanoes,  like  other  mountains,  are  subject  to  the  destructive 
activity  of  the  atmosphere,  of  rivers  and  of  the  sea,  and,  when 
eruptions  have  ceased,  this  destruction  may  go  on  with  great 
rapidity,  especially  in  the  case  of  cones  made  up  of  loose  materials. 


FIG.  29.  — Mt.  Shasta,  California.     (U.  S.  G.  S.) 

Very  ancient  cones  can  seldom  be  found,  for  this  reason,  and  often 
the  lava-filled  pipe  is  the  only  record  left  of  an  ancient  volcano. 

Fissure  Eruptions.  — There  is  much  reason  to  believe  that  the 
mode  of  volcanic  eruption  from  a  single  vent,  described  in  the 
foregoing  pages,  is  not  the  only  method  by  which  molten  lava 
may  reach  the  surface.  It  would  seem  that  in  past  times  lava  has 
welled  up  through  great  fissures  and  overflowed  immense  areas  in 
successive  floods.  As  an  example  of  this  may  be  mentioned  the 
vast  fields  of  lava  which  occur  in  Idaho,  Oregon,  and  Washington, 


CAUSES   OF  VOLCANIC  ACTIVITY  85 

covering  more  than  100,000  square  miles  to  the  depth  of  several 
hundred  feet.  On  an  even  larger  scale  is  the  lava  plateau  of  the 
Deccan  in  India,  while  similar  but  smaller  lava  fields  occur  in 
Patagonia,  Iceland,  Scotland,  and  other  regions. 

Eruptions  of  this  type  are  rare  in  modern  times  and  are  best 
displayed  in  Iceland,  where  lava  wells  out  through  great  fissures, 
some  of  which  are  20  miles  in  length,  and,  in  some  cases,  repeatedly 


FIG.  30.  —  Vesuvius  and  Monte  Somma 

through  the  same  fissure.  Small  craters  which  eject  scoriae  are 
ranged  along  the  fissures.  At  Schemakha,  near  the  west  coast  of 
the  Caspian  Sea,  the  earthquake  of  1902  was  accompanied  by  the 
formation  of  a  fissure,  through  which  lava  was  extruded,  very 
unexpectedly  because  igneous  rock  had  been  previously  unknown 
in  that  area. 

The  Causes  of  Volcanic  Activity.  — Many  theories  have  been 
advanced  to  explain  the  causes  of  vnlcanism,  but,  it  must  be  can- 


86 


VOLCANOES 


didly  admitted,  none  of  them  is  satisfactory.  In  an  elementary 
book,  like  this,  no  adequate  discussion  of  this  most  difficult  prob- 
lem can  be  given,  but  merely  a  brief  sketch  of  some  of  the  ways 
in  which  its  solution  has  been  attempted.  This  problem  is  in- 
timately connected  with  those  concerning  the  origin  of  the  Solar 
System  and  the  planetary  evolution  of  the  earth,  which  are  astro- 
nomical rather  than  geological  in  their  nature. 


FIG.  31.  — Mt.  Wrangel,  Alaska.     (U.  S.  G.  S.) 

The  principal  questions  for  which  an  answer  must  be  found  in 
any  complete  and  adequate  theory  of  vulcanism  are  as  follows: 
(i)  What  is  the  depth  of  the  reservoir  whence  the  volcanic  ma- 
terials are  derived?  and,  consequently,  what  are  the  relations  of 
the  different  vents,  near  and  remote,  to  one  another?  (2)  What 
causes  the  high  temperature  of  volcanic  materials?  (3)  What  is 
the  origin  of  the  steam  and  other  vapours  and  gases?  (4)  What 
produces  the  ascensive  force  of  the  lava?  (5)  Why  should  vol- 
canic action  be  so  generally  intermittent?  (6)  The  past  and 


CAUSES  OF  VOLCANIC  ACTIVITY  87 

present  distribution  of  volcanoes  should  be  explained,  as  also  the 
shifting  of  activity,  which  dies  out  in  one  region  and  appears  in 
another. 

(i)  No  certain  answer  can  yet  be  given  to  any  of  these  ques- 
tions, chiefly  because  we  can  observe  only  what  goes  on  at  the 
surface  of  the  earth  and  still  remain  ignorant  concerning  the  physi- 
cal condition  of  the  interior.  Hence,  nothing  is  known  as  to  the 


FIG.  32.  — Truncated  tuff  cone,  island  of  Oahu.     (Photograph  by  Libbey) 

depths  from  which  the  volcanic  materials  rise.  According  to 
one  view  which  is  quite  widely  held,  the  reservoirs  of  lava  are  local 
and  comparatively  superficial,  which  would  explain  the  fact  that 
vents  which  are  quite  near  together  may  be  entirely  independent  of 
each  other  and  eject  very  different  materials.  This  view  is  further 
confirmed  by  the  speedy  exhaustion  of  many  volcanic  vents,  a 
large  number  of  which  have  had  but  a  single  eruption.  Accord- 
ing to.  this  hypothesis,  the  length  of  time  during  which  a  volcanic 


88  VOLCANOES 

region  remains  active  is  determined  by  the  size  of  the  reservoir 
which  supplies  it. 

On  the  other  hand,  it  is  maintained  by  many  students  of  vulcan- 
ism  that  the  material  is  derived  from  a  deep-seated  layer  of  ac- 
tually or  potentially  fused  material,  which  everywhere  underlies 
the  surface  of  the  earth.  In  support  of  this  opinion,  it  is  pointed 
out  that  often  widely  separated  volcanoes  are  evidently  connected 
in  some  manner,  and  that  the  volcanic  products  of  all  regions 
are  closely  similar.  These  two  hypotheses  are  not  altogether 
contradictory,  for  it  is  quite  possible  that  some  volcanoes  may  be 
supplied  from  shallow  reservoirs  which  are  soon  exhausted,  and 
others  from  a  deeper  and  general  source  of  supply. 

(2)  The  high  temperature  has  been  explained  in  two  principal 
ways.  If  we  accept  the  nebular  hypothesis  of  the  origin  of  the 
Solar  System,  we  must  grant  that  the  earth  was  once  a  globe  of 
glowing  gas,  which  subsequently  condensed,  in  part  at  least,  to  a 
molten  globe,  and  then  solidified  on  the  surface  and  to  unknown 
depths.  One  explanation  of  volcanic  heat  is  that  it  is  due  to  the 
originally  high  temperature  of  the  earth,  not  yet  lost  by  radiation, 
whether  in  local  reservoirs  or  in  a  universal,  deep-seated  layer. 
By  those  who  accept  the  latter  view,  it  is  generally  assumed  that 
the  interior  of  the  earth  is  exceedingly  hot,  but  solidified  by 
pressure,  and  that  when,  by  folding  or  fracturing  of  the  overlying 
rocks,  this  pressure  is  partially  relieved,  the  highly  heated  masses 
become  liquefied  along  the  line  of  diminished  pressure. 

In  the  second  class  of  hypotheses  on  this  subject  of  temperature, 
it  is  assumed  that  the  earth  never  was  in  a  molten  condition,  or 
that  it  has  already  so  far  cooled  that  its  proper  heat  is  no  longer 
sufficient  to  produce  fusion  of  rock.  From  this  point  of  view,  the 
great  heat  is  believed  to  be  generated  mechanically,  by  the  friction 
of  internal  masses  under  compression  and  contraction,  or,  with 
much  less  probability,  to  be  due  to  chemical  processes,  or  even  to 
radio-activity: 

Similar  divergences  of  opinion  obtain  with  regard  to  the  nature 
and  origin  of  the  lavas  ejected  by  volcanoes.  The  view  most 


CAUSES   OF  VOLCANIC  ACTIVITY  89 

commonly  held  is  that  they  are,  for  the  most  part,  the  original, 
unaltered  material  of  the  globe,  whether  this  has  always  remained 
fluid,  or  has  been  remelted  by  release  of  pressure,  or  otherwise. 
According  to  another  opinion,  volcanic  products  are  formed  from 
the  fusion  of  sedimentary  material  which  was  laid  down  under 
water,  but  has  been  deeply  buried  within  the  crust  of  the  earth  by 
subsidence.  A  third  view  recognizes  both  sources  of  supply. 
/  (3)  The  problem  as  to  the  origin  of  the  steam  which  plays  so 
important  a  part  in  volcanic  eruptions  is  likewise  very  differently 
solved  by  different  investigators.  One  opinion  is  that  the  steam, 
like  the  lava  itself,  is  primordial  and  was  absorbed  from  the  atmos- 
phere (which  then  contained  all  the  waters  of  the  sea)  when  the 
surface  of  the  globe  was  still  molten.  Melted  substances  will,  it 
is  known,  absorb  many  times  their  own  volume  of  steam  and 
gases,  when  in  contact  with  them  under  pressure.  From  this  it  is 
inferred  that  the  lava  has  contained  the  steam  ever  since  the  first 
cooling  of  the  surface  crust.  A  second  opinion  derives  the  water 
from  the  surface  of  the  earth,  supposing  that  it  descends  partly 
through  fissures  and  partly  through  the  pores  of  the  overlying 
rocks  by  capillarity.  The  nearness  of  most  volcanoes  to  the  sea  is 
looked  upon  as  favouring  this  view.  Others,  again,  employ  both 
methods  of  explanation,  regarding  the  ordinary  steam  which  im- 
pregnates all  lavas  as  primordial,  but  believing  that  the  violently 
explosive  eruptions  are  caused  by  the  sudden  access  of  large 
bodies  of  water  to  the  lava  masses.  The  evidence  of  known  facts 
is  at  present  distinctly  in  favour  of  the  view  that  the  steam  is  essen- 
tially primordial. 

•J  (4)  The  causes  of  the  ascensive  force  of  the  lava  column  are 
sought  by  various  writers  in  several  different  agencies.  Some  find 
an  all-sufficient  cause  in  the  steam  pressure,  while  others  maintain 
that  some  other  force  must  be  at  work  and  find  this  partly  in  the 
weight  of  overlying  masses,  especially  in  the  case  of  sinking  blocks, 
and  partly  in  the  unequal  contraction  of  the  earth,  and  consequent 
pressure  upon  the  molten  or  plastic  layer  beneath.  It  has  been 
calculated  that  a  radial  contraction  of  one  millimetre  "  would 


90  INTERNAL  CONSTITUTION  OF  THE  EARTH 

suffice  to  supply  matter  for  five  hundred  of  the  greatest  known 
volcanic  eruptions."  (Prestwich.)  That  steam  is  an  essential 
factor  in  this,  as  in  other  volcanic  phenomena,  appears  to  be  well 
established.  Steam  and  other  gases  and  vapours,  under  great  pres- 
sures and  at  high  temperatures,  have  a  remarkable  penetrating 
power  and,  when  suddenly  released,  will  perforate  metal  or  rock  like 
a  projectile.  Even  under  a  pressure  of  only  a  few  hundred  at- 
mospheres, superheated  steam  corrodes  and  abrades  like  the 
sand-blast,  as  a  substitute  for  which  it  is  now  frequently  em- 
ployed. 

(5) .  The  intermittency  of  volcanoes  and  their  mode  of  distribu- 
tion add  to  the  difficulty  of  the  whole  subject,  but  any  complete 
theory  must  explain  them.  The  views  which  bring  volcanic  action 
into  relation  with  the  mechanical  changes  in  the  crust  are  those 
which  seem  most  consonant  with  the  known  facts  of  the  past  and 
present  distribution  of  the  vents. 

Here,  for  lack  of  space,  we  must  leave  the  subject.  Enough 
has  been  said  to  show  how  far  we  still  are  from  understanding  the 
mystery 'of  volcanoes. 

THE  INTERNAL  CONSTITUTION  OF  THE  EARTH 

The  interior  of  the  earth  is  completely  beyond  the  reach  of 
direct  observation  and  what  is  known,  or  may  be  reasonably  in- 
ferred, as  to  its  physical  constitution,  is  derived  from  various 
lines  of  indirect  evidence.  The  deepest  boring  ever  made  is  less 
than  -guVo"  part  of  the  earth's  radius,  and  we  have  no  experience  with 
such  enormous  pressures  as  obtain  within  the  mass  of  the  globe, 
and  can  therefore  form  but  imperfect  conceptions  of  their  effects. 

From  observations  with  the  pendulum  and  plumb-line  it  is 
calculated  that  the  specific  gravity  of  the  earth  as  a  whole  is  5.6, 
while  the  average  specific  gravity  of  the  rocks  which  form  the  ac- 
cessible parts  of  the  crust  is  only  2.6.  It  follows  that  the  in- 
terior of  the  globe  is  composed  of  much  denser  materials  than  the 
superficial  portion,  and  this  fact,  together  with  the  phenomena  of 


TEMPERATURE  OF  THE  EARTH'S   INTERIOR  9 1 

terrestrial  magnetism,  has  led  many  to  the  belief  that  the  earth 
is  substantially  a  globe  of  iron.  This  inference  is  also  supported 
by  the  occurrence  of  native,  or  unoxidized,  iron  in  certain  igneous 
rocks. 

Temperature  of  the  Earth's  Interior.  —  Volcanoes,  which  eject 
molten  and  white-hot  lavas,  and  thermal  springs,  which  pour  out 
floods  of  hot  and  even  boiling  water,  plainly  indicate  that  the  in- 
terior of  the  earth  is  highly  heated,  at  least  along  certain  lines. 
Direct  observations  tend  to  prove  that  this  high  temperature  is 
universally  diffused  through  the  earth's  mass.  For  a  short  dis- 
tance below  the  surface  of  the  ground  the  temperature  varies,  like 
that  of  the  air,  though  not  so  greatly,  between  day  and  night. 
Farther  down,  the  daily  variation  ceases,  but  there  is  a  seasonal 
variation,  also  with  a  less  extreme  range  than  the  seasonal  differ- 
ences of  the  air- temperatures.  The  almost  constant  temperature 
of  deep  cellars  and  ice-houses  is  a  familiar  fact.  At  a  still  greater 
depth  is  reached  a  level  where  the  temperature  remains  the  same 
throughout  the  year,  and  is  but  slightly  in  excess  of  the  average 
annual  temperature  of  the  air  above  ground  at  a  given  locality. 
Evidently  the  temperature  at  the  level  of  no  variation  is  deter- 
mined by  the  solar  heat  and  other  climatic  factors,  and  its  depth 
depends  upon  the  range  of  temperature  changes  in  the  air.  In  the 
tropics,  with  their  uniform  degree  of  heat,  the  level  of  no  variation 
is  only  three  or  four  feet  below  the  surface,  and  much  the  same  is 
true  of  the  polar  regions,  where  the  ground  is  permanently  frozen 
to  a  depth  of  several  hundred  feet,  but  in  the  temperate  zones  this 
level  is  much  deeper;  generally  speaking,  the  depth  increases  with 
the  latitude  and  at  New  York  is  about  fifty  feet,  but  the  level  again 
rises  toward  the  surface  as  the  polar  regions  are  approached. 

Even  at  the  level  of  no  variation  the  inherent  heat  of  the  earth 
makes  itself  apparent,  and  below  this  level  the  temperature  in- 
creases with  the  depth,  though  at  very  different  rates  in  different 
places.  Thus,  in  Great  Britain  the  rate  of  increment  varies  be- 
tween i°  F.  for  every  30  feet  of  descent  and  i°  for  every  90  feet. 
Increasing  heat  with  increasing  depth  is  observed  in  all  deep 


92  INTERNAL  CONSTITUTION  OF  THE   EARTH 

borings,  tunnels  and  mines,  and  often  has  completely  checked 
any  further  penetration.  The  levels  at  which  the  great  tunnels 
under  the  Alps  were  placed  were  determined  chiefly  by  considera- 
tions of  temperature,  as  it  was  necessary  to  avoid  a  degree  of  heat 
in  which  men  could  not  work. 

The  deepest  borings  in  the  world  are  those  in  Prussia,  some  of 
them  considerably  exceeding  a  mile  in  depth.  Observations  made 
in  these  borings  give  an  average  increment  of  about  i°  F.  for  every 
60  feet.  Should  this  rate  be  continued  regularly,  it  would  reach,  at 
a  depth  of  35  miles,  a  heat  sufficient  to  melt  almost  any  known 
rock,  at  atmospheric  pressure.  However,  the  available  observa- 
tions are  much  too  superficial  to  permit  the  formulation  of  any 
general  law,  farther  than  the  establishment  of  the  fact  of  the  uni- 
versal increment  of  temperature  with  descent  into  the  earth. 

Physical  State  of  the  Earth's  Interior.  —  Opinions  concerning  the 
internal  constitution  of  the  earth  differ  very  radically  and  only 
within  the  last  few  years  has  evidence  begun  to  accumulate  which 
permits  the  drawing  of  certain  inferences  with  a  considerable 
degree  of  probability. 

Many  hypotheses  as  to  the  condition  of  the  earth's  interior  have 
been  proposed,  of  which  the  following  are  the  most  important: 
(i)  That  the  earth  is  a  molten  globe,  covered  only  by  a  relatively 
thin  crust.  (2)  That  it  is  substantially  a  solid  body.  (3)  That 
the  interior  passes  gradually  from  a  solid  crust  to  a  gaseous  core, 
heated  beyond  the  critical  temperature  and  yet  under  ruch  enor- 
mous pressure  that  the  core  is  as  rigid  as  a  solid  body,  but  still  a 
gas  in  molecular  condition.  According  to  this  theory,  the  tem- 
perature of  the  earth  at  the  centre  is  about  180,000°  F.  and  the 
pressure  3,000,000  atmospheres.  (4)  That  it  has  a  very  large 
solid  nucleus  surrounded  by  a  layer  of  fused  material,  upon  which 
the  crust  floats  in  equilibrium. 

In  the  present  imperfect  state  of  knowledge,  it  is  not  possible  to 
decide  definitely  between  these  conflicting  hypotheses,  but,  as 
mentioned  above,  evidence  has  been  obtained  which  seems  to 
point  clearly  to  certain  conclusions. 


PHYSICAL  STATE  93 

(1)  The  first,  or  "  thin  crust  "  hypothesis,  is  now  almost  en- 
tirely abandoned,  for  there  is  really  no  evidence  in  its  favour  and 
very  much  against  it.    The  velocity  and  character  of  the  earth- 
quake waves  which  traverse  the  mass  of  the  globe  and   the  as- 
tronomical relations  of  the  earth  as  a  planet,  especially  the  tidal 
phenomena,  are  strongly  opposed  to  this  view. 

(2)  That  the  earth  is  substantially  a  solid  body,  is  the  opinion 
held  at  present  by  many  geologists  and  astronomers.     In  support 
of  it  may  be  cited  the  astronomical  evidence  just  mentioned,  and 
the  earthquake  waves,  the  speed  of  which  requires  a  medium 
more  rigid  than  steel,  while  the  very  transmission  of  the  trans- 
verse  or    distortional    waves    would    seem    to    require    a    solid 
medium. 

(3)  Between  the  second  and  third  hypotheses  the  distinction  is 
one  not  easy  to  explain  in  an  elementary  manner,  and  there  are 
many  modifications  of  the  latter.     According  to  Arrhenius,  "  the 
rigidity  of  the  earth  is  greater  rather  than  less  than  that  of  steel, 
but  the  interior  forms  an  extremely  viscous   mass,  with  qualities 
somewhat  like  those  of  asphalt  at  a  low  temperature,  of  pitch, 
sealing-wax  and  glass."    These  bodies  behave  under  forces  of  de- 
formation, which  act  quickly  or  with  constantly  changing  direc- 
tion, like  solids ;  but  under  slow,  long-continued  pressures,  acting 
in  a  constant  direction,  they  behave  like  fluids.     Observations  and 
records  of  very  distant  earthquakes  show  that  when  the  path  of 
the  mass-waves  penetrates  to  a  depth  of  more  than  three-fifths  of 
the  earth's  radius,  the  transverse  waves  of  distortion  are  either 
extinguished  or  greatly  retarded.    This  points  to  a  change  in  the 
character  of  the  medium  and  decidedly  supports  the  notion  of  a 
gaseous  core  postulated  by  this  hypothesis. 

(4)  The  fourth  hypothesis,  which  assumes  the  presence  of  a  fused 
layer  between  the  crust  and  the  solid  nucleus,  with  gradual  tran- 
sitions from  one  to  the  other,  is  believed  to  avoid  the  astronomical 
objections  to  a  molten  globe,  as  well  as  certain  geological  difficul- 
ties in  accepting  the  hypothesis  of  an  entirely  solid  earth.    The 
earthquake  observations,  so  frequently  cited,  are  decidedly  op- 


94  INTERNAL  CONSTITUTION  OF  THE  EARTH 

posed  to  the  belief  that  a  layer  of  actually  fused  matter  can  exist 
at  a  moderate  depth  below  the  surface. 

It  is  thus  probable  that  below  the  superficial  crust,  only  a  few 
miles  in  thickness,  the  great  mass  of  the  earth  is  composed  of  very 
dense  material,  which  transmits  elastic  waves  like  a  very  perfectly 
elastic  solid,  and  yet  is  so  highly  heated  and  under  such  enormous 
pressure  that  it  is  potentially  fused  and  liquefies  upon  sufficient 
release  of  pressure,  and  yields  plastically  to  slow,  long-continued 
stresses  which  act  in  a  constant  direction.  Furthermore,  there  is 
evidence  that  a  core,  two-fifths  of  the  earth's  diameter  and  com- 
posed of  matter  in  a  different  state  of  aggregation,  which  may  be 
gaseous,  occupies  the  centre. 

In  this  connection  something  should  be  said  concerning  the 
important  theory  of  isostasy,  which  may  be  thus  defined:  "The 
earth  is  composed  of  heterogeneous  material  which  varies  con- 
siderably in  density.  If  this  heterogeneous  material  were  so  ar- 
ranged that  its  density  at  any  point  depended  simply  upon  the 
depth  of  that  point  below  the  surface,  ...  a  state  of  equilibrium 
would  exist,  and  there  would  be  no  tendency  toward  a  rearrange- 
ment of  masses. 

"  If  the  heterogeneous  material  composing  the  earth  were  not 
arranged  in  this  manner  at  the  outset,  the  stresses  produced  by  grav- 
ity would  tend  to  bring  about  such  an  arrangement ;  but  as  the 
material  is  not  a  perfect  fluid,  .  .  .  the  rearrangement  will  be 
imperfect.  .  .  .  The  excess  of  material  represented  by  that  por- 
tion of  the  continent  which  is  above  sea-level  will  be  compensated 
for  by  a  defect  of  density  in  the  underlying  material.  The  conti- 
nents will  be  floated,  so  to  speak,  because  they  are  composed  of 
relatively  light  material;  and,  similarly,  the  floor  of  the  ocean  will, 
on  this  supposed  earth,  be  depressed,  because  it  is  composed  of 
unusually  dense  material.  This  particular  condition  of  approxi- 
mate equilibrium  has  been  given  the  name  isostasy."  (Tittmann 
and  Hayford.) 

The  recent  very  extensive  and  exact  operations  of  the  United 
States  Coast  Survey  have  brought  strong  confirmation  of  the 


PHYSICAL  STATE  95 

theory  of  isostasy.  "The  United  States  is  not  maintained  in 
its  position  above  sea-level  by  the  rigidity  of  the  earth,  but  is,  in 
the  main,  buoyed  up,  floated,  by  material  of  deficient  density." 
(Tittmann  and  Hayford.) 

It  should  be  noted  that  isostasy,  being  a  condition  of  approximate 
equilibrium,  is  conservative  in  tendency  and  does  not  explain  the 
active  movements  of  elevation  and  depression  of  the  crust. 

The  whole  subject  of  the  temperatures  and  physical  state  of 
the  earth's  interior  has  been  complicated  and  obscured  by  the 
discovery  of  radio-activity,  and  already  some  very  far-reaching 
inferences  have  been  drawn  from  the  distribution  of  radio-active 
substances  in  the  rocks.  At  present,  however,  it  would  be  prema- 
ture to  give  any  extended  discussion  of  this  problem. 

Summary.  — The  study  of  the  subterranean,  or  igneous,  agen- 
cies has  proved  to  be  very  unsatisfactory  in  the  way  of  explaining 
the  phenomena  and  referring  them  to  the  operation  of  understood 
physical  agents,  because  so  little  is  really  known  and  so  much 
remains  to  be  discovered.  Nevertheless,  we  have  learned  much 
that  is  of  great  importance  in  geological  reasoning.  We  have 
seen  that  the  earth  contains  within  itself  a  great  store  of  energy, 
and  that  its  interior,  in  whatever  physical  state  that  may  be,  is 
highly  heated,  and  possesses  great  quantities  of  material  which  is 
either  actually  or  potentially  molten,  and  is  permeated  with  super- 
heated steam  and  other  gases.  •  This  molten  material  is  often 
forced  upward,  and  is  either  poured  out  at  the  surface,  or  fills  up 
fissures  and  cavities  in  the  rocks,  or  pushes  its  way  between  them. 
Cooling  under  various  circumstances,  the  molten  masses  con- 
solidate into  a  great  variety  of  characteristic  rocks,  frothy,  glassy,  or 
crystalline.  Explosive  discharges  of  steam  blow  the  melted  rock 
into  fragments  of  all  grades  of  fineness,  and  these  fragments  like- 
wise accumulate  either  on  the  land  or  under  water,  and  form 
rocks,  the  nature  and  origin  of  which  may  be  readily  recognized. 

We  have  further  seen  that  the  operation  of  these  subterranean 
forces  produces  shocks  and  jars  in  the  interior,  which  are  propa- 
gated to  the  surface  as  earthquakes,  and  there  bring  about  per- 


96  SUMMARY   OF  SUBTERRANEAN  AGENCIES 

manent  changes,  associated  with  the  fissuring  and  dislocation  of 
the  rocks,  landslips,  alteration  in  the  course  of  rivers,  formation 
of  lakes,  and  the  like.  The*  frequency  of  earthquakes,  their  wide 
geographical  range,  and  the  constant  tremor  of  the  ground  de- 
tected by  delicate  instruments,  led  us  to  infer  that  the  crust  of  the 
earth  is  decidedly  unstable. 

This  conclusion  we  found  strengthened  by  the  oscillations  of 
level  between  land  and  sea,  which,  though  extremely  slow,  are 
seen  to  be  still  in  progress.  Historical  geology  will  show  us  that 
these  changes  of  level  have,  in  the  course  of  ages,  been  effected 
on  the  grandest  scale.  Almost  all  the  great  continents  are  com- 
posed of  rocks  which,  for  the  most  part,  were  laid  down  in  the  sea 
and  still  contain  the  fossils  of  marine  animals,  and  this  shows  that 
these  continents  have  been  under  the  sea.  Not  that  all  parts 
of  any  continent  were  submerged  at  the  same  time,  but  now  one 
part  and  now  another  was  overflowed  and  again  emerged,  until 
nearly  all  have  been  covered  in  their  turn. 

In  brief,  the  principal  geological  functions  of  the  subterranean 
agencies  are  two:  (i)  they  bring  up  from  below  and  form  at  the 
surface,  and  at  all  depths  beneath  it,  certain  characteristic  kinds  of 
rocks;  and  (2)  they  tend  to  increase  the  inequalities  of  the  earth's 
surface,  and  thus  to  counteract  the  agencies  which  are  cutting  down 
the  land  and  steadily  tending  to  reduce  it  to  the  level  of  the  sea.  . 

• 


SECTION    II 

SURFACE  AGENCIES 

THE  superficial,  or  surface,  agents,  all  of  which  are  manifestations 
of  solar  energy,  are  those  which  act  upon  or  near  the  surface  of  the 
ground;  only  one,  circulating  water,  is  able  to  penetrate  to  con- 
siderable depths  within  the  earth.  The  work  of  the  surface  agents 
may  all  be  summed  up  in  two  categories,  the  destruction  and 
reconstruction  of  rock.  These  two  processes  are  complementary; 
for,  since  matter  is  indestructible,  and  can  have  only  its  position 
and  physical  and  chemical  relations  changed,  it  is  obvious  that 
what  is  removed  in  one  place  must  be  laid  down  in  another. 
Neither  process,  therefore,  can  go  on  without  the  other,  and  recon- 
struction necessarily  implies  antecedent  destruction  to  furnish  the 
materials.  Ceaseless  cycles  of  change  are  everywhere  in  progress, 
new  combinations  continually  formed,  and  older  rocks  worked  over 
into  newer.  It  is  this  circulation  of  matter  upon  and  within  the 
crust  of  the  earth,  which  we  have  already  compared  to  the  physio- 
logical changes  in  the  body  of  a  living  organism. 

The  work  of  destruction  and  reconstruction  is  in  a  continuous 
series  of  changes,  beginning  with  the  mechanical  disintegration  or 
chemical  decomposition  of  an  older  rock,  followed  by  the  trans- 
portation of  the  material  thus  supplied,  for  longer  or  shorter  dis- 
tances, its  deposition  in  a  new  place,  and  finally,  if  the  series  is  com- 
plete, the  consolidation  of  the  loose  debris  into  rock. 

The  processes  of  rock  destruction  and  removal,  which  are 
grouped  together  under  the  general  name  of  denudation,  or  erosion, 
are  chiefly  confined  to  the  land  surfaces,  while  those  of  recon- 
struction take  place  principally  under  bodies  of  water  and,  most 

H  Q7 


98  SURFACE  AGENCIES 

of  all,  in  the  sea.  An  important  work  of  reconstruction  is  also 
performed  on  the  land,  but  on  a  less  extensive  scale  than  in 
the  sea. 

The  surface  agents  all  act  both  destructively  and  reconstruc- 
tively  according  to  circumstances,  but  with  very  different  degrees 
of  efficiency.  Certain  agents  are  preeminently  destructive,  others 
as  preeminently  reconstructive,  while  others  again  operate  most 
efficiently  as  agents  of  transportation.  Again,  as  was  pointed  out 
in  the  Introduction,  the  depth  below  the  surface  at  which  operations 
take  place  has  a  very  important  bearing  upon  the  effect  produced. 
From  this  point  of  view,  we  may  regard  the  earth's  crust  as  being 
composed  of  a  number  of  concentric  shells,  of  somewhat  irregular 
thickness  and  rather  indefinite,  or  even  fluctuating,  boundaries. 
The  superficial  shell,  which  extends  from  the  surface  down  to  the 
level  of  the  ground  water  (see  p.  124),  is  called  the  shell  of 
weathering  (Van  Rise's  "  belt  of  weathering  "),  and  is  character- 
ized by  the  oxidation,  carbonation,  and  hydration  of  minerals,  and 
great  quantities  of  material  are  removed  in  solution.  As  a  result 
of  these  operations  the  rocks  are  decomposed,  becoming  soft  and 
friable;  the  minerals  produced  are  few  in  number,  of  simple  com- 
position, and  are  usually  imperfectly  crystallized.  The  second 
shell,  that  of  cementation,  which  extends  downward  from  the  ground- 
water  level  to  a  varying  depth  with  undetermined  lower  limit, 
is  largely  saturated  with  water,  and  hence  has  but  a  limited  supply 
of  oxygen  and  carbon  dioxide.  Therefore,  while  oxidation  and 
carbonation  occur,  they  are  less  important  than  hydration,  and  the 
resultant  minerals  are  more  crystalline  than  in  the  shell  of  weather- 
ing. Solution  goes  on,  but  deposition  becomes  very  important 
and  fills  the  openings  in  the  rocks  with  mineral  matter,  and  the 
general  effect  of  the  various  processes  is  to  harden  the  rocks. 

The  work  of  the  surface  agents,  in  its  threefold  aspect'  of  erosion, 
transportation,  and  deposition,  is  profoundly  affected  by  the  dia- 
strophic  movements  of  the  earth's  crust.  In  a  given  case  the 
effects  produced  will  vary  greatly  in  accordance  with  the  elevation, 
subsidence,  or  stationary  character  of  the  region.  In  general,  ele- 


SURFACE  AGENCIES  99 

vation  favours  denudation,  and  subsidence  favours  deposition  of 
transported  material. 

In  studying  the  work  of  the  surface  agents,  the  logical  order  of 
treatment  requires  that  the  destructive  operations  be  considered 
first.  The  agencies  to  be  examined  are:  (i)  the  atmosphere, 
(2)  running  water,  (3)  ice,  (4)  lakes,  (5)  the  sea,  (6)  animals  and 
plants.  Of  these  various  agents  the  work  is  principally  mechanical, 
but  water,  in  its  various  forms,  is  a  slow  but  extremely  efficient 
agent  of  chemical  changes. 


CHAPTER  IV 
DESTRUCTIVE  PROCESSES.  —  THE   ATMOSPHERE 

THE  atmospheric  agencies  are  by  far  the  most  important  of  the 
destructive  or  denuding  agents,  because  no  part  of  the  land  surface 
is  altogether  exempt  from  their  activity.  Their  work  is  described 
by  the  general  term  weathering,  and  is  shown  at  once  by  the  dif- 
ferent appearance  of  freshly  quarried  stone  from  that  which  has 
been  long  exposed  in  the  face  of  a  cliff,  or  even  in  ancient  build- 
ings. While  such  agents  as  rivers  and  the  sea  do  work  that  is 
much  more  apparent  and  striking  than  that  of  the  atmosphere, 
yet  they  are  more  locally  confined,  and  even  in  their  opera- 
tions the  atmosphere  renders  important  aid.  Though  no  part 
of  the  land  surface  is  entirely  free  from  the  destructive  activity  of 
the  atmosphere,  the  rapidity  and  intensity  of  this  activity  vary 
much  in  different  places.  There  are,  in  the  first  place,  the  great 
differences  of  climate  to  be  considered,  differences  in  the  amount 
and  distribution  of  the  rainfall,  of  temperature,  and  of  the  winds. 
In  the  second  place,  the  resistance  offered  by  the  various  kinds 
of  rocks  to  the  disintegrating  processes  differs  very  greatly,  in 
accordance  with  the  differences  of  hardness  and  chemical  compo- 
sition. Again,  the  presence  or  absence  of  a  covering  of  protective 
vegetation  has  an  important  influence  upon  the  amount  and  char- 
acter of  the  destruction  effected. 

The  outcome  of  all  these  varying  factors  is  to  produce  very 
irregular  land  surfaces.  While  the  tendency  of  the  atmospheric 
agencies  is  gradually  to  wear  down  the  land  to  the  level  of  the 
sea,  yet  in  that  process  some  parts  are  cut  away  much  more  rapidly 
than  others;  and  hence  the  first  effect  of  denudation  is  an  increas- 
ingly irregular  surface.  The  overlying  screen  of  soil  conceals  many 

100 


RAIN  Ibl 

of  these  irregularities,  especially  the  minor  ones;  and  were  that 
screen  removed,  the  rock  surface  would  be  seen  to  be  much  more 
irregular  and  rugged  than  the  actual  surface  of  the  ground. 

So  long  as  the  land  surface  is  varied  by  hill  and  valley,  it  is  said 
to  possess  relief;  and  when  it  has  all  been  planed  down  to  a  flat 
or  gently  sloping  surface,  raised  but  slightly  above  the  level  of  the 
sea,  it  is  said  to  have  reached  the  base-level  of  erosion,  or  to  be 
base-levelled. 

The  atmospheric  agents  may  be  conveniently  divided  into 
(i)  rain,  (2)  frost,  (3)  changes  of  temperat  Tre,  (4)  wind. 

i.  RAIN 

The  work  of  the  rain,  which  is  both  chemical  and  mechanical, 
varies  greatly  in  accordance  with  climatic  factors.  The  annual 
precipitation  of  two  regions  may  be  the  same  in  amount,  but  in  one 
the  rainfall  may  be  in  very  frequent,  gentle  showers,  and  in  the 
other  in  less  frequent,  but  far  heavier  and  more  violent  downpours; 
under  such  different  conditions  the  destructive  work  of  the  rain 
will  be  very  different.  Effects  of  still  another  kind  are  produced  in 
those  regions  which  have  regularly  alternating  rainy  and  dry 
seasons.  Temperature  also  modifies  the  work  of  the  rain  to  an 
important  degree,  so  that  results  are  brought  about  in  warm  coun- 
tries quite  different  from  those  observed  in  temperate  and  cold 
regions.  Thus,  each  climatic  zone  exhibits  the  work  of  the  rain 
with  characteristic  differences. 

Perfectly  pure  water  would  act  upon  rocks  with  extreme  slow- 
ness, but  such  water  is  not  known  to  occur  in  nature.  The  rain- 
drops, formed  by  the  condensation  of  the  watery  vapour  of  the 
atmosphere,  absorb  certain  gases  which  very  materially  increase 
the  solvent  power  of  the  water.  Of  these  gases  the  most  important 
are  oxygen  (O)  and  carbon  dioxide  (CO2),  and  all  rain-water 
contains  them. 

It  was  formerly  supposed  that  rain-water  in  percolating  through 
the  soil  acquired  additional  destructive  efficiency  by  absorbing 


102  THE  ATMOSPHERE 

certain  products  of  vegetable  decomposition  called  humous  acids. 
Recent  exact  investigations,  however,  have  thrown  grave  doubts 
upon  the  existence  of  these  acids  in  the  soil,  and  the  effects 
which  had  been  ascribed  to  them  are  now  referred  to  carbon 
dioxide  acting  out  of  immediate  contact  with  the  atmosphere. 

One  of  the  first  and  simplest  effects  of  atmospheric  moisture 
consists  in  the  hydration  of  the  minerals  exposed  to  it.  Hydra- 
tion,  the  taking  up  of  water  into  chemical  union,  is  an  important 
agency  of  decay;  it  causes  an  increase  of  volume  and  thus  greatly 
increases  the  pressure  in  the  lower  parts  of  rock  masses  which 
contain  hydrating  minerals.  In  the  District  of  Columbia  "  granite 
rocks  have  been  shown  to  have  become  disintegrated  for  a  depth  of 
many  feet,  with  loss  of  but  some  13.46  per  cent  of  their  chemical 
constituents.  .  .  .  Natural  joint  blocks  brought  up  from  shafts 
were,  on  casual  inspection,  sound  and  fresh.  It  was  noted,  how- 
ever, that  on  exposure  to  the  atmosphere,  such  not  infrequently  fell 
away  to  the  condition  of  sand."  (Merrill.) 

Oxidation  affects  especially  the  iron  minerals  and  thus  brings 
about  conspicuous  colour  changes,  for  jron  compounds  form  the 
principal  colouring  matter  of  the  rocks  and  soils.  Ferrous  com- 
pounds give  little  colour,  but  the  rocks  in  which  they  occur  are 
apt  to  have  a  blue  or.grey  tint,  due  to  other  substances,  both  organic 
and  inorganic.  When  such  rocks  are  exposed  to  the  action  of  air 
and  water,  the  ferrous  compounds  are  oxidized,  producing  ferric 
oxide  and  ferric  hydrates,  the  former  giving  a  red  colour  and  the 
latter  various  shades  of  yellow  and  brown. 

When  fired  in  a  kiln,  a  blue  clay  will  yield  red  bricks,  by  the 
conversion  of  FeCO3  into  Fe2O3.  In  nature,  rain-water  effects  a 
similar  change,  and  the  contrast  between  the  superficial  and  deep- 
seated  parts  of  the  same  rock  is  often  as  great  as  between  blue  clay 
and  red  brick.  Weathered  blocks  stained  red  on  the  outside  are 
often  blue,  grey,  or  nearly  black  on  the  inside,  because  the  change 
has  not  affected  the  whole  mass. 

An  especially  important  and  wide-spread  change  is  carbonation, 
due  to  the  carbon  dioxide  which  all  natural  waters  contain  in 


RAIN  103 

greater  or  less  quantity.  The  silicates  are  attacked  and  decom- 
posed in  a  manner  that  will  be  explained  below,  and,  under 
certain  circumstances,  the  insoluble  ferric  hydrates  are  converted 
into  soluble  ferrous  carbonate. 

Finally,  solution  plays  a  highly  important  role  in  the  destructive 
work  of  the  rain.  All  rocks  contaiif  some  soluble  material,  and 
when  this  soluble  material  is  removed,  the  rock  crumbles  into  a 
friable  mass,  which,  on  complete  disintegration,  forms  soil.  This 
may  be  illustrated  by  a  block  of  frozen  earth,  which  is  as  hard 
as  many  rocks,  being  cemented  by  the  ice  crystals,  which  bind 
the  particles  of  soil  together.  When  the  ice  is  melted,  the  mass 
immediately  becomes  incoherent.  So,  in  the  rocks,  the  re- 
moval of  even  a  small  quantity  of  soluble  material  often  causes 
the  whole  to  crumble.  Except  vegetable  moulds,  all  soils  are 
derived  from  the  decay  and  disintegration  of  rocks. 

The  chemical  composition  of  the  rock-forming  minerals  varies 
so  much,  that  the  processes  which  destroy  them  must  vary  corre- 
spondingly, differing  in  the  case  of  the  igneous  rocks,  on  the  one 
hand,  from  that  of  the  sedimentary  rocks,  on  the  other. 

Most  igneous  rocks  are  made  up  of  crystals  of  some  kind  of 
felspar  (see  p.  13),  associated  with  such  minerals  as  augite  (p.  16), 
hornblende  (p.  16),  and  quartz  (p.  12).  In  granite,  for  example, 
which  is  composed  of  orthoclase  felspar,  quartz,  and  mica  or  horn- 
blende, rain-water  slowly  attacks  the  orthoclase  by  dissolving  out 
the  potash,  probably  as  a  carbonate,  and  also  a  considerable 
proportion  of  the  combined  silica.  The  aluminous  silicate,  which 
forms  the  residue,  is  hydrated  as  kaolinite  (A12O3,  2  SiO2,  2  H2O), 
forming  clay,  while  the  quartz  is  little  or  not  at  all  affected. 
The  mica,  if  present,  is  very  slowly  attacked,  the  hornblende 
more  readily. 

The  decomposition  of  granite,  then,  results  in  the  formation  of 
clay,  through  which  are  scattered  flakes  of  mica  (if  mica  were  orig- 
inally present)  and  the  unaltered  grains  of  quartz.  In  the  other 
igneous  rocks  the  manner  of  decomposition  is  essentially  similar; 
the  complex  silicates  are  broken  up  into  simpler  compounds,  clay 


104 


THE  ATMOSPHERE 


being  derived  from  the  aluminous  silicates,  especially  the  felspars, 
while  the  quartz,  if  present,  is  broken  up  into  fragments  and  forms 
sand.  The  bases,  potash,  soda,  lime,  magnesia,  iron,  etc.,  are 
removed  in  solution,  chiefly  as  carbonates,  and  more  or  less  of  the 
silica  is  also  dissolved  and  carried  away.  Even  when  an  igneous 
rock  is  yet  firm  and  hard  and,  to  the  naked  eye,  appears  to  be 
quite  unchanged,  the  microscope  often  reveals  the  first  stages  of 
decay. 


FIG.  33.  —  Boulders  of  weathering,   Eldon  Mt.,  Arizona. 
Hackett,  Flagstaff,  Arizona) 


(Photograph  by  A.  E. 


In  most  tropical  regions,  where  there  is  a  long  dry  season,  fol- 
lowed by  a  wet  season  of  violent  rainfall,  the  manner  of  decay  is 
characteristically  different.  The  felspars  and  other  aluminous 
silicates  lose  nearly  all  of  their  silica,  so  that  the  residue  is  the 
hydrated  oxide  of  aluminium,  not  the  silicate,  and  the  iron  is  oxi- 
dized, forming  nodules  and  masses  and  staining  the  whole  a  deep 
red.  This  characteristic  warm-country  soil  is  called  laterite. 


RAIN  105 

Very  many  igneous  rocks  weather  into  heaps  and  masses  of 
rounded  boulders,  which  are  often  mistaken  for  the  deposits  made 
by  glaciers.  The  spheroidal  shape  is  due  to  the  more  rapid  decay 
of  the  edges  of  the  original  joint  blocks,  which  are  attacked  on  both 
sides  at  once.  As  the  edges  and  angles  are  removed  more  quickly 
than  the  broad  faces  of  the  blocks,  a  rounded  form  results.  Once 
acquired,  the  round  shape  is  long  retained,  because  then  decay 
penetrates  at  a  nearly  equal  rate  from  all  parts  of  the  surface. 

Rocks  which  are  themselves  composed  of  substances  derived 
from  the  decay  of  older  rocks  are  attacked  in  their  turn  and  yield 
material  for  new  formations.  These  derivative  rocks,  such  as 
sandstones,  slates,  and  limestones,  are  affected  in  characteristic 
ways  by  the  rain. 

Sandstones  are  composed  of  grains  of  sand  (quartz,  SrO2)  ce- 
mented together;  the  cementing  substance  may  be  silica  itself, 
some  compound  of  iron,  such  as  Fe2O3,  or  carbonate  of  lime 
(CaCO3),  and  the  dissolving  away  of  the  cement  causes  the  rock 
to  crumble  into  sand.  In  a  sandstone  with  siliceous  cement  the 
action  is  excessively  slow,  atmospheric  waters  having  very  little 
effect  upon  silica,  but  underground  it  is  slowly  attacked.  Ferric 
oxide  (Fe2O3)  is  likewise  unchanged  by  rain-water,  but  beneath 
the  soil  it  is  converted  into  the  soluble  carbonate  and  removed. 
The  uppermost  layers  of  red  sandstone  are  often  thus  completely 
disintegrated  into  loose  sand,  bleached  by  the  removal  of  the  iron 
which  gave  it  its  colour.  Carbonate  of  lime  is  very  soluble  in  water 
containing  carbon  dioxide,  as  all  rain-water  does,  and  in  sandstones 
with  calcareous  cement,  disintegration  is  rapid.  In  sandstones 
and  slates  it  is  the  cementing  substance  which  is  removed,  leaving 
the  grains  of  sand  or  particles  of  clay  unchanged.  This  is  because 
the  materials  of  these  rocks  were,  for  the  most  part,  originally 
derived  from  the  decomposition  of  the  igneous  rocks,  and  the 
minerals  which  compose  them  are  already  of  a  very  simple  and 
stable  character. 

The  sandstones  are  largely  employed  for  building  materials,  and 
their  value  and  permanence  for  such  purposes  depend  principally 


io6 


THE  ATMOSPHERE 


upon  the  character  of  the  cementing  substances  in  them.     For 
this  reason,  the  siliceous  and  ferruginous  sandstones  are  the  most 


FIG.  34.  —  Soil  originating  in  place  by  the  decomposition  of  sandstone 

durable,  those  with  calcareous  cements  usually  yielding  with  com- 
parative rapidity  to  the  attacks  of  the  weather. 


RAIN  107 

Slates  and  shales,  by  removal  of  their  soluble  constituents, 
crumble  down  into  clay. 

Limestones  are  among  the  few  rocks  which  are  chiefly  or  entirely 
made  up  of  soluble  material,  the  carbonate  of  lime  (CaCO3). 
This  is  attacked  by  the  rain-water,  dissolved  and  carried  away  in 
solution,  while  the  insoluble  impurities  contained  in  the  rock 
remain  to  form  soil.  The  proportion  of  such  impurities  varies 
greatly  in  different  limestones,  and  hence  the  residual  soil  will 
vary,  but  it  is  generally  a  clay,  since  that  is  much  the  commonest 
of.  the  impurities  in  limestone.  Sand  also  occurs  in  limestones, 
either  with  or  without  clay.  When  the  sand  forms  a  coherent 
mass,  out  of  which  the  calcareous  material  has  been  dissolved,  it 
is  called  rotten  stone. 

The  gradual  formation  of  soil  by  the  disintegration  of  rock  may 
be  easily  observed  in  excavations,  even  shallow  ones,  such  as  cel- 
lars, wells,  railroad  cuttings,  and  the  like.  At  the  surface  is  the 
true  soil,  which  is  usually  dark-coloured,  due  partly  to  the  admix- 
ture of  vegetable  mould,  partly  to  the  complete  oxidation  and  hy- 
dration  of  its  minerals.  Next  follows  the  subsoil,  which,  owing  to 
the  absence  of  vegetable  matter  and  the  less  complete  oxidation 
and  hydration,  is  of  a  lighter  colour.  The  subsoil  is  frequently 
divided  into  distinct  layers,  and  often  contains  unaltered  masses 
of  the  parent  rock,  which  have  resisted  decomposition,  while  the 
surrounding  parts  have  become  entirely  disintegrated.  By  im- 
perceptible gradations  the  subsoil  shades  into  what  looks  like 
unaltered  rock,  but  is  friable  and  crumbles  in  the  fingers;  this  is 
rotten  rock.  From  this  to  the  firm,  unchanged  rock  the  passage 
is  equally  gradual. 

In  the  northern  portions  of  the  United  States  the  soil  is,  in  most 
localities,  of  only  moderate  depths,  because  at  a  late  period  (geo- 
logically speaking)  this  region  was  covered  with  a  great  ice-sheet, 
which  swept  away  much  of  the  accumulations  of  ancient  rock 
decay.  In  the  parts  of  the  country  where  the  ice-sheet  did  not 
come,  the  soil  is  much  deeper,  and  in  tropical  lands  it  attains 
remarkable  depths.  In  our  Southern  States  the  felspathic  rocks  are 


108  THE  ATMOSPHERE 

often  found  thoroughly  disintegrated  to  depths  of  50  or  100  feet 
while  in  Brazil  the  soil  is  often  200  to  300  feet  deep. 

The  mechanical  effect  of  rain  is  less  extensive,  perhaps,  than  its 
chemical  work  of  disintegration,  but  is  very  important,  neverthe- 
less. Under  ordinary  conditions,  this  mechanical  work  consists  in 
the  washing  of  soil  from  higher  to  lower  levels.  How  consider- 
able is  the  movement  of  soil  that  has  thus  been  brought  about,  may 
be  imagined  when  one  sees,  after  a  heavy  rain,  the  rills  which 
run  over  the  slopes,  muddy  and  charged  with  sediments,  and  how 
turbid  the  streams  become  with  the  soil  which  the  rain  washes  into 
them.  Bare  soil  is  rapidly  torn  up  and  washed  away  by  the  action 
of  rain,  but  a  covering  of  vegetation,  and  especially  of  the  elastic  and 
matted  stems  and  roots  of  grasses,  much  retards  the  action. 

Other  things  being  equal,  the  rapidity  with  which  the  rain 
sweeps  away  the  soil  depends  upon  the  steepness  of  the  slope 
upon  which  the  soil  is  formed;  for  gravity  largely  determines 
these  movements.  On  cliffs  and  steep  hillsides  the  soil  is 
quickly  removed,  and  in  such  places  it  is  thin  or  quite  lacking, 
while  in  the  valleys  it  often  accumulates  to  great  depths.  Even 
on  gentle  slopes  and  almost  level  stretches  the  rains  slowly  wash  it 
downward,  and  eventually  into  the  streams  which  carry  it  to  the 
sea.  The  soil  is  thus  not  stationary,  but  under  the  influence  of  the 
rains  and  streams  is  slowly  but  steadily  travelling  seaward.  Dis- 
regarding the  alluvial  deposits  made  by  rivers,  and  soils  accu- 
mulated by  the  action  of  ice  or  wind,  the  soil  of  any  district  is 
thus  a  residual  product,  and  its  quantity  represents  the  surplus  of 
chemical  disintegration  over  mechanical  removal. 

The  mechanical  action  of  rain  is  greatly  increased  by  extreme 
violence  and  volume  of  precipitation;  a  single  "cloud-burst"  will 
do  far  more  damage  than  the  same  quantity  of  rain  falling  in 
gentle  showers.  Those  who  know  only  the  temperate  regions  can 
form  but  imperfect  conceptions  of  the  violence  of  tropical  rains. 
On  the  southern  foot-hills  of  the  Himalayas,  for  example,  the  rain- 
fall is  exceedingly  great  (in  some  localities  as  much  as  500  inches 
per  annum),  and  almost  all  of  it  is  precipitated  in  six  months  of  the 


RAIN  ICQ 

year;  especially  remarkable  is  the  quantity  which  often  falls  in  a 
single  day.  "  The  channel  of  every  torrent  and  stream  is  swollen 
at  this  season,  and  much  sandstone  and  other  rocks  are  reduced 
to  sand  and  gravel  by  the  flooded  streams.  So  great  is  the  super- 
ficial waste,  that  what  would  otherwise  be  a  rich  and  luxuriantly 
wooded  region  is  converted  into  a  wild  and  barren  moorland." 
(Lyell.) 


FIG.  35.  —  Bad  lands  of  South  Dakota.     (U.  S.  G.  S.) 

The  action  of  rain  is  thus  by  no  means  uniform,  the  results  de- 
pending upon  so  many  and  such  varying  factors,  that  we  may  find 
marked  differences  in  closely  adjoining  regions,  and  even  in  one 
and  the  same  mass  of  rock.  One  of  the  most  remarkable  monu- 
ments of  rain  erosion  is  exhibited  by  the  curious  districts  in  the 
far  Western  States  known  as  the  bad  lands,  which  cover  many 
thousands  of  square  miles  in  the  Dakotas,  Nebraska,  Wyoming, 
Utah,  etc.  The  bad-land  rocks  are  mostly  rather  soft  sandstones 
and  clays,  with  prevailingly  calcareous  cements,  and  formed  in 
nearly  horizontal  beds  or  layers.  The  rainfall  is  light,  though 
torrential  showers  sometimes  occur;  but  the  absence  of  vegeta- 


no 


THE  ATMOSPHERE 


tion  is  favourable  to  its  efficiency,  and  the  present  aridity  of  the 
climate  is  not  of  very  long  standing,  from  a  geological  point  of 
view.  The  chemical  action  of  the  rain  has  disintegrated  the  rocks 
by  dissolving  out  the  calcareous  cement,  and  then  the  debris 
so  formed  has  been  mechanically  washed  away. 

At  the  present  time  the  action  of  the  rain  is  very  slow,  because 
the  debris  which  covers  the  sides  of  the  cliffs  and  slopes  is  almost 


FlG.  36.  —  Bad  lands  near  Adelia,  Nebraska.  The  rock  in  the  middle  distance  is 
sandstone  formed  in  a  stream-channel  and  the  bluffs  are  flood-plain  deposits. 
(U.  S.  G.  S.) 

impervious  to  water,  and  holes  left  after  the  excavation  of  fossil 
skeletons  often  remain  visible  for  many  years;  but  where  the 
bare  rock  is  exposed,  the  disintegration  often  proceeds  with  ex- 
traordinary rapidity,  and  a  single  shower  will  produce  notable 
effects.  The  different  layers  of  rock  resist  decay  differently,  and 
even  in  the  same  bed  some  parts  are  much  more  durable  than 


RAIN 


III 


others.  This  differential  weathering  has  resulted  in  that  remark- 
able variety  and  grotesqueness  of  form,  resembling  the  ruins  of 
gigantic  towers  and  castles,  for  which  the  bad-land  scenery  is 
famous.  The  sculpture  of  the  rain  produces  this  variety  in  ac- 
cordance with  the  arrangement  of  the  more  and  less  durable 
layers.  The  varying  arrangement  of  these  layers  produces  a 
fantastic  topography.  A  variant  of  bad-land  topography  is  given 


FIG.  37. —  Bad  lands  in  Wyoming,  with  talus  slopes.     (U.  S.  G.  S.) 

by  the  pillars  of  Monument  Park,  Colorado,  which  are  due  to 
weathering,  the  capping  of  hard  rock  protecting  the  soft  sandstone 
below.  (See  Fig.  38.) 

The  mechanical  wash  of  rain  is  greatly  retarded  when  the  ground 
is  covered  with  dense  vegetation,  which  protects  the  soil  against 
the  impact  of  raindrops  and  the  wear  of  rain  rills.  The  removal 
of  the  vegetation  is  often  speedily  followed  by  disastrous  results, 


112 


THE  ATMOSPHERE 


and  especially  the  reckless  and  wanton  destruction  of  the  forests, 
which  has  gone  on  in  this  country  ever  since  its  settlement  by 
Europeans,  has  been  followed  by  the  loss  of  valuable  soil  on  a 
vast  scale.  Speaking  of  the  soil  destruction  in  the  old  fields  of 


FIG.  38.  — Monument  Park,  Colorado.     (U.  S.  G.  S.) 

southern  Mississippi,  McGee  says  that  they  are  washed  away, 
"  leaving  mazes  of  pinnacles  divided  by  a  complex  network  of 
runnels  glaring  red  toward  the  sun  and  sky  in  strong  contrast  to  the 


FROST  113 

rich  verdure  of  the  hillsides  never  deforested.  .  .  .  Whole  villages, 
once  the  home  of  wealth  and  luxury,  are  being  swept  away  at  the 
rate  of  acres  for  each  year." 

It  is  to  be  hoped  that  the  work  of  the  United  States  Bureau  of  For- 
estry, in  endeavouring  to  check  this  terrible  destruction,  may  re- 
ceive the  support  it  so  well  deserves  from  every  intelligent  lover  of 
his  country. 

2.   FROST 

The  term  frost,  in  this  connection,  is  restricted  to  the  freezing  of 
water.  Water  is  one  of  the  comparatively  few  substances  which 
expand  considerably  on  solidifying.  This  expansion  amounts  to 
about  one-eleventh  of  the  original  bulk  of  the  water,  and,  exert- 
ing a  pressure  of  somewhat  more  than  2000  pounds  per  square 
inch,  takes  place  with  irresistible  power,  bursting  thick  iron  vessels 
like  egg-shells. 

Excepting  loose,  incoherent  masses,  like  sand  and  gravel,  no 
rocks  are  formed  of  continuous  sheets  of  material,  but  are  rather 
to  be  considered  as  masses  of  blocks,  divided  by  joints  (see 
pp.  5  and  136).  In  addition  to  these  visible  clefts,  the  blocks 
are  traversed  by  minute  crevices,  rifts,  and  pores,  all  of  which 
openings  take  up  and  retain  quantities  of  water,  as  may  readily  be 
seen  by  examining  freshly  quarried  stone.  When  exposed  to  a 
low  temperature,  the  water  freezes  and  forces  out  the  large  blocks 
and  shatters  them  into  pieces  of  smaller  and  smaller  size.  The 
fragments  thus  formed  are  called  talus,  and  great  accumulations  of 
such  blocks  are  found  at  the  foot  of  cliffs  in  all  regions  where  the 
winters  are  at  all  severe.  Talus  accumulations  are  also  formed  by 
other  agencies,  as  will  be  seen  in  the  sequel.  Alternate  freezings 
and  thawings  not  only  break  up  rocks,  but  cause  the  broken 
fragments  and  soil  to  work  their  way  down  slopes.  Each  freezing 
causes  the  fragments  to  rise  slightly  at  right  angles  to  the  inclined 
surface,  and  each  thawing  produces  a  reverse  movement;  hence 
the  slow  creep  down  the  slope. 

The  action  of  frost  is,  of  course,  practically  absent  in  the  low- 
l 


THE  ATMOSPHERE 


lands  of  the  tropics,  but  in  high  mountains  and  in  all  countries 
which  have  cold  winters,  frost  is  an  agent  of  great  importance  in 
the  mechanical  shattering  of  rocks  and  slow  destruction  of  cliffs. 
The  hardest  rocks  are  shivered  into  fragments  and,  dislodged  from 
their  places,  the  fragments  roll  down  the  mountain  side  till  they 


FlG.  39.  —  Shales  "  cree 


under  the  action  of  frost.     (U.  S.  G.  S.) 


come  to  rest,  perhaps  thousands  of  feet  below.  Immense  accumu- 
lations of  frost-made  talus  are  to  be  found  in  such  places  as  the 
foot  of  the  Palisades  of  the  Hudson,  the  abrupt  southern  slope  of 
the  Delaware  Water  Gap,  and -wherever  cliffs  or  peaks  of  naked 
rock  are  exposed  to  severe  cold.  Many  mountain  passes  are  so 
bombarded  by  falling  stones  as  to  be  extremely  dangerous;  in  the 


FROST  1 1 5 

Sierra  Nevada  of  California,  talus  slopes  as  much  as  4000  feet  high 
are  reported,  all  the  work  of  frost.  At  Sherman,  where  the  Union 
Pacific  Railroad  crosses  the  "continental  divide,"  the  ground  is 
covered  for  miles  with  small,  angular  fragments  of  granite  broken 
up  by  the  frost. 

In  the  polar  regions  frost  is  probably  the  most  important  of  the 
disintegrating  agents.  In  Spitzbergen  Beechy  found  that  in  sum- 
mer the  mountain  slopes  absorb  quantities  of  water,  which  freezes 


FIG.  40.  —  Cliff  and  talus  slope,  Delaware  Water  Gap,  NJ. 

in  winter  with  very  destructive  effect.  "Masses  of  rock  were,  in 
consequence,  repeatedly  detached  from  the  hills,  accompanied  by 
a  loud  report,  and  falling  from  a  great  height,  were  shattered  to 
fragments  at  the  base  of  the  mountain,  there  to  undergo  more 
rapid  disintegration."  Similar  phenomena  are  reported  from  the 
Aleutian  Islands  of  Alaska. 

The  action  of  frost  is,  in  itself,  purely  mechanical ;  no  chemical 
change  is  occasioned  by  it,  and  the  smallest  fragments  into  which  a 
block  may  be  riven  are  sharp  and  angular,  and  the  minerals  have 


116  THE  ATMOSPHERE 

unaltered  and  shining  faces.  But,  on  the  other  hand,  frost  pre 
pares  the  way  for  the  more  rapid  action  of  rain  and  percolating 
waters.  The  effects  of  these  agents  are  produced  upon  the  surface 
of  the  rocks  and  the  walls  of  the  crevices  which  run  through  them. 
By  breaking  up  the  blocks,  the  frost  greatly  increases  the  surface 
and  thus  facilitates  the  work  of  the  rain.  A  breaking  up  of  one 
cubic  foot  into  cubic  inches  multiplies  the  exposed  surface  by  12. 
Frost  is  an  extremely  superficial  agent  and  acts  effectively  only 
a  few  feet  below  the  surface.  In  the  polar  regions  the  ground 
is  permanently  frozen  to  a  depth  of  several  hundred  feet,  but  the 
shattering  of  rocks  requires  alternate  freezings  and  thawings. 

Rain  and  frost  are  agents  whose  effects  are  most  important  in 
regions  of  moist  climate  and  abundant  rainfall,  for  both  are  forms 
of  the  activity  of  water.  Few  regions  of  the  earth's  surface  are 
altogether  rainless,  but  nearly  all  of  the  continents  have  great 
desert  areas  in  which  atmospheric  precipitation  is  very  light.  It 
might  seem  that  in  such  deserts  the  work  of  rock  disintegration 
must  be  practically  at  a  standstill,  and  that  the  circulation  of  ma- 
terial must  be  so  slow  as  to  be  hardly  distinguishable  from  com- 
plete stagnation.  Even  in  these  regions,  however,  the  rain 
accomplishes  something,  and  it  is  aided  by  other  agencies  which  in 
moist  climates  play  a  much  more  modest  role;  these  are  the  changes 
of  temperature  and  the  wind. 

3.  CHANGES  OF  TEMPERATURE 

In  moist  and  equable  climates  these  temperature  changes  are  of 
very  subordinate  importance  as  a  destructive  agent  and  act  chiefly 
in  giving  an  easier  passage  to  percolating  waters.  In  arid  regions, 
on  the  other  hand,  especially  on  high  mountains  and  plateaus, 
where  there  are  great  differences  of  temperature  between  day  and 
night,  this  agency  becomes  much  more  important.  During  the 
day  the  naked  rocks  are  heated  very  hot  by  the  full  blaze  of  the 
sun,  while  at  night  the  rapid  radiation  which  occurs  in  dry  and 


CHANGES   OF  TEMPERATURE 


117 


thin  air  chills  the  outer  layers  of  rock  very  quickly,  and  they  at- 
tempt to  contract  upon  the  still  heated  and  expanded  interior. 
Thus,  stresses  are  set  up  which  the  rock  cannot  resist,  and  pieces, 
great  and  small,  are  split  off  from  the  surface.  In  this  manner 
great  talus  slopes,  like  those  due  to  frost  action,  accumulate  at 
the  foot  of  cliffs  and  on  mountain  slopes  in  all  dry  regions  which 
have  hot  days  and  cool  nights.  Even  when  the  rocks  are  not 
shattered  to  pieces,  their  crevices  and  fissures  are  slowly  widened. 


i&    H^HRi 


FIG.  41.  —  Smooth  exfoliated  surface  of  granite,  Matopos  Hills,  Rhodesia,  South  Africa 

Certain  rocks,  notably  granites,  exfoliate  under  extreme  tem- 
perature changes,  that  is,  the  surface  portions  split  off  in  large 
sheets,  which  may  be  of  almost  any  thickness,  and  are  either  flat 
or,  more  commonly,  are  curved.  In  this  way  are  produced  the 
granite  domes  which  are  found  in  so  many  parts  of  the  world, 
such  as  those  of  the  Yosemite,  Stone  Mountain  in  Georgia,  the 
Matopos  Hills  in  South  Africa.  The  smooth  slopes,  due  to  ex- 
foliation, are  often  deceptively  like  those  worn  and  smoothed  by 


u8 


THE  ATMOSPHERE 


glaciers,  a  resemblance  which  is  heightened  by  the  large  boulders, 
remnants  of»  exfoliated  masses,  which  often  occur  upon  these 
slopes. 

The  effect  of  temperature  changes  is  frequently  the  disintegration 
of  rocks  into  minute  fragments.  This  extreme  effect  is  especially 
noteworthy  in  those  igneous  rocks  which  are  coarsely  crystalline. 
A  rock  of  this  kind  is  made  up  of  several  different  minerals,  each 


FlG.  42. — Slope  of  exfoliating  granite,  Matopos  Hills 


of  which  has  its  own  particular  rate  of  expansion  and  contraction, 
and  thus  the  particles  are  subjected  to  stresses  which  gradually 
separate  them  and  cause  the  rock  to  crumble.  In  Egypt  one 
may  pick  up  granite  fragments  from  the  ancient  monuments  which 
will  break  into  small  pieces  upon  very  slight  pressure.  The 
Egyptian  obelisk  in  New  York  seemed,  when  first  brought  to  this 
country,  to  be  perfectly  sound  and  fresh,  but  the  severe  winters  of 


WIND 


119 


our  Atlantic  seaboard  speedily  showed  how  the  granite  had  been 
rifted  and  weakened  by  the  centuries  of  exposure  to  temperature 
changes  in  the  dry  climate  of  Egypt. 

Changes  of  temperature  do  work  of  a  purely  mechanical  kind, 
as  does  frost,  and  are  even  more  entirely  superficial  than  the  latter, 
for  a  thin  covering  of  debris  suffices  to  put  an  end  to  their  efficiency. 


FIG.  43.  —  Exfoliation  of  glaciated  granite,  Sierra  Nevada.     (U.  S.  G.  S.) 


4.   WIND 

The  wind,  of  itself  and  unassisted  by  hard  particles,  can  ac- 
complish but  little  in  disintegrating  firm  rocks,  but  on  high  moun- 
tain crests  and  "  knife-edges,"  where  the  wind  blows  with  great 
velocity,  it  may  accomplish  considerable  destruction.  When,  how- 
ever, the  wind  is  able  to  drift  along  quantities  of  sand  and  fine 
gravel,  it  becomes  a  disintegrating  agent  of  importance.  Except 
on  sandy  coasts,  this  agency  is  of  small  efficiency  in  regions  of 
ordinary  rainfall,  because  in  these  the  soil  is  protected  and  held 


120 


THE  ATMOSPHERE 


together  by  its  covering  of  vegetation.  On  sandy  coasts  we  may 
often  observe  the  abrading  effects  of  wind-driven  sand.  In  a 
Cape  Cod  light-house  a  single  heavy  gale  so  ground  a  plate-glass 
window  as  to  render  it  opaque  and  useless,  and  on  that  same  coast 
window-panes  are  sometimes  drilled  through  by  the  sand  flying 
before  a  storm.  Fragments  of  glass  lying  on  the  sand  dunes  are 
soon  worn  as  thin  as  a  sheet  of  paper. 


FIG.  44.  —  Exfoliating  granite  dome,  Yosemite  Valley,  California.     ^U.  S.  G.  S.) 

In  arid  regions,  and  more  especially  in  sandy  deserts,  high 
winds  sweep  along  much  sand  and  fine  gravel,  which  are  hurled 
against  any  obstacle  and  gradually  cut  it  away. 

Very  hard  rocks  yield  but  slowly  to  the  cutting  action  of  wind- 
driven  sand,  and  in  them  the  chief  effect  to  be  observed  is  a 
scratching  and  polishing  of  the  surface.  The  same  principle  is 
employed  in  the  sand-blast,  which  is  a  jet  of  sand,  driven  at  a 
high  velocity  and  used  to  engrave  glass,  polish  granite,  and  do 
other  work  of  the  kind.  Soft  rocks  are  quite  rapidly  abraded  and 


WIND 


121 


cut  down  by  the  drifting  sand,  and  go  to  increase  the  mass  of 
cutting  material.    The  softer  parts  are  cut  away  first,  leaving  the 


FIG.  45.  —  Wind-sculptured  sandstone,  Black  Hills,  South  Dakota.     (U.  S.  G.  S.) 

harder  layers,  streaks,  or  patches  standing  in  relief.     In  this  way 


122 


THE  ATMOSPHERE 


very  fantastic  forms  of  rocks  are  frequently  shaped  out;  pot-holes 
and  caverns  are  excavated  by  the  eddying  drift,  and  archways 
cut  through  projecting  masses.  (See  Frontispiece  and  Figs. 

45-46.) 

As  the  wind  does  not  lift  the  harder  and  heavier  particles  to 
any  great  height,  the  principal  effect  is  produced  near  the  level 
of  the  ground,  and  thus  masses  of  rock  are  gradually  undermined 


FIG.  46.  —  Honey-combed  rock,  due  partly  to  wind  erosion  and  partly  to  the  solvent 
action  of  rain.     (U.  S.  G.  S.) 


and  fall  in  ruins,  which  in  their  turn  are  slowly  abraded.  Isolated 
blocks  are  sometimes  so  symmetrically  cut  away  on  the  under  side, 
that  they  come  to  rest  upon  a  very  small  area  and  form  rocking 
stones,  which,  in  spite  of  their  size  and  weight,  may  be  swung  by 
the  hand. 

The  fine  particles  abraded  from  the  rocks  by  drifting  sand  have 
undergone  no  chemical  change,  the  process  being  entirely  me- 
chanical. 


WIND  123 

The  abrading  effects  of  wind-driven  sand  may  be  observed  in 
any  desert  region  where  naked  rocks  are  exposed,  as,  for  example, 
in  the  arid  parts  of  Utah  and  Arizona.  One  very  characteristic 
effect  of  this  natural  sand-blast  is  found  in  the  appearance  of  the 
pebbles  shaped  by  it.  Pebbles  of  very  hard  and  homogeneous 
materials,  such  as  quartz  or  chalcedony,  are  highly  polished. 
Those  made  from  igneous  rocks  have  the  softer  minerals  worn 
away,  leaving  the  harder  to  stand  in  relief  in  curious  patterns, 
while  limestone  is  carved  into  beautiful  arabesques. 

The  wind-driven  sand,  which  does  the  work  of  abrading,  is 
itself  abraded  and  grows  finer,  the  longer  the  distance  which  it  trav- 
erses. 

We  have  seen  that  the  rain  is  slowly  shifting  the  soil  seaward, 
and  in  dry  countries  the  wind  acts  in  similar  fashion.  Strong 
winds,  blowing  steadily  in  one  direction,  carry  great  quantities  of 
dust  and  fine  sand  with  them,  sometimes  directly  into  the  sea  or 
other  bodies  of  water,  sometimes  into  rivers,  or  again  to  moister 
regions,  where  it  comes  under  the  influence  of  the  rain. 

Slowly  as  they  work,  the  wind  and  temperature  changes  prevent 
any  complete  stagnation  in  the  circulation  of  material,  and  thanks 
to  them,  the  processes  of  disintegration  of  rock  and  transporta- 
tion of  soil  are  kept  up  even  in  the  dryest  deserts. 


CHAPTER  V 
DESTRUCTIVE  PROCESSES.  —  RUNNING  WATER 

THE  source  of  all  running  water,  whether  surface  or  under- 
ground, is '  atmospheric  precipitation.  All  springs  and  streams 
are  merely  rain  (or  snow)  water  collected  and  fed  from  reservoirs. 
The  rain-water  which  falls  upon  the  land  is  disposed  of  in  three  ways : 
one  part  is  returned  to  the  atmosphere  by  evaporation;  another 
part  flows  over  the  surface  to  the  nearest  watercourse.  The  re- 
mainder sinks  into  the  soil  to  a  greater  or  less  depth,  and  though 
some  of  it  is  returned  to  the  surface  in  springs,  yet  a  great  part 
must  reach  the  sea  by  subterranean  channels.  The  surface  flow, 
together  with  the  supply  from  springs,  constitutes  the  "  run-off." 

The  relative  proportions  of  these  three  parts  of  the  total  pre- 
cipitation vary  much  in  accordance  with  the  climate  and  with  the 
topography  of  the  land  surface.  In  a  moist  climate  with  heavy 
rainfall  the  run-off  may  amount  to  one-half  of  the  precipitation, 
and  the  loss  by  evaporation  is  at  a  minimum.  In  arid  regions, 
where  evaporation  is  very  great,  the  run-off  is  from  one-fifth  to 
zero.  Climatic  factors  being  equal,  run-off  increases  with'  the 
steepness  of  the  slopes  and  is  thus  relatively  less  in  large  drainage 
basins  than  in  small  ones. 

i.   THE  GROUND  WATER 

Within  the  soil  the  movement  of  water  is  in  different  directions 
according  to  circumstances.  In  climates  of  considerable  rainfall 
which  have  no  long  dry  season,  the  movement  is  chiefly  downward, 
due  to  gravity,  but  if  there  are  long  periods  of  drought,  evaporation 
from  the  surface  causes  an  upward  movement  of  the  water  by  cap- 

124 


THE  GROUND    WATER 

illarity;  and  in  the  tropics  this  upward  movement  produces  cer- 
tain important  and  characteristic  effects. 

At  a  depth  below  the  surface,  which  varies  greatly  at  different 
times  and  places,  the  soil  and  rocks  are  saturated  with  water, 
which  is  called  the  ground  water.  Near  the  sea,  or  other  bodies 
of  surface  water,  the  ground  water  may  be  very  little  below  the 
surface  of  the  ground,  while  in  arid  regions,  with  irregular  topog- 
raphy, it  may  sink  to  great  depths.  In  the  eastern  United  States 
the  ground  water  is  encountered  at  depths  of  i-ioo  feet,  as  is 
shown  by  the  countless  wells  which  are  supplied  by  it.  In  the 
limestone  plateau  of  eastern  Kentucky  and  Tennessee  the  ground 
water  is  from  200-300  feet  below  the  surface  and  is  determined 
by  the  level  at  which  the  surface  streams  flow,  that  is,  the  drainage 
level  of  the  region.  In  the  plateau  of  the  Colorado  River,  which 
is  dissected  by  profoundly  deep  canons,  the  ground  water  is,  in 
places,  nearly  3500  feet  from  the  surface. 

The  level  of  the  ground  water  is  thus  highly  irregular  and 
depends  upon  the  amount  of  precipitation  and  upon  the  topo- 
graphical features.  As  a  general  rule,  the  level  of  ground  water  is 
at  that  of  the  streams  and  rises  toward  the  divides,  but  less  steeply 
than  the  surface  of  the  ground.  Similarly,  the  ground  water  level 
fluctuates  with  the  rainfall,  rising  in  wet  seasons  and  sinking  in  dry, 
as  is  shown  by  the  failure  of  wells  after  a  long  drought. 

It  is  usual  to  regard  the  ground  water  as  everywhere  penetrating 
to  great  depths,  and,  from  this  point  of  view,  it  is  frequently  called  the 
"  sea  of  ground  water,"  but  there  is  reason  for  much  hesitation  in 
accepting  this  belief.  In  a  large  number  of  very  deep  mining  shafts 
in  various  parts  of  the  world,  and  in  both  humid  and  arid  regions, 
water  is  found  only  in  the  upper  levels,  within  2500  feet  or  less  of 
the  surface,  while  below  the  mines  are  dry,  even  dusty.  Such 
shafts  frequently  encounter  water  in  the  lower  levels,  when  they  in- 
tersect large  fissures,  and  this  indicates  that  water  descends  to  great 
depths  principally  through  such  fissures.  The  character  of  the 
rocks  themselves  has  a  great  effect  upon  the  depths  to  which  water 
can  penetrate, — some  rocks  being  porous  and  with  such  open 


126  RUNNING   WATER 

joints  as  to  permit  a  free  passage  of  water,  while  others  are  almost 
impervious.  "It  is  probable  that  the  universal  presence  of 
ground  water  is  characteristic  of  a  comparatively  shallow  surface 
belt,  below  which  the  water  which  has  not  been  again  drawn  off  at 
the  surface,  at  a  lower  level,  or  has  not  been  used  up  in  hydration 
processes,  is  concentrated  into  the  larger  fissures."  (Spurr.) 

Aside  from  the  extremely  slow  movement  of  water  through  the 
mass  of  porous  rock,  underground  waters  follow  the  larger  open- 
ings, such  as  joint-cracks,  bedding  planes,  etc.  The  inclination  of 
the  stratified  rocks,  the  alternation  of  porous  and  impervious  beds, 
and  the  character  of  the  joints  and  fissures  are  thus  the  factors 
which  determine  the  direction  of  flow,  especially  in  the  shell  of 
weathering,  where  the  rocks  are  not  saturated  with  water.  In 
soluble  rocks,  such  as  limestones,  the  water  may  dissolve  out  its 
own  channels.  Surface  topography  has  but  a  subordinate  effect 
upon  the  course  of  underground  waters,  and  it  often  happens  that, 
for  considerable  distances,  the  surface  and  subterranean  move- 
ments of  water  are  in  exactly  opposite  directions. 

The  factors  which  determine  the  movement  of  underground 
waters  are  of  great  practical  importance  in  all  problems  of  drain- 
age and  water  supply.  Serious  evils  have  followed  from  carelessly 
taking  for  granted  that  the  underground  flow  would  be  in  the  same 
(direction  as  that  on  the  surface. 

As  the  movement  of  underground  waters  is  almost  always 
excessively  slow,  their  mechanical  work  is  trifling,  but  chemically 
they  bring  about  important  changes.  The  water,  making  its  way 
downward  through  the  joints  and  bedding  planes  of  the  rocks, 
exerts  its  solvent  and  decomposing  action  upon  the  walls  of  these 
crevices,  in  the  manner  already  described  in  connection  with  the 
work  of  rain.  Down  to  the  level  of  the  ground  water,  or  in  the  shell 
of  weathering,  percolating  waters  are  the  great  agent  of  decom- 
position and  therefore  always  contain  more  or  less  mineral  matter 
in  solution,  the  nature  and  quantity  of  which  depend  upon  the 
character  of  the  rocks  traversed.  Below  the  ground  water  level 
in  the  shell  of  cementation,  the  effects  are  more  reconstructive  than 


THE   GROUND   WATER 


127 


destructive,  though  solution  and  alteration  of  minerals  continue 
at  these  lower  levels. 

In  passing  through  limestones  in  the  shell  of  weathering,  per- 
colating waters  dissolve  channels,  great  and  small,  through  the 
rock.  Pipes  and  sink-holes  are  dissolved  downward  from  the  sur- 
face, and  in  the  mass  of  the  rock  great  caverns  are  formed  by  the 
solvent  power  of  the  carbonated  waters.  Such  caverns,  as  the 


FIG.  47.  —  Sink-hole  in  limestone,  near  Cambria,  Wyoming.     (U.  S.  G.  S.) 


Mammoth  Cave  of  Kentucky,  for  example,  are  often  many  miles 
in  extent  and  have  considerable  rivers  flowing  in  them.  Indeed,  in 
limestone  regions  the  smaller  streams  generally  have  a  longer  or 
shorter  underground  course.  The  lower  level  of  the  caverns  is 
determined  by  the  general  drainage  level  at  which  the  surface 
streams  flow.  In  the  shell  of  cementation  the  movement  of  water 
is  very  much  slower  and  its  solvent  effects  are  much  lessened.  The 
beds  of  rock-salt,  which  would  long  ago  have  been  dissolved  away 


128 


RUNNING   WATER 


by  moving  waters,  are  found  at  depths  which  may  be  reached  by 
mining  or  boring. 

When  underground  waters  become  highly  heated  through  con- 


FIG.  48.  —  Canon  and  lower  falls  of  the  Yellowstone  River.     (U.  S.  G.  S/l 

tact  with  hot  volcanic  masses,  or  by  descending  to  great  depths 
along  channels  which  permit  a  return  to  higher  levels,  their  solvent 
efficiency  is  greatly  increased.  Rocks  penetrated  by  such  thermal 


THE  GROUND   WATER 


129 


Paleozoic  limestones 


FIG.  49. —  Profile  of  Turtle  Mt.,  showing  the  amount  of  material  removed  in  the 
Frank  rock-slide.     (Brock) 

waters  are  profoundly  altered  in  character  and  composition.  The 
complex  minerals  of  the  igneous  rocks  are  decomposed;  the 
felspars  become  opaque  from  the  formation  of  kaolin,  or  are 
altered  to  hydrated  micas;  minerals  containing  magnesia  and 
iron  give  rise  to  talc,  chlorite,  serpentine,  and  the  like,  while  the 
lime  compounds  are  converted  into  the  bicarbonate  and  carried 
away  in  solution.  Some  of  the  minerals  are  altered  in  place,  and 
others  are  deposited  in  the  crevices  of  the  rocks.  Thermal  waters 
also  alter  minerals  by  bringing  in  new  material  in  solution.  In 
the  Yellowstone  Park  the  lavas  of  the  great  volcanic  plateau,  which 
has  been  deeply  trenched  by  the  Yellowstone  River,  are  pro- 
foundly decomposed  and  altered  by  the  hot  waters  which  traverse 
it. 

Except  in  caverns,  underground  waters  flow  too  slowly  to  ac- 
complish direct  mechanical  erosion,  but  indirectly  they  may  bring 
about  important  mechanical  changes.  Masses  of  soil  or  talus, 
lying  on  steep  slopes,  saturated  by  long-continued,  heavy  rains, 
may  have  their  weight  so  increased  and  their  friction  so  reduced, 


130 


RUNNING  WATER 


SPRINGS  131 

as  to  glide  downward  in  land-slips,  which  are  sometimes  disastrous. 
Of  this  kind  was  the  great  land-slip  of  1826  in  the  White  Mountains 
of  New  Hampshire. 

Rock- slides  occur  when  the  rocks  forming  a  slope  become  satu- 
rated with  water,  until  they  can  no  longer  support  themselves. 
The  movement  is  much  facilitated  by  underlying  beds  of  clay,  or 
clay  rocks,  which  become  very  slippery  when  lubricated  with 
water.  Mountain  valleys  in  all  parts  of  the  world  show  plain  evi- 
dence of  such  rock-slides,  and  often  a  vast  quantity  of  rock  is  thus 
displaced.  At  Elm,  Switzerland,  in  1881,  more  than  12,000,000 
cubic  yards  of  rock  were  carried  down  for  a  distance  of  2000  feet. 
In  1903  a  great  rock-slide  occurred  at  Frank  in  the  Canadian  prov- 
ince of  Alberta,  when  the  entire  face  of  Turtle  Mountain  fell  and 
rushed  across  the  valley  in  a  huge  avalanche  of  rock  fragments,  es- 
timated at  40,000,000  cubic  yards.  The  causes  of  this  great  rock- 
slide  were  several,  but  an  unusual  amount  of  ground  water  and  a 
severe  frost  following  warm  weather  were  the  chief  agents. 

2.   SPRINGS 

Springs  are  the  openings  of  the  ground  water  upon  the  surface, 
and  could  not  be  formed  were  the  land  perfectly  free  from  irregu- 
larities, for  gravity  controls  the  movement  of  underground  waters, 
and  the  source  of  a  spring  must  be  higher  than  its  mouth.  It 
must  be  remembered,  however,  that  a  subterranean  stream  is  often 
confined  as  in  a  pipe,  and  that  the  pressure  to  which  it  is  subjected 
may  seem  to  make  it  flow  upward,  as  when  a  spring  rises  from  a 
deep  fissure,  or  bursts  out  upon  the  top  of  a  hill.  But  these  are 
not  real  exceptions,  and  here  also  the  source,  which  may  be  many 
miles  distant,  is  above  the  spring,  and  it  is  this  which  produces 
the  necessary  pressure. 

The  commonest  type  of  spring  is  formed  when  a  relatively  im- 
pervious bed  of  rock  (usually  clay  in  some  form)  overlaid  by 
porous  rocks,  crops  out  on  a  hillside.  The  ground  water  saturates 
the  lower  layers  of  the  porous  beds,  until  its  descent  is  arrested 
by  the  impervious  bed,  and  then  the  water  follows  the  upper  sur- 


132  RUNNING   WATER 

face  of  the  latter.  When,  by  some  irregularity  of  the  ground,  the 
impervious  bed  comes  to  the  surface,  the  water  will  issue  as  a 

spring,  or  a  line  of 

"$>^?3?^  springs    (see     Fig. 

51). 

A  second  class  of 
springs  are  those 
which  rise  through 
a  crack  or  fissure 
in  the  rocks.  In- 
clined porous  beds, 

FIG.  51.— Arrangement  of  strata  which  causes  hillside  encl°sed  between 
springs.  The  lower  close-lined  bed  impervious  more  impervious 

ones,  allow  the 

water  to  follow  them  downward,  until  in  its  lower  course  such 
water  is  under  great  pressure,  or  "  head  "  (Fig.  52).  On 
reaching  a  fissure  opening  upward,  the  water  will  rise  through  it 
and,  if  under  sufficient  pressure,  will  come  to  the  surface. 

An  artesian  well  is  an  "artificial  fissure-spring."  It  is  a  boring 
which  taps  a  sheet  or  stream  of  the  ground  water,  when  the  water 
is  under  sufficient  pressure  to  rise  to  the  surface  or  even  spout 
high  above  it. 

In  limestone  districts  depressions  of  the  surface  may  intersect  the 
course  of  considerable  underground 'streams,  which  thus  reach  the 
surface  in  springs  of  unusual  volume.  Very  striking  and  beau- 
tiful examples  are  the  Giant  Spring  in  the  canon  of  the  upper  Mis- 
souri, near  Great  Falls,  Montana,  and  Silver  Spring,  Florida, 
which  is  navigable  for  steamboats. 

Springs,  as  such,  do  little  in  the  way  of  rock  disintegration,  but 
they  accomplish  something  by  undermining  the  rocks  at  the  point 
where  they  issue,  and  thus  working  their  way  backward.  This 
process  is  known  as  the  recession  of  spring-heads.  The  under- 
ground streams,  of  which  springs  are  the  outlets,  have  often  ef- 
fected much  in  the  way  of  dissolving  rock  material,  and  hence 
spring-water  always  contains  dissolved  minerals,  principally  the 


SPRINGS 


133 


carbonates  and  sulphates  of  lime  and  magnesia,  and  the  chlo- 
rides of  magnesium  and  sodium.  In  mineral  springs  the  quan- 
tity of  dissolved  materials  is  larger  and  perceptible  to  the  taste. 

Thermal  Springs  are  those  whose  temperature  is  notably  higher 
than  that  of  ordinary  springs  in  the  same  region,  and  they  range 
from  a  lukewarm  to  a  boiling  state.  This  increase  of  temperature 
may  be  caused  in  either  of  two  ways :  (i)  In  volcanic  regions,  water 
coming  into  contact  with  uncooled  masses  of  lava  is  highly  heated 
and  reaches  the  surface  as  a  hot  spring.  Of  this  class  are  the  in- 


FlG.  52.  —  Diagram  of  fissure-spring.     The  heavy  line  represents  the  fissure  along 
which  the  water  rises 

numerable  thermal  springs  of  the  Yellowstone  Park.  (2)  Wher- 
ever the  disposition  of  the  rocks  is  such  that  water  may  descend 
to  great  depths  within  the  earth  and  yet  return  to  the  surface  by 
hydrostatic  pressure,  thermal  springs  appear.  These  conditions 
are  found  only  in  regions  where  the  rocks  have  been  much  folded 
and  fractured.  In  this  case  the  temperature  of  the  water  is  raised 
by  the  interior  heat  of  the  earth,  which,  as  we  have  seen,  increases 
with  the  depth.  Springs  of  this  class  occur  numerously  along  the 
Appalachian  Mountains,  and  in  larger  numbers  and  of  higher 


134  RUNNING   WATER 

temperatures  they  accompany  the  various  ranges  of  the  Rocky 
Mountains  and  Sierra  Nevada. 

Geysers  are  thermal  springs  which  periodically  erupt,  throwing 
up  hot  water  in  beautiful  fountains,  accompanied  by  clouds  of 
steam.  Though  of  great  scientific  interest,  geysers  are  not  im- 
portant geological  agents,  because  of  their  rarity,  since  they  occur 
only  in  Iceland,  the  Yellowstone  Park,  and  New  Zealand. 


FIG.  53.  —  An  artesian  well.     (U.  S.  G.  S.) 

The  destructive  effects  of  thermal  springs  are  principally  ac- 
complished below  the  surface,  and  have  already  been  considered 
under  the  head  of  underground  waters.  The  high  percentages  of 
dissolved  materials  which  such  springs  usually  contain  are  evi- 
dence of  the  important  work  of  rock  disintegration  which  they 
perform. 


EROSION  BY   RIVERS 


135 


3.   RIVERS 

Erosion  by  Rivers.  — The  destructive  work  of  rivers,  includ- 
ing in  that  term  all  surface  streams,  is  far  less  extensive,  in  the 
aggregate,  than  that  of  the  atmospheric  agencies,  but  because  the 
work  of  a  stream  is  concentrated  along  its  narrow  course,  it 
appears  much  more  striking  and  impressive. 


FlG.  54. ~- The  "Bottomless  Pit,"  Arizona.     The  stream  disappears  in  a  limestone 
.     cavern  and  is  not  known  to  reappear.     (Photograph  by  A.  E.  Hackett,  Flagstaff, 
Ariz.) 


A  certain  amount  of  solution  and  decomposition  is  performed  by 
rivers  upon  the  rocks  of  the  bed,  and  in  limestones  this  may  be 
considerable,  especially  if  the  water  be  charged  with  organic  acids 
from  a  swamp  or  peat-bog.  Limestone  regions  are  characterized 
by  a  paucity  of  surface  streams,  most  of  which  pass  into  caverns 
and  underground  channels  which  they  have  made  by  dissolving 


136  RUNNING   WATER 

the  limestone.  Such  subterranean  streams  may  or  may  not 
reappear  on  the  surface,  according  to  circumstances. 

The  mechanical  work  of  a  river  is  much  greater  than  the  chemi- 
cal, and  is  dependent  upon  the  velocity  of  the  current,  varying 
directly  as  the  square  of  that  velocity.  The  velocity  of  a  stream 
is  the  rather  complex  resultant  of  several  factors,  the  chief  of  which 
is  gravity;  the  steeper  the  slope  of  the  bed,  the  swifter  the  flow  of 
the  water.  A  second  factor  is  the  volume  of  water,  the  velocity 
varying  as  the  cube  root  of  the  volume.  That  is  to  say,  if  one  of 
two  streams  which  flow  down  the  same  slope  has  eight  times  as 
much  water  as  the  other,  it  will  flow  twice  as  fast.  Other  factors 
enter  into  the  result,  but  slope  of  bed  and  volume  of  water  are 
much  the  most  important. 

Pure  water  can  do  little  to  abrade  hard  rocks,  though  it  can 
wash  away  sand,  gravel,  and  other  loose  materials.  When  the 
Colorado  River  broke  into  the  Salton  Sink  in  southeastern  Cali- 
fornia in  1905,  it  cut  a  deep  trench  with  incredible  rapidity  through 
the  soft  alluvial  soils.  Streams  also  take  advantage  of  the  joint- 
blocks,  into  which  all  rocks  are  divided,  and  often  loosen  and 
carry  down  such  blocks.  This  process  is  called  plucking  and  is 
important  in  the  destructive  work  of  glaciers  and  the  sea.  As  in 
the  case  of  the  wind,  the  stream  merely  supplies  the  power;  the 
implement  with  which  the  cutting  is  performed  is  the  sand,  pebbles, 
and  other  hard  particles  which  the  water  sets  in  motion.  These 
abrade  the  rocks  against  which  they  are  cast,  just  as  the  wind- 
driven  sand  does,  but  more  effectively,  because  of  the  ceaseless 
activity  of  the  stream,  and  because  many  rocks  are  rendered 
softer  and  more  yielding  by  being  wet.  The  cutting  materials  are 
themselves  abraded  and  worn  finer  and  finer  by  continued  friction 
against  the  rocks  and  against  one  another.  In  the  case  of  complex 
minerals  this  abrasion  is  accompanied  by  more  or  less  chemical 
decomposition,  as  has  been  shown  experimentally  by  rotating 
crystals  of  felspar  in  a  drum  half  filled  with  water.  When  the 
felspar  was  ground  down  to  mud,  the  water  showed  the  presence 
of  potash  and  soda  in  solution.  Angular  blocks  are  speedily  worn 


EROSION   BV   RIVERS  137 

into  cobblestones  and  these  into  pebbles  of  spheroidal  or  flat, 
discoidal  form.  A  process  of  selection  goes  on,  by  which  the 
softer  materials  are  ground  into  mud,  the  harder  remaining  as 
.pebbles  and  sand. 

An  example  of  exceedingly  rapid  wear  of  hard  rock  by  running 
water,  under  favourable  conditions,  is  given  by  the  Sill  tunnel  in 
Austria,  which  is  provided  with  a  pavement  of  granite  slabs  more 
than  a  yard  thick.  Great  quantities  of  debris  are  swept  over  this 
pavement  at  a  high  velocity  and  so  rapid  is  the  abrasion,  that  it 
was  found  necessary  to  renew  the  granite  slabs  after  a  single  year. 

A  river  which  is  subject  to  sudden  fluctuations  of  volume,  being 
now  a  rushing  torrent  and  again  almost  dry,  is  a  much  more 
efficient  agent,  both  of  erosion  and  of  transportation,  than  is  one 
which  carries  nearly  the  same  quantity  of  water  at  all  times,  or 
which  fluctuates  only  slowly. 

The 'velocity  of  a  stream  differs  much  in  its  various  parts,  dimin- 
ishing, as  a  rule,  from  the  head  waters  to  the  mouth.  In  very 
many  cases  there  are  also  local  variations  of  speed,  falls,  rapids, 
and  eddies  alternating  with  quiet  reaches.  In  eddies  and  at  the 
foot  of  cascades  the  water  acquires  a  rotary  motion,  which  is 
transmitted  to  stones  lying  on  the  bottom.  In  a  rocky  bed  these 
revolving  stones  excavate  cylindrical  holes,  often  of  remarkable 
regularity,  called  pot-holes,  or  giant  kettles.  The  diameter  and 
depth  of  pot-holes  are  determined  by  the  volume  and  velocity 
of  the  water  and  by  the  length  of  time  during  which  the  eddy  or 
fall  remains  at  the  same  point. 

Since  the  velocity  of  a  stream  is  so  largely  dependent  upon 
gravity,  it  is  obvious  that  the  deeper  a  stream  cuts  its  channel,  the 
less  steep  does  its  slope  become,  and  that  so  long  as  the  region  is 
neither  upheaved  nor  depressed,  the  river  performs  its  vertical 
erosion  at  a  constantly  decreasing  rate.  Unless,  therefore,  the 
work  is  done  under  very  exceptional  conditions,  as  in  the  case  of 
the  Niagara,  we  cannot  reason  from  the  present  rate  of  excavation 
to  the  length  of  time  involved  in  cutting  out  a  given  gorge. 

Unless  the  region  through  which  a  river  flows  is  upheaved,  and 


138 


RUNNING  WATER 


FIG.  55.  —  Undermined  pot-hole,  Little  Falls,  N.Y.  The  arrow  points  to  the  upper 
opening  and  the  partly  concealed  figure  stands  in  the  lower  part.  (Photograph 
by  van  Ingen^ 


EROSION  BY  RIVERS 


139 


thus,  by  increasing  the  fall,  renewed  power  is  given  to  the  stream, 
a  stage  must  sooner  or  later  be  reached  when  the  vertical  cutting 
of  the  stream  must  cease.  This  stage  is  called  the  base-level  of 
erosion,  or  regimen  of  the  river,  and  it  approximates  a  parabolic 
curve,  rising,  toward  the  head  of  the  stream.  Elevation  of  the 
country  will  start  the  work  afresh,  until  a  new  base-level  is  reached, 


FIG.  56.  —  Pot-hole  in  stream,  Mill  Creek,  Oklahoma.    (U.  S.  G.  S.) 

while  depression  will  have  a  contrary  effect  and  may  put  a  stop  to 
vertical  erosion  where  it  was  in  active  progress  before.  When 
the  base-level  is  reached,  the  river  cuts  laterally,  undermining  its 
banks  (see  Fig.  55)  and  working  like  a  horizontal  tool  upon  the 
country-side. 

As  valleys  are  also  excavated  by  other  agents,  it  is  important  to 
note  the  characteristic  features  of  river-formed  valleys.     Unas- 


140  RUNNING  WATER 

sisted  by  other  agencies,  a  river  cuts  a  narrow,  steep-sided  trench 
or  gorge,  the  possible  depth  of  which  depends  upon  the  height 
above  base-level  at  which  the  river  began  its  work,  disregarding 
any  subsequent  elevation  of  the  land.  As  soon  as  the  gorge  begins 
to  form,  its  sides  are  attacked  by  the-  atmospheric  destructive  forces 
and  a  process  of  widening  is  begun ;  but  this  is  very  slow  and  to 
widen  out  the  gorge  or  canon  into  a  broad  valley,  with  gentle 
slopes,  requires  a  very  long  period  of  time,  determined  by  the 
activity  of  the  climate  and  the  resistant  power  of  the  rocks.  Even 
in  the  gorge  stage,  a  river  valley  tends  to  have  a  V-shaped  cross- 
section,  because  the  upper  part  of  the  gorge,  having  been  longest 
exposed  to  weathering,  has  suffered  the  greatest  loss. 

A  river  valley  is  rarely  straight  for  any  considerable  distance, 
but  takes  a  sinuous  course,  with  rocky  spurs  projecting  alternately 
from  opposite  sides  of  the  stream,  and  these  spurs  have  a  continu- 
ous (or  a  terraced)  slope  from  top  to  bottom.  This  is  true  even  of 
swift  streams  flowing  through  hard  rocks,  and  the  tendency  is 
much  exaggerated  when  the  velocity  of  the  current  is  diminished 
and  the  river-bed  is  in  soft  materials,  as  in  the  lower  Mississippi. 
Under  such  conditions  the  stream  meanders  to  an  extraordinary 
degree  and  often,  by  cutting  through  the  narrow  neck  of  a  meander, 
abandons  part  of  its  channel  and  leaves  an  "  ox-bow  lake  "  to 
mark  its  former  course. 

Still  another  feature  of  river  valleys  is  the  accordant  relation  be- 
tween the  main  valley  and  its  tributaries.  Normally,  tae  tribu- 
taries lower  their  beds  at  the  same  rate  as  the  main  stream  and 
enter  the  latter  on  the  same  level.  Exceptions  to  this  rule  do 
occur,  as  in  the  case  of  the  tributaries  below  a  great  cataract  in 
the  trunk  river,  which  enter  high  up  on  the  gorge  walls,  but  such 
exceptions  admit  of  a  ready  explanation. 

When  a  river  enters  the  sea  or  a  lake,  its  velocity  is  checked 
and  it  is  no  longer  able  to  excavate  a  channel,  so  that  the  con- 
tinuation of  the  rive*-  channel  across  the  sea-floor,  like  that  of 
the  Hudson  (see  p.  34),  is  a  proof  that  the  lower  course  of  the 
stream  has  been  submerged  under  the  sea.  There  are,  however, 


EROSION  BY  RIVERS 


141 


two  known  instances  of  the  excavation  of  a  utream  channel  in 
the  bed  of  a  lake,  the  upper  Rhone  in  Lake  Geneva  and  the 
upper  Rhine  in  Lake  Constance.  These  streams  are  both  gla- 
cial streams  of  very  cold  and  dense  water  and  charged  with  great 
loads  of  coarse  sediments.  Discharging  into  bodies  of  warmer 
and  lighter  water,  the  river  currents  are  able  to  maintain  them- 
selves for  some  distance  from  shore  and  thus  to  cut  trenches. 
Other  examples  will,  no  doubt,  be  found  under  similar  condi- 
tions, but  must  be  very  uncommon. 


FIG.  57,  —  A  meandering  stream;  ox-bow  lakes  af  the  right :  Alashuk  River,  Alaska. 

(U.  S.  G.  S.) 

Having  learned  the  general  character  of  river  erosion,  we  may 
illustrate  it  with  a  few  concrete  examples. 

i.  A  particularly  interesting  case  is  that  of  the  little  river 
Simeto  in  Sicily,  since  the  history  of  its  gorge  is  so  well  known. 
In  1603  a  great  lava  flood  from  ^Etna  was  poured  out  across  the 
course  of  the  stream,  a,nd,  when  cold,  solidified  into  a  barrier  of 


142 


RUNNING  WATER 


the  hardest  rock.     When  Sir  Charles  Lyell  visited  the  spot   in 
1828,  he  found  that  in  a  little  more  than  two  centuries  the  stream 


FIG.  58.  —  The  Au  Sable  Chasm,  N.Y.     (Copyright  by  S.  R.  Stoddard,  Glens  Falls,  N.Y.) 


EROSION  BY   RIVERS 


143 


had  cut  a  gorge  through  this  barrier  of  40  to  50  feet  deep,  and 
varying  in  width  from  50  to  several  hundred  feet.  The  lava  which 
had  thus  been  trenched  is  not  porous  or  slaggy,  but  homogeneous, 
dense,  and  very  hard. 

2.  In  the  northern  parts  of  the  United  States  the  great  ice- 
sheet,  which  in  late  geological  times  covered  the  country,  brought 
down  with  it  vast  quantities  of  drift,  that  filled  up  the  channels  of 
many  streams  and  quite  revolutionized  the  drainage  of  certain  dis- 


FlG.  59.  —  Old,  high-level  channel  of  the  Niagara  River,  below  the  present  falls. 

(U.  S.  G.  S.) 

tricts.  Since  that  time  the  displaced  streams  have  cut  out  new 
channels  for  themselves,  often  through  hard  rocks,  and  many  now 
flow  in  quite  deep  gorges,  with  nearly  vertical  walls.  Au  Sable 
Chasm,  New  York,  is  an  example  of  these  geologically  modern 
river  gorges,  the  atmosphere  not  having  had  time  to  widen  it. 
3.  The  Niagara  is  an  exceptional  case,  the  gorge  being  cut, 
not  only  by  the  direct  abrasion  of  the  running  water,  but  also  by 
the  action  of  the  spray  and  frost  at  the  falls.  In  the  ravine  the 


144  RUNNING  WATER 

upper  rock  is  a  hard,  massive  limestone,  which  is  underlaid  by  a 
soft  clay  shale.  The  latter  is  continually  disintegrated  by  the 
spray  of  the  cataract  and  by  the  severe  winter  frosts  undermining 
the  limestone,  which,  when  no  longer  able  to  bear  its  own 
weight,  breaks  off  in  tabular  masses.  Thus  the  falls  are  steadily 
receding,  leaving  behind  them  a  gorge,  which  is  deepened  by  the 
river  and  especially  by  the  plunging  masses  of  water  at  the  foot  of 
the  cataract. 

4.  One  of  the  most  remarkable  known  examples  of  river  ero- 
sion is  seen  in  the  canons  of  the  Colorado.    The  Grand  Canon  is 
over  200  miles  long  and  from  4000  to  6500  feet  deep,  with  pre- 
cipitous walls.     It  is  extremely  probable  that  the  river  has  been 
rendered   able  to  cut  to  such  profound   depths  by  the  gradual 
uplifting  of  the  whole  region,  which  is  now  a   lofty  plateau,  in 
places  more  than  8000  feet  above  the  sea.    The  erosive  power  of 
the  river  has  thus  been  continually  renewed  and  a  more  or  less 
uniform  rate  of  excavation  secured. 

5.  Finally,  an  extremely  curious  example  of  river  erosion  is 
the  gorge  of  the  Zambesi  River  in  South  Africa,  at  and  below  the 
Victoria  Falls.     Above  the  falls  the  river  is  more  than  a  mile 
wide,  and  at  the  cataract  it  plunges  into  a  narrow  chasm  which 
is  transverse  to  the  course  of  the  river.     From  the  chasm  the  only 
outlet  is  by  a  narrow  gorge  of  only  50-60  yards  in  width,-  and 
below  this  gateway  the  gorge  continues  for  many  miles  in  a  series 
of  sharp  zigzags,  which  are  highly  exceptional  in  such  a  hard 
rock  as  the  basaltic  lava  which  the  river  has  trenched  to  a  depth 
of  400  feet.     This  remarkable  result  is  due  to  the  fact  that  the 
excavating  work  of  the  river  has  been  controlled  by  the  lines  of 
joints  in  the  rock. 

Transportation ^by  Rivers.  — The  main  importance  of  rivers  as 
geological  agents  is  not  so  much  their  work  of  erosion,  but  lies 
rather  in  what  they  accomplish  as  carriers  of  the  results  of  their 
own  destructive  activity  and  that  of  the  atmosphere,  comprising 
both  the  materials  which  are  mechanically  swept  along  in  sus- 
pension and  those  which  are  carried  in  solution. 


TRANSPORTATION  BY  RIVERS  145 

Materials  mechanically  Carried.  — The  transporting  power  of 
running  water  is  dependent  upon  the  velocity  of  the  current,  and 
both  mathematical  and  experimental  treatment  of  the  problem 
brings  out  the  surprising  result  that  the  transporting  power  varies 
directly  as  the  sixth  power  of  the  velocity.  If  the  rapidity  of  a 
stream  be  doubled,  it  can  carry  64  times  as  much  as  before.  The 
destructiveness  of  sudden  and  violent  floods  is  thus  explained.  In 
the  terrible  flood  which  overwhelmed  Johnstown,  Pennsylvania,  in 
1889,  great  locomotives  and  massive  iron  bridges  were  swept  off, 
it  is  hardly  an  exaggeration  to  say,  like  straws,  and  huge  boulders 
carried  along  like  pebbles.  The  formula  as  to  the  relation  of  veloc- 
ity to  transporting  power  refers  more  particularly  to  the  coarser 
materials  which  are  pushed  along  the  bottom  of  the  stream.  No 
relation  has  yet  been  determined  for  very  fine  particles  of  silt  and 
clay,  some  of  which  remain  suspended  indefinitely  even  in  still 
water.  Transporting  power  also  increases  as  the  temperature  of 
the  water  decreases. 

It  obviously  follows  from  the  relation  obtaining  between  velocity 
and  transporting  power,  that  a  slight  increase  in  the  rapidity  of  a 
stream  will  largely  augment  the  load  which  it  carries,  provided  the 
stream  obtains  as  much  material  as  it  can  transport,  while  a 
slight  reduction  of  velocity  will  cause  the  deposition  of  a  large  part 
of  that  load.  The  buoyancy  of  water  adds,  in  an  important  degree, 
to  its  ability  to  sweep  along  sediment,  because  when  any  substance 
is  immersed  in  water,  it  loses  weight  to  an  amount  equal  to  the 
weight  of  an  equal  bulk  of  water.  The  specific  gravity  of  most 
rocks  is  from  two  and  one-half  to  three,  so  that  when  immersed 
they  lose  from  one-third  to  two-fifths  of  their  weight  in  air.  The 
shape  of  the  fragments  is  likewise  a  factor  in  determining  the 
velocity  requisite  to  move  them;  the  larger  the  surface  of  the 
fragment  in  proportion  to  its  weight,  the  more  easily  it  is  carried 
in  suspension.  Thus  flat  grains  or  scales  are  carried  farther  than 
round  ones;  while,  on  the  other  hand,  rounded  fragments  are  more 
easily  rolled  along  the  bottom,  when  too  heavy  for  the  current 
to  lift. 

L 


146  RUNNING  WATER 

The  greater  part  of  the  debris  or  sediment  which  a  stream  car- 
ries is  furnished  to  it  by  the  destructive  activity  of  the  atmosphere; 
the  rains  wash  in  the  finer  materials,  while  frost  and  land-slips  bring 
in  the  larger  masses  which  are  carried  down  by  mountain  torrents. 
To  this  material  the  river  adds  that  which  is  derived  from  its  own 
work  in  the  cutting  away  of  its  banks  and  bed. 

Materials  in  Solution.  —  In  addition  to  what  the  river  carries 
down  mechanically  in  suspension  or  sweeps  along  the  bottom, 
there  is  a  third  class  of  material;  namely,  that  which  is  dissolved 
in  the  waters  of  the  stream.  Dissolved  matters  are  always  present 
in  greater  or  less  quantity,  and  are  the  same  in  kind  as  those 
which  we  have  already  found  to  occur  in  spring-waters,  whence 
they  are,  for  the  most  part,  derived  by  the  rivers.  River-water 
is,  however,  usually  more  dilute  than  that  of  springs ,  because  of 
the  rain  which  falls  into  it,  or  pours  in  from  the  banks.  In  very 
dry  regions,  where  this  additional  rain  supply  is  at  a  minimum, 
and  where  the  streams  are  concentrated  by  continual  evapora- 
tion, they  are  frequently  undrinkable,  on  account  of  the  quantity 
of  matter  in  solution  which  they  contain.  Examples  of  this 
are  the  salt  and  so-called  "  alkali  "  (a  very  comprehensive  term) 
streams  of  the  arid  West,  which  contain  a  great  variety  of  dis- 
solved minerals. 

The  quantity  of  material  which  rivers  are  continually  sweeping 
into  the  sea  is  enormously  great.  Every  year  the  Mississippi  car- 
ries into  the  Gulf  of  Mexico  nearly  7,500,000,000  cubic  feet  of 
solid  sediment,  either  in  suspension  or  pushed  along  the  bottom, 
an  amount  sufficient  to  cover  one  square  mile  to  a  depth  of  268 
feet.  In  addition  to  this  is  the  quantity  brought  down  in  solution, 
which  is  estimated  at  2,850,000,000  cubic  feet  annually. 

Different  rivers  vary  much  in  the  proportion  of  suspended  and 
dissolved  materials  which  they  carry  and  discharge  into  the  sea;  a 
roughly  approximate  average  makes  the  amount  of  material  re- 
moved equal  to  about  1 1 ,400  cubic  feet  (600  tons)  of  annual  waste 
for  every  .square  mile  of  the  land  surface  of  the  globe,  that  is,  under 
existing  conditions  of  slope,  temperature,  rainfall,  etc.  How  great 


TRANSPORTATION   BY   RIVERS  147 

a  difference  in  the  result  a  change  in  these  factors  may  produce 
will  be  seen  from  a  comparison  of  the  Mississippi  and  the  Ganges. 
The  amount  of  suspended  matter  discharged  by  the  former  repre- 
sents a  lowering  of  the  surface  of  the  entire  drainage  area  at  the 
rate  of  one  foot  in  4920  years,  while  in  the  case  of  the  Ganges  it 
is  one  foot  in  1880  years,  or  more  than  twice  as  fast.  The  amount 
of  material  carried  by  the  Amazon  has  not  been  determined,  but 
there  can  be  little  doubt  that  it  is  far  greater  than  that  discharged 
by  the  Mississippi.  The  area  drained  by  the  Amazon  is  less  than 
twice  as  large  as  the  drainage  basin  of  the  Mississippi,  and  yet  it 
brings  to  the  sea  five  times  as  much  water  as  does  the  great  river 
of  North  America. 


CHAPTER  VI 

DESTRUCTIVE  PROCESSES.  —  SNOW  AND  ICE,  THE  SEA, 
LAKES,  ANIMALS  AND  PLANTS 

Avalanches  are  great  masses  of  snow  which  descend  from  the 
mountain  tops  at  a  very  high  velocity,  and  are  frequent  in  all  high 
mountains  with  heavy  snowfall,  and  occur,  though  less  commonly, 


FIG.  60. — Summit  of  Mt.  Blanc,  Switzerland,  showing  the  great  accumulations  of 

snow 

on  mountains  of  medium  height.  Winter  avalanches  of  dry  and 
powdery  snow  do  comparatively  little  destructive  work,  but  in 
thawing  weather,  when  the  snow  is  heavily  charged  with  water, 

148 


GLACIERS  149 

great  masses  of  earth  and  rock  are  brought  down  in  the  avalanche, 
which  sweeps  everything  before  it.  Though  acting  only  occa- 
sionally, avalanches  are  efficient  agents  in  the  removal  of  material 
from  higher  to  lower  levels. 

On  a  small  scale,  snow-slides  remove  unprotected  soil  from 
slopes.  In  the  bad  lands  (see  p.  109),  where  the  rain  wash 
produces  comparatively  little  effect  upon  the  debris-covered 
buttes,  sliding  masses  of  snow  strip  off  the  covering  of  soil  and 
expose  fresh  surfaces  of  rock  to  the  destructive  action  of  the  water. 

Glaciers  are  much  the  most  important  form  of  ice  as  a 
geological  agent.  A  glacier  is  a  stream  of  ice  which  flows  as  if 
it  were  a  very  tough  and  viscous  fluid,  and  does  not  merely  glide 
down  a  slope,  as  snow  slides  from  the  roof  of  a  house.  Glaciers 
play  a  very  important  part  in  keeping  up  the  circulation  of  the 
atmospheric  waters,  and  produce  geological  results  of  an  extremely 
characteristic  kind.  Their  contribution  to  the  sum  total  of  rock 
destruction  and  reconstruction  is,  it  is  true,  relatively  small,  but  it 
often  becomes  important  to  trace  the  former  extension  of  glaciers, 
which,  in  its  turn,  has  a  wide  bearing  upon  some  of  the  most  far- 
reaching  of  cosmical  problems. 

As  we  ascend  into  the  atmosphere  from  any  point  on  the  earth's 
surface,  we  find  that  it  becomes  continually  colder  with  increasing 
height.  In  this  ascent  a  level  is  eventually  reached  where  the 
temperature  of  the  air  never  rises  for  any  length  of  time  above 
the  freezing-point,  and  above  this  level  no  rain,  but  only  snow,  falls. 
This  level  is  called  the  limit  of  perpetual  snow,  or  simply  the  snow- 
line.  While  the  height  of  the  snow-line  above  the  sea-level  is,  like 
climate  in  general,  much  affected  by  local  factors,  yet,  speaking 
broadly,  its  elevation  is  determined  by  latitude.  In  the  tropics  the 
snow-line  is  15,000  or  16,000  feet  above  the  sea,  —  descending  more 
and  more,  as  we  go  toward  the  poles,  and  coming  down  nearly  to 
sea-level  within  the  polar  circles,  but  does  not  actually  reach  that 
level  at  any  known  point  in  the  northern  hemisphere. 

Were  there  no  means  of  bringing  the  snow  which  accumulates 
above  the  snow-line  to  some  place  where  it  may  melt,  it  would 


ISO 


SNOW  AND   ICE 


evidently  gather  indefinitely,  and  at  last  nearly  all  the  moisture  of 
the  earth  would  be  thus  locked  up.  As  a  matter  of  fact,  there  is 
no  such  indefinite  accumulation.  In  very  dry  regions  the  excess  of 
snow  is  disposed  of  by  direct  evaporation,  and  on  high  mountains 
avalanches  carry  the  snow  down  to  lower  levels,  where  it  melts. 


FlG.  61. — Two  valley  glaciers  descending  Mt.  Blanc,  showing  the  terminal  moraine 
at  the  foot  of  each.  On  account  of  the  foreshortening  the  glaciers  appear  to  be 
unduly  steep 

In  places  where  the  excess  of  snow  cannot  be  disposed  of  in  either 
of  these  ways,  glaciers  are  formed  and  thus  keep  up  the  circula- 
tion of  the  waters,  by  carrying  the  surplus  snow  down  to  lower 
levels  at  which  it  can  melt,  or  by  entering  the  sea  and  in  the 


GLACIERS  151 

shape  of  icebergs  (which  are  fragments  of  glaciers)  being  floated 
to  warmer  latitudes. 

Though  even  at  the  present  time  there  are  in  various  parts  of 
the  world  great  tracts  of  glacier  ice,  they  cannot  be  called  com- 
mon and  are  found  only  where  certain  conditions  concur.  The 
nature  of  these  conditions  will  be  best  understood  by  examining 
the  process  of  glacier  formation. 

Snow  is  made  up  of  minute,  hexagonal  crystals  of  ice,  which 
are  intimately  mixed  with  air  and  thus  separated  from  one  another. 
Though  the  individual  crystals  are  transparent,  snow  is  white  and 
opaque,  as  always  results  when  a  transparent  body  is  intimately 
mixed  with  a  gas,  as  in  the  foam  on  water,  or  in  powdered  glass. 
Ice  is  composed  of  the  same  kind  of  crystals  as  is  snow,  but  they 
are  in  contact  with  one  another,  not  separated  by  air.  To  con- 
vert snow  into  ice,  therefore,  it  is  only  necessary  to  expel  the  air 
and  bring  the  crystals  into  contact,  for  which  pressure  alone  is 
not  ordinarily  sufficient. 

The  first  step  in  the  transformation  is  the  partial  melting  of  the 
upper  layers  of  snow,  for  which  a  change  of  temperature  is  neces- 
sary, though  the  change  need  not  warm  the  air,  but  may  be  due 
to  the  direct  rays  of  the  sun.  Glaciers  are  rare  in  the  tropics 
because  of  the  constancy  of  the  temperature,  and  the  small  area 
which  extends  above  the  snow-line,  which  seldom  permits  the  for- 
mation of  extensive  snow-fields.  Sometimes,  however,  the  condi- 
tions of  glacier  formation  are  fulfilled  even  in  the  equatorial  zone; 
for  example,  there  is  a  glacier  on  one  of  the  peaks  of  Ecuador. 

When  the  surface  layers  of  snow  have  been  partially  melted,  the 
water  thus  formed  trickles  down  into  the  snow  beneath,  expelling 
much  of  the  air.  This  underlying  snow  has  still  a  temperature 
much  below  the  freezing-point,  and  the  percolating  water  is  soon 
refrozen  into  little  spherules  of  ice.  This  substance,  midway  be- 
tween snow  and  ice,  is  called  neve^  and  may  be  seen  every  winter 
wherever  the  snow  lies  for  any  length  of  time.  The  hardened 
"  crust "  which  forms  by  the  refreezing  of  partly  melted  snow  is 
neve.  The  air,  which  is  now  in  the  form  of  discrete  bubbles,  is 


152  SNOW  AND   ICE 

largely  expelled  by  the  increasing  pressure  of  the  overlying  snow 
masses,  which  are  continually  added  to  by  renewed  falls,  and  the 
neve  is  thus  converted  into  ice. 

The  structure  of  glacial  ice  is  characteristically  different  from 
that  produced  by  the  freezing  of  a  body  of  water.  The  latter  is 
made  up  of  parallel  crystals  with  optical  axes  perpendicular  to 
the  surface  of  the  water.  Glacial  ice,  on  the  other  hand,  consists 
of  crystalline  grains,  which  increase  in  size  toward  the  lower  end 
of  the  glacier,  with  optical  axes  disposed  irregularly.  The 
banded'  structure  of  the  glacier,  often  so  conspicuous,  is  a  kind 
of  stratification  and  is  derived  from  the  successive  snow  layers  of 
the  neVe. 

The  temperature  of  the  interior  of  a  glacier  corresponds  at  every 
depth  to  the  melting-point  of  the  ice  for  the  pressure  at  that  depth. 
The  melting-point  of  ice  is  lowered  by  pressure,  and  therefore  pres- 
sure changes  within  and  at  the  bottom  of  the  glacier  cause  melting 
and  refreezing  without  corresponding  temperature  changes.  This 
fact  that  glacial  ice  is  so  nearly  at  the  melting-point  indicates 
that  the  maximum  thickness  of  the  glacier  cannot  exceed  1600 
feet,  which  in  truth  appears  to  be  the  thickness  of  the  Antarctic 
ice-cap.  A  greater  thickness  would  cause  melting  by  pressure  of 
the  bottom  parts. 

It  follows  from  their  mode  of  formation  that  glaciers  can  be 
formed  only  where  the  snow  accumulates  to  great  thicknesses,  and 
cannot  be  disposed  of  by  either  melting  or  evaporation  Hence, 
glaciers  are  rare  or  absent  in  dry  regions,  as  in  most  of  the  Rocky 
Mountains  within  the  limits  of  the  United  States.  It  also  follows 
that  the  ground  upon  which  the  snow  lies  must  be  so  shaped  as 
to  allow  great  masses  of  it  to  gather. 

A  glacier  moves  in  much  the  same  way  as  a  river,  but  at  a  very 
much  slower  rate.  The  middle  portion  moves  faster  than  the  sides, 
because  the  latter  are  retarded  by  the  friction  of  the  banks,  and,  for 
the  same  reason,  the  top  moves  faster  than  the  bottom.  While 
behaving  like  a  plastic  substance  under  pressure,  ice  yields  readily 
to  tension,  and  even  a  slight  change  in  the  slope  of  the  bed  will 


GLACIERS 


153 


cause  a  great  transverse  crack,  or  crmasse,  to  form,  which,  like  an 
eddy  in  a  stream,  seems  to  be  stationary,  because  always  formed 
again  at  the  same  spot.  Other  systems  of  cracks,  the  marginal 


FlG.  62.  —  A  hanging  glacier,  Cascade  Pass,  Wash.     Note  the  terminal  moraine  and 
the  crevasses.     (U.  S.  G.  S.) 


154 


SNOW  AND   ICE 


crevasses,  are  formed  along  the  sides  of  the  glacier,  and  are  due 
to  the  more  swiftly  moving  middle  pulling  away  from  the  retarded 
sides. 

The  rate  of  glacier  movement  depends  upon  the  snow  supply, 
upon  the  slope  of  the  ground,  and  the  temperature  of  the  season. 
The  comparatively  small  glaciers  of  the  Alps  move  at  rates  varying 
from  two  to  fifty  inches  per  day  in  summer  and  at  about  half  that 
rate  in  winter,  while  the  vastly  larger  glaciers  of  the  polar  lands 


FlG.  63.  —  Moraine-covered  surface  of  the  Malaspina  Glacier,  Alaska.   (U.  S.  G.  S.) 

have  a  correspondingly  swifter  flow.  The.  great  stream  of  ice 
which  enters  Glacier  Bay  in  Alaska  has  a  summer  velocity  of 
seventy  feet  per  day  in  the  middle. 

Southeastern  Alaska  is  a  region  where  glaciers  are  developed  on 
a  very  extensive  scale.  The  Malaspina  is  an  immense  ice-sheet, 
having  an  area  of  1500  square  miles,  which  is  formed  at  the  foot  of 
the  St.  Elias  Alps  by  the  confluence  of  several  great  glaciers  from 
the  neighbouring  mountains.  Parts  of  this  vast  accumulation  of 


GLACIERS  155 

ice  are  stagnant  and  deeply  covered  with  rock  debris,  upon  which 
there  is  a  luxuriant  growth  of  vegetation,  with  not  less  than  1000 
feet  of  ice  beneath  it. 

In  Greenland  and  the  Antarctic  continent  the  accumulations  of 
ice  are  on  a  scale  not  elsewhere  found,  and  these  regions  present 
conditions  of  great  geological  interest.  Greenland,  except  for  a 
narrow  strip  along  the  coasts,  is  buried  beneath  a  vast  ice-sheet, 
from  which  great  glaciers  descend  to  the  sea.  In  the  interior  only 


FIG.  64.  —  Nunatak  rising    through    the    ice-cap,  Greenland.     (Photograph    by 

Libbey) 

a  few  isolated  mountain  peaks,  or  nunataks,  rise  through  the  ice 
mantle;  except  for  these,  nothing  is  visible  but  illimitable  fields 
of  snow.  The  snowfall  is  not  very  great;  but  so  little  of  it  is 
disposed  of  by  evaporation  or  melting,  that  there  is  a  large  excess 
which  goes  to  the  growth  of  the  ice-sheet,  and  keeps  up  the  supply 
for  the  innumerable  glaciers  which  flow  to  the  sea. 

The  Antarctic  ice-cap  is  estimated  to  be  nearly  seven  times  as 
large  as  that  of  Greenland. 

The  source  of  a  glacier  is  always  above  the  snow-line,  but  the 


1 56 


SNOW  AND   ICE 


ice-stream  itself  may  descend  far  below  that  line,  slowly  melting 
and  diminishing  in  thickness  as  it  flows.  The  lower  end  is  at  the 
point  where  the  rate  of  melting  and  the  rate  of  flow  balance,  so 
that  changes  in  the  temperature  of  the  seasons  or  in  the  amount 
of  the  snow  supply  will  cause  the  glacier  to  advance  or  retreat,  as 
one  or  other  of  these  factors  prevails.  Thus  the  Alaskan  glaciers 
have  retreated  notably  within  the  last  century,  while  some  of  the 
Norwegian  ones  are  advancing.  From  the  lower  end  of  a  glacier 


FIG.  65.  —  Edge  of  the  Greenland  ice-sheet,  with  a  glacier  descending  from  it. 
The  dark  line  is  a  medial  moraine.     (Photograph  by  Libbey) 

there  always  issues  a  stream  of  water,  which  flows  under  the  ice, 
often  in  great  volume,  and  even  in  winter,  for  the  thick  ice  is  a 
non-conductor  and  protects  the  stream  from  the  intense  cold  of  the 
air. 

There  are  various  forms  of  moving  bodies  of  land  ice  correspond- 
ing to  bodies  of  water.  We  have  (i)  Alpine  glaciers,  of  which 
those  in  the  Alps  are  types,  and  are  relatively  small  streams  occu- 


GLACIERS 


157 


pying  narrow  mountain  valleys,  each  connected  with  a  particular 
basin  or  gathering  ground  of  snow.  (2)  H&ngi/n^  glaciers .  which 
descend  but  little  below  the  snow-line  and  are  small  glaciers  occupy- 
ing steep  clefts  near  the  mountain  tops.  In  some  cases  they  are 
not  connected  with  snow-fields,  but  are  fed  by  avalanches.  The 
glaciers  of  the  northern  Rocky  Mountains  and  the  Sierra  Nevada- 
are  mostly  of  this  class.  (3)  Ice  fields,  exemplified  in  Scandi- 
navia; these  are  extensive  and  continuous  areas  of  thick  ice, 


FIG.  66.  —The  Columbia  Glacier,  Alaska.     (U.  S.  G.  S.) 

with  gently  curved  surface,  from  the  margins  of  which  numerous, 
but  mostly  small,  glaciers  descend  through  rocky  gorges.  (4)  Pied- 
mont glaciers,  like  the  Malaspina  of  Alaska.  These  are  great  ac- 
cumulations or  lakes  of  ice  which  form  at  the  foot  of  mountains,  by 
the  coalescence  of  numerous  glaciers  of  the  Alpine,  or  valley,  type. 
(5)  Continental  glaciers  are  those  which  cover  enormous  areas  of 
land,  such  as  the  ice-sheet  under  which  nearly  all  of  Greenland  is 
buried  and  that  which  covers  the  Antarctic  land.  This  is  a  type 


158 


SNOW  AND   ICE 


of  especial  interest  and  significance  to  the  geologist,  because  of  the 
light  which  it  throws  upon  the  often  mysterious  operations  of  the 
ice-sheets  which  once  covered  large  portions  of  North  America  and 
Europe. 

Glacier  Erosion  is  highly  characteristic,  and  enables  us  to  detect 
the  former  extension  of  ice-streams  which  have  greatly  shrunken 
and  their  former  presence  in  regions  whence  they  have  long  van- 


FlG.  67. —  Glaciated  surface,  Sierra   Nevada,  Cal.     The  angular  recesses  indicate 
places  where  blocks  have  been  removed  by  plucking.     (U.  S.  G.  S.) 

ished.  The  erosive  capabilities  of  moving  ice  have  been  and 
still  are  the  subject  of  much  dispute,  but  the  researches  of  the  last 
ten  or  fifteen  years  in  many  parts  of  the  world  have  brought 
together  a  great  body  of  evidence  which  strongly  supports  the  view 
that  glaciers,  under  favouring  conditions,  are  extremely  efficient 
and  powerful  agents  of  erosion.  Just  as  in  the  case  of  water,  the 
destructive  power  of  ice  depends  upon  the  velocity  with  which  it 
moves  and  the  pressure  exerted  on  its  bed,  so  that  a  glacier  may 


GLACIER   EROSION 


159 


erode  actively  at  one  part  of  its  course,  and  little  or  not  at  all  at 
another.  It  is  not  surprising,  therefore,  that  advancing  glaciers 
have  been  observed  to  override  loose  masses  of  gravel  without 
moving  them. 

When  acting  effectively,  newly  formed  glaciers  remove  the  soil, 
talus,  and  other  loose  materials  from  the  surface,  which  thus,  in  the 
first  instance,  is  rendered  more  irregular  than  before,  because 


FIG.  68.  —  Steeply  inclined  strata,  with  edges  roughened  by  glacial  plucking,  overlaid 
by  glacial  drift,  Iron  Mt.,  Mich.     (U.  S.  G.  S.) 


of  the  varying  depths  to  which  the  effects  of  weathering  penetrate 
(see  p.  101).  Bare  rocks  are  eroded  by  a  double  process :  (i)  The 
joint-blocks  are  torn  away  (plucking)  by  the  advancing  ice,  an 
operation  which  is  much  facilitated  by  the  continual  liquefaction 
and  regelation  of  the  ice  at  the  bottom  of  the  glacier,  owing  to 
changes  of  pressure.  As  has  been  already  remarked,  the  tempera- 
ture of  the  ice  within  and  at  the  bottom  of  the  glacier  is  near  the 


i6o 


SNOW  AND   ICE 


melting-point  for  the  pressure.  Thus,  slight  changes  of  pressure, 
due  to  the  motion  of  the  glacier  and  inequalities  of  the  bed,  cause 
the  ice  now  to  melt  and  again  to  solidify,  and  in  this  manner 
the  joint-blocks  of  the  bed-rock  are  loosened.  (2)  The  bottom  of 
the  glacier  is  a  mass  of  ice  mingled  with  recks,  pebbles,  boulders, 
sand,  and  debris  of  all  sizes,  and  by  their  means  the  bed-rock  is 
worn  down,  smoothed,  polished,  and  scored  with  parallel  marks, 


FIG.  69.  —  Glacial  striae  on  limestone,  overlaid  by  drift ;  Pillar  Point,  Lake 
Ontario.     (U.  S.  G.  S.) 

in  a  fashion  which  forms  the  unmistakable  autograph  of  the  glacier. 
The  rock  fragments,  firmly  held  by  the  immense  weight  of  the  ice, 
are  slowly  pushed  over  the  rocky  bed  and  cut  grooves  correspond- 
ing to  the  size  of  the  fragments,  from  hair-like  scratches  to  deep 
troughs.  The  scorings  are,  of  course,  in  the  direction  of  the  move- 
ment and  keep  parallel  often  for  considerable  distances. 
The  smaller  particles  act  as  a  polishing  powder  and  smooth  the 


GLACIER  EROSION 


161 


bed,  and,  if  the  bed-rock  is  sufficiently  hard,  it  receives  a  high 
polish.  Hummocks  of  rock,  over  which  the  ice  has  flowed,  are 
worn  and  rounded  into  the  form  called  "  roches  moutonnees," 
with  the  upstream  side  gently  sloping  and  polished,  but  with  the 
downstream  side  abrupt  and  often  rough. 


FIG.  70. — Ancient  glacial  striae  (Permian);  Riverton,  on  the  Vaal  River,  South 
Africa.     (R.  B.  Young) 

As  in  the  case  of  the  river,  the  abrading  material  is  itself  abraded 
in  its  journey,  and  much  of  it  is  ground  to  the  rock  powder  which 
heavily  loads  the  stream  flowing  from  the  foot  of  the  glacier.  The 
pebbles  and  boulders  are  scratched  with  parallel  or  intersecting 
lines,  smoothed  and  polished,  but,  as  they  are  not  rolled  over  and 
over  like  river  pebbles,  they  are  not  spheroidal  in  shape,  but  are 
more  or  less  angular  and  sometimes  facetted,  with  smooth  faces 
which  meet  at  a  distinct  angle.  This  peculiar  shape  is  due  to  the 


1 62  SNOW  AND   ICE 

shifting  or  turning  of  the  pebble  in  the  ice,  so  that,  after  one  side 
is  worn  flat,  another  is  similarly  worn,  and  this  may  be  several 
times  repeated. 

The  amount  of  debris  produced  and  carried  by  a  glacier  is  often 
very  great.  In  summer  the  glaciers  of  the  Justedal  in  Norway 
together  bring  down  nearly  2,400,000  cubic  yards  of  material  daily. 

If  glaciers  are  powerful  eroding  agents  and  not  merely  a  means 
of  rounding  and  polishing  the  rocks,  we  should  expect  to  find  that 


FIG.  71.  —  Glacial  grooves  on  sandstone  cliff;  Delaware  Water  Gap,  Pa. 

glaciated  regions  display  topographical  features  not  to  be  found 
elsewhere,  and  this  is  actually  the  case.  Not  only  are  rounded  and 
flowing  outlines  and  forms  produced,  in  marked  contrast  to  the 
craggy  and  angular  shapes  due  to  ordinary  weathering,  but  the 
character  of  glacially  excavated  valleys  is  extremely  significant, 
when  compared  with  that  of  river  valleys  (see  p.  140). 

(i)  In  cross-section  a   glacial  valley  is  U-shaped,  with  broad 
bottom  and  steep  or  vertical  sides.     If  the  upper  portion  of  the 


GLACIER  EROSION 


163 


valley  was  not  occupied  by  the  ice,  the  slopes  may  be  gradual 
down  to  the  former  level  of  the  ice,  where  they  become  abrupt. 
(2)  Glacial  valleys  are  often  straight  and  open  for  long  distances; 
they  may  have  spurs  alternating  from  the  opposite  sides,  but 
these  spurs  are  truncated  by  the  ice,  and,  if  the  action  persisted 
sufficiently  long,  the  spurs  will  have  been  entirely  removed.  (3) 
The  tributary  valleys  do  not  enter  the  main  valley  at  grade,  as 


FIG.  72.  — U-shaped  glacial  valley;  Kern  Canon,  Cal.     (U.  S.  G.  S.) 

they  normally  do  in  the  case  of  rivers,  but  enter  at  the  sides  of  the 
main  valley  much  above  its  bottom  and  hence  are  called  hanging 
valleys.  The  explanation  of  this  peculiar  arrangement  is  that  the 
smaller,  tributary  glaciers  could  excavate  their  beds  much  less 
rapidly  than  the  trunk  ice-stream.  "  Although  there  is  no  uni- 
form height  at  which  these  side  valleys  enter  the  main  trough, 
in  general  it  is  true  that,  the  smaller  the  tributary  valley,  the 


1 64  SNOW  AND   ICE 

higher  its  mouth  lies  above  the  main  valley  bottom."  (Tarr.) 
Hanging  valleys  may  be  formed  in  other  ways  than  by  glaciers, 
but  while  they  are  common  in  glaciated  regions,  elsewhere 
they  are  only  occasional.  (4)  Glaciers  may  erode  their  valleys 
below  sea-level,  because  they  exclude  the  sea  water,  as  rivers 
cannot  do. 

Glaciers  flowing  from  high  mountains  head  usually  in  large 
amphitheatres,  or  "  cirques,"  caused  by  the  lateral  plucking  of  the 
rocky  walls  by  the  fields  of  ice  and  snow.  Each  cirque  slowly  re- 
treats upward,  thus  often  reducing  the  divides  between  them  to 
extremely  sharp  "  knife-edges,"  as  is  conspicuously  displayed  in 
the  Sierra  Nevada  of  California. 

The  part  of  a  glacier  which  descends  below  the  snow-line  is  in 
summer  exposed  to  continual  melting  and  may  be  more  or  less 
completely  covered  with  running  water.  Surface  streams  flow 
in  channels,  which  they  melt  for  themselves,  until  they  meet  with 
a  crevasse,  down  which  they  pour  in  cataracts.  As  crevasses  are 
continually  formed  at  the  same  spot  (see  p.  153),  such  a  cataract 
may  remain  stationary  for  a  long  period  and  wear  out  a  cylindri- 
cal pot-hole,  exactly  as  is  done  by  a  cascade  in  a  stream. 

Glacier  Transportation.  — The  transporting  power  of  a  glacier 
is- not  determined  by  its  velocity,  at  least  so  far  as  the  material 
carried  on  its  surface  is  concerned.  This  is  because  the  rocks 
may  be  regarded  as  floating  bodies  with  reference  to  the  ice,  and 
thus  a  rock  weighing  many  tons  is  carried  with  as  mu^h  ease  as  a 
grain  of  sand.  The  masses  of  material  transported  by  a  glacier 
are  known  as  moraines.  The  moraines  which  are  carried  on  the 
top  of  the  glacier  are  derived  from  the  cliffs  and  peaks  which 
overhang  the  ice,  and  the  action  of  frost  and  land-slips  is  con- 
tinually showering  down  earth,  sand,  and  rocks  of  all  sizes,  from 
small  blocks  up  to  masses  the  size  of  houses.  This  material  is 
heaped  up  along  the  sides  of  the  glacier  in  disorderly  array,  and 
here  forms  the  lateral  moraines.  When  a  glacier  is  composed  of 
branch  streams,  it  will  have  a  corresponding  number  of  medial 
moraines  (see  Fig.  65),  in  the  middle  of  the  glacier.  When  two 


GLACIER  TRANSPORTATION 


i6S 


branches  unite,  their  coalesced  lateral  moraines  form  a  single 
medial  moraine. 

The  quantity  of  material  thus  carried  on  the  top  of  the  glacier 
depends  upon  the  amount  of  rock  surface  which  extends  above 
the  level  of  the  ice  and  is  subject  to  the  action  of  the  ice  and 
the  atmosphere.  In  the  Alps,  where  the  glaciers  flow  in  deep 
ravines,  the  moraines  are  large,  and  some  of  the  great  Alaskan 


FIG.  73.  —  Front  of  Bowdoin  Glacier,  Greenland.    The  dark  bands  are  made  by 
englacial  drift.     (Photograph  by  Libbey) 

glaciers  have  their  lower  reaches  so  covered  with  rubbish,  that 
the  ice  is  visible  only  in  the  crevasses.  In  Greenland,  on  the 
contrary,  the  inland  ice-cap  has  very  little  material  on  its  surface, 
because  only  scattered  nunataks  rise  above  it. 

The  bottom  part  of  the  glacier  is  a  confused  mass  of  ice,  stones, 
etc.,  and  this  debris  is  the  ground  moraine,  which  is  to  be  regarded, 
not  as  so  much  material  pushed  along  between  the  ice  and  the  rocky 


1 66  SNOW  AND  ICE 

bed,  but  as  an  integral  part  of  the  glacier.  At  the  foot  or  end  of  the 
glacier  is  the  terminal  moraine  (see  Fig.  62),  where  all  the  materials 
carried  are  dumped  in  a  promiscuous  heap,  except  so  much  as  is 
swept  away  by  the  stream  of  water.  Besides  the  moraines  proper, 
there  is  a  certain  amount  of  englacial  drift,  carried  in  the  body  of 
the  ice.  This  is  derived  from  debris  that  comes  from  the  surface, 
but  does  not  work  its  way  entirely  to  the  bottom,  as  well  as  from 
that  which  gathers  upon  the  surface  of  the  snow  or  neve  and  is 
covered  up  by  subsequent  snowfalls.  The  materials  carried  by 
a  glacier  are  as  characteristic  as  the  marks  left  upon  the  rocks 
over  which  the  ice  has  flowed.  Aside  from  the  substances  swept 
along  by  the  subglacial  stream,  the  various  fragments  are  not 
rounded  and  water-worn,  as  is  the  sediment  of  rivers.  The  mo- 
raines on  the  top  of  the  ice  (lateral  and  medial)  are  little  or  not 
at  all  abraded,  but  are  deposited  as  angular  blocks  and  frag- 
ments. The  ground  moraine,  on  the  other  hand,  is  abraded  in 
the  peculiar  way  already  described.  In  all  this  work  of  glacial 
denudation  the  process  is  entirely  mechanical,  —  chemical  de- 
composition plays  no  part  in  it. 

Certain  other  forms  of  transportation  by  ice  may  be  conven- 
iently mentioned  here. 

Ground  Ice  forms  in  rivers  and  ponds  on  the  bottom,  freezing 
around  stones  and  boulders,  and  when  broken  up  by  thaws,  this 
ice  may  float  for  long  distances,  carrying  with  it  burdens  far 
greater  than  the  stream  which  transports  the  ice  could  carry 
unassisted.  The  shores  of  the  St.  Lawrence  River  are  fringed 
with  lines  of  large  boulders  which  have  thus  been  brought  down. 

Lake  Ice  produces  some  curious  effects  in  northern  regions. 
When  the  lake  is  covered  with  cakes  of  ice  as  the  result  of  an  early 
thaw,  refreezing,  by  expanding  the  water  between  the  floating 
blocks,  causes  the  ice  to  press  strongly  upon  the  shore.  In  case 
the  lake  beach  is  covered  with  boulders,  the  push  of  the  ice  heaps 
up  the  boulders  into  a  ring  wall. 

Coast  Ice.  —  In  Arctic  regions  the  shallow  water  along  the  coast 
is  frozen  in  winter  into  a  broad  shelf  of  ice  called  the  ice-foot.  In 


THE   SEA  167 

the  spring  land-slips  cover  the  ice  with  debris,  while  the  bottom  is 
studded  with  stones  and  pebbles.  When  the  ice-foot  is  broken  up 
in  summer,  part  of  it  is  drifted  away  and  transports  its  load  of  rock 
for  long  distances.  Other  parts  are  worked  backward  and  forward 
by  the  waves  and  tides,  scoring  the  rocks  of  the  coast  and  grind- 
ing and  polishing  the  fragments  of  rock  frozen  in  the  ice,  in  much 
the  same  fashion  as  glacial  pebbles  are  scored  and  ground.  Over 
comparatively  limited  areas  the  marks  of  coast  ice  often  have  a 
deceptive  resemblance  to  those  left  by  glaciers. 

Icebergs.  —  When  a  glacier  enters  the  sea,  it  ploughs  along  the 
bottom  until  the  buoyant  power  of  the  water  breaks  off  great  frag- 
ments of  it,  which  float  away  as  icebergs.  These  are  often  of 
gigantic  size,  veritable  islands  of  ice,  and  huge  as  they  appear,  only 
about  one-ninth  of  their  bulk  is  above  water.  As  icebergs  are 
derived  from  glaciers,  they  carry  away  whatever  debris  the  parent 
glacier  had  upon  or  within  it. 

2.  THE  SEA 

The  destructive  work  of  the  sea  is  accomplished  mainly  by 
means  of  the  waves  which  the  wind  raises  upon  its  surface  and  by 
wind  and  tidal  currents.  The  great  ocean  currents  are,  as  a  rule, 
so  far  from  shore,  and  flow  in  such  deep  water,  that  their  erosive 
power  is  comparatively  small.  The  Gulf  Stream  is  said  to  scour 
the  bottom  in  the  Florida  Straits  and  off  the  Carolina  coast,  but  this 
is  exceptional. 

Waves  act  continually  upon  all  coasts,  but  with  very  different 
force  at  different  times  and  places.  According  to  observations 
made  for  the  Scotch  Lighthouse  Board,  the  average  wave  pres- 
sure on  the  coast  of  Scotland  is  for  the  five  summer  months  6n 
pounds  per  square  foot,  and  for  six  winter  months  2086  pounds. 
These  are  average  figures  and  are  greatly  exceeded  in  storms,  when 
the  force  of  the  breakers  often  rises  to  many  tons  per  square  foot. 

The  effect  produced  by  this  great  force  depends  upon  the  char- 
acter of  the  rocks  of  the  coast,  its  height,  and  the  angle  at  which 


1 68 


THE  SEA 


it  rises  out  of  the  water;  also,  in  the  case  of  stratified  rocks,  upon 
the  attitude  of  the  beds,  whether  they  are  horizontal,  or  inclined 
toward  or  away  from  the  sea.  When  the  coast  is  high,  steep,  and 
rocky,  the  waves  continually  wear  away  its  base,  partly  by  dis- 
lodging the  blocks  into  which  all  consolidated  rocks  are  divided, 
and  partly  by  using  as  projectiles  the  blocks  which  it  has  dis- 


FlG.  74.  —  Wave  erosion ;  Etretat,  France 

lodged,  or  which  have  been  loosened  by  the  frost.  In  heavy  gales 
great  masses,  weighing  tons,  it  may  be,  are  hurled  with  tremendous 
violence  against  the  base  of  the  cliffs,  cutting  them  into  caverns, 
which  are  further  excavated  by  the  ordinary  surf.  Eventually, 
the  cliff  is  undermined,  and  the  unsupported  masses  above  fall  in 
ruins. 

Waves  are  not  so  entirely  dependent  for  their  effectiveness  as 


THE  SEA 


169 


rivers  are  upon  the  hard  materials  which  they  dash  upon  the 
coast  for  their  efficiency  as  destructive  agents.  The  force  of  the 
mere  blow  given  by  a  storm  breaker  is  very  great,  and  the  hydro- 
static pressure  which  first  forces  the  water  into  every  fine  crevice 
of  the  rock,  and  then  withdraws  it,  together  with  the  sudden 
compression  and  reexpansion  of  the  air  contained  in  these  fissures, 
assists  materially  in  the  loosening  of  the  blocks. 


FlG.  75.  —  Wave-cut  arch,  coast  of  California.     (U.  S.  G.  S.) 

Along  coasts  which  are  composed  of  hard  rocks  the  work  of 
cutting  back  the  land  by  the  sea  is  comparatively  slow,  but  when 
the  rocks  are  soft  and  yielding,  and  yet  rise  abruptly  from  the 
ocean,  the  waste  is  so  rapid  as  to  attract  every  one's  attention. 
The  coast  of  Yorkshire  in  England  is  washed  away  at  an  average 
rate  of  nearly  seven  feet  per  annum.  The  island  of  Heligoland, 
near  the  German  coast,  has  suffered  great  loss  from  the  attacks  of 
the  sea  within  historic  times;  the  small  eastern  island  was  cut  off 
from  the  larger  island,  Heligoland  proper,  by  a  great  storm  in  1 720. 


170 


THE  SEA 


At  Long  Branch,  New  Jersey,  the  sandy  bluffs  must  be  artificially 
protected  against  the  attacks  of  the  sea ;  yet  in  spite  of  such  protec- 
tion, almost  every  severe  gale  does  considerable  damage. 

Sandy  coasts  which  are  low-lying  and  flat  often  suffer  less  from 
the  inroads  of  the  sea  than  rocky  and  precipitous  ones,  especially 
as  they  are. apt  to  be  lines  along  which  material  is  accumulating. 
Even  such  coasts  may,  however,  be  rapidly  cut  back,  as  is  shown 


FIG.  76.  —  Wave  erosion,  strata  dipping  seaward;  Orkney  Islands,  Scotland 

in  the  familiar  example  of  Coney  Island,  where  great  damage  has 
been  done  of  late  years.  When  the  sea  is  eating  away  a  sandy 
shore,  the  homogeneous  material  prevents  the  occurrence  of  such 
irregularities  of  the  coast-line  as  occur  in  rocky  districts.  So  long 
as  the  coast  is  neither  elevated  nor  depressed,  the  surf  cuts  it  back 
at  a  continually  decreasing  rate,  because  the  retreat  of  the  coast- 
line leaves  a  shelf  covered  with  shallow  water,  in  passing  over 


THE  SEA 


I/I 


which  the  waves  are  retarded  by  friction  and  strike  the  shore 
with  greatly  diminished  force.  Just  how  far  such  a  coast  may  be 
cut  back  is  not  definitely  known,  but  it  probably  does  not  exceed 
a  few  miles,  at  most.  On  the  other  hand,  if  the  land  in  question 
be  slowly  sinking,  the  sea  gains  a  great  advantage  and  may  con- 
tinue its  destructive  work  indefinitely.  Indeed,  several  high 
authorities  are  of  the  opinion  that  this  is  the  only  method  by 


FIG.  77.  —  Wave  erosion,  strata  dipping  landward ;  Duncansby  Head,  Orkney  Islands 

which  great  areas  can  be  planed  down  to  an  approximately  level 
surface.  Again,  it  should  be  noted  that  when  the  sea  is  advancing 
over  an  ancient  land  surface,  it  finds  ready  to  hand  an  immense 
body  of  soft  materials  which  are  speedily  removed.  The  deep 
decay  of  the  rocks  into  soil  and  the  deposits  made  by  the  wind,  rivers, 
lakes,  etc.,  have  all  prepared  the  way  for  the  erosive  action  of  the 


172 


THE   SEA 


sea.     When  the  coast  is  elevated,  the  sea  cuts  a  succession  of 
terraces. 

Besides  cutting  back  its  shores,  the  sea  continually  grinds  up  the 
material  which  is  brought  into  it  by  the  rivers,  and  that  which  it 
obtains  by  its  own  wear  of  the  coast.  The  great  blocks  on  the 
shore  are  rolled  about  in  storms,  and  worn  into  rounded  boulders, 
which  are  gradually  reduced  to  smaller  and  smaller  size.  All  the 


FIG.  78.  —Joint-block  partly  dislodged  by  the  surf  on  wave-cut  terrace;  Wick, 
Orkney  Islands 

minerals  softer  than  quartz  are  rapidly  ground  into  fine  particles 
and  swept  away  by  the  undertow  into  deeper  and  quieter  waters, 
leaving  the  larger  quartz  fragments  "to  form  the  pebbles  and  sand 
of  the  beach. 

The  action  of  the  waves  is  limited  vertically,  ceasing  to  be  effec- 
tive in  quite  shallow  water,  not  far  below  the  low-tide  mark.  In 
violent  storms  the  waves  often  accomplish  much  destruction  far 


THE   SEA 


173 


above  high  tide,  but  the  principal  work  of  the  waves  is  confined  to 
a  belt  extending  from  a  little  above  high  tide  to  somewhat  below 
low  tide.  However,  Graham  Island,  near  Sicily  (see  p.  67), 
which  was  thrown  up  in  1831,  has  been  so  completely  removed 
by  the  waves  that  not  even  a  shoal  remains.  Below  the  low- 
water  mark  the  wave  work  is  often  efficiently  supplemented  by 
tidal  currents,  which  under  favourable  circumstances  acquire 
great  velocity  and  depth,  scouring  away  loose  materials  and 
even  cutting  into  solid  rock.  When  an  island  of  considerable 


FlG.  79.  —  Igneous  rock,  corroded  by  sea-water,  about  i/2  natural  size.     (Photo- 
graph by  van  Ingen) 

extent  is  exposed  to  the  incoming  tide,  the  latter  travels 
around  the  island  in  both  directions,  and  if  the  shape  of  the 
mainland  is  favourable,  one  of  these  currents  will  be  much  higher 
than  the  other,  which  will  produce  a  "  race  "  between  the  island 
and  the  mainland.  Hell  Gate,  New  York,  is  an  example  of  this; 
the  tide  advances  through  New  York  Bay  and  Long  Island  Sound, 
being  higher  at  flood,  lower  at  ebb,  in  the  sound  than  in  the  bay. 
The  consequence  is  a  swift  current  into  the  bay  at  flood-tide  and 


LAKES 

into  the  sound  at  ebb.  The  floor  of  the  British  Channel,  over 
which  the  tidal  currents  run  very  rapidly,  is  swept  bare  of  sand, 
which  is  carried  into  the  North  Sea.  By  such  means  as  this,  the  sea 
cuts  away  the  land  to  depths  much  greater  than  unassisted  waves 
can  effectively  reach. 

Rocks  are  also  attacked  chemically  by  the  solvent  and  decom- 
posing action  of  sea-water.  The  silicates,  such  as  .the  felspars, 
augite,  hornblende,  etc.,  are  attacked  much  more  rapidly  than  in 
fresh  water.  In  shoal  water  and  on  the  shore  this  action  is  obvious 
only  in  spots  sheltered  from  the  direct  assault  of  the  waves,  because 
the  products  of  decomposition  are  immediately  removed  by  the  surf 
and  the  mechanical  work  is  so  much  more  striking.  Limestone 
coasts  suffer  from  solution  by  sea-water,  and  are  characterized  by 
long  caverns  and  tunnels,  though  sea  caves  are  worn  by  the  surf  in 
all  classes  of  rocks  In  the  profound  depths  of  the  oceanic  basins, 
where  the  water  is  never  disturbed  and  where  its  motion  is  ex- 
tremely slow,  chemical  activity  becomes  relatively  very  impor- 
tant. Calcareous  shells  are  completely  dissolved,  and  the  volcanic 
debris,  which  covers  the  sea-bottom  over  vast  areas,  is  disinte- 
grated into  a  characteristic  red  clay. 

3.   LAKES 

In  comparison  with  the  long  life  of  the  earth,  lakes  must  be 
regarded  as  merely  temporary  bodies  of  water,  which  '.rill  sooner 
or  later  disappear,  either  by  being  drained  of  their  waters  or  by 
being  filled  up  with  the  sediments  which  are  washed  into  them. 
The  general  term  lake  is  employed  for  any  inland  body  of  water, 
which  does  not  form  part  of  the  sea,  but  lakes  are  formed  in  very 
different  ways  and  have  correspondingly  different  histories.  Most 
lakes  occupy  depressions  below  the  general  drainage  level  of 
the  country,  whether  these  depressions  be  due  to  movements  of  the 
earth's  crust,  to  glacial  excavations,  to  unequal  decomposition  by 
the  atmospheric  agencies,  or  to  some  other  factor.  Others,  again, 
are  held  back  by  dams,  such  as  lava  streams,  glacial  moraines,  or 


LAKES 


175 


the  glaciers  themselves,  by  the  debris  of  land-slips,  or  by  the  deltas 
of  tributary  streams  which  bring  in  more  material  than  the  main 
river  can  dispose  of.  Others  still  are  enlarged  basins  cut  out  by 
rivers.  Great  lakes  that  persist  for  long  periods  of  time  are  con- 
tained in  basins,  often  of  great  depth,  which  were  formed  by 
movements  of  the  earth's  crust;  the  other  kinds  are  more  evanes- 
cent and  usually  of  rather  small  size. 


FIG.  80.  —  Wave-cut  bluff  on  Lake  Ontario.     (U.  S.  G.  S.) 

Small  lakes  accomplish  very  little  in  the  way  of  rock  destruction, 
but  are  rather  places  of  accumulation.  The  waves,  even  in  storms, 
are  not  heavy  enough  effectively  to  cut  back  the  shores,  while  the 
current  of  water  through  the  lake  is  too  slow  and  the  sediment 
transported  too  small  and  light  to  erode  the  bottom  as  a  river 
does.  In  great  lakes,  such  as  those  which  drain  into  the  St. 


1/6  ORGANIC  AGENCIES 

Lawrence,  storms  develop  a  very  heavy  surf,  and  such  lakes 
eat  into  their  shores  as  the  ocean  does,  but  the  very  small  tide 
confines  the  work  of  the  waves  within  narrower  limits,  and  the 
lighter  breakers  are  less  effective.  Lakes  are  subject  to  various 
accidents  which  cause  great  fluctuations  of  the  water-level.  De- 
serted shore-lines  are  marked  by  beaches  and  terraces.  The 
method  of  denudation  by  lakes  is  the  same  as  that  of  the  sea, 
but  the  modes  of  accumulation  of  material  are  characteristically 
different. 

4.  ORGANIC  AGENCIES 

The  organic  agencies  are  animals  and  plants,  both  living  and 
after  death.  In  some  respects  these  agencies  tend  to  counteract 
the  destructiveness  of  others,  and  the  protective  effects  may  be 
taken  up  first. 

(i)  Protective  Effects.  — These  have  already  been  considered 
in  part,  in  connection  with  the  processes  of  weathering  (see  p.  100). 
The  protective  effects  of  organisms  are  almost  entirely  those  of 
plants,  since  animals,  on  land  at  least,  are  not  sufficiently  abundant 
to  be  of  any  importance  in  this  connection.  A  thick  covering 
of  vegetation,  especially  the  elastic,  matted  roots  of  grassy  turf, 
protects  the  soil  against  the  mechanical  wash  of  rain.  How  com- 
plete this  protection  often  is,  may  be  seen  in  the  different  effects 
produced  by  a  heavy  fall  of  rain  upon  a  grass  field  and  on  the 
adjoining  ploughed  lands,  or  even  on  the  roads.  The  roads  may 
be  so  washed  out  as  to  be  impassable,  while  the  grass  fields  have 
not  suffered  at  all.  In  certain  of  the  western  bad  lands,  the  effi- 
cient protection  given  by  grass  is  very  well  shown;  where  the  grass 
has  established  itself  thickly,  the  country  is  gently  rolling,  but  where 
it  is  absent,  the  wild  and  broken  bad  lands  are  developed. 

Forests  also  are  very  important  conservers  of  the  soil,  especially 
on  mountain  sides  and  other  steep  slopes.  The  removal  of  forests 
only  too  often  is  followed  by  calamitous  results. 

Vegetation,  especially  grass,  protects  loose,  light  soils  from  the 
wind,  and  often  this  is  the  only  means  by  which  sand  dunes  can 


ORGANIC  AGENCIES 


177 


de  held  in  place  and  prevented  from  overwhelming  valuable  lands. 
Even  the  banks  of  rivers  and  the  seacoast  may  be  efficiently  pro- 
tected by  plants.  Dense  masses  of  seaweed  growing  on  the  rocks 
form  an  elastic  buffer  against  the  surf,  and  along  low-lying  tropical 


FIG.  81.  —  Erosion  following  removal  of  forest ;  Great  Smoky  Mts.,  Tenn. 
(U.S.  Bureau  of  Forestry) 

coasts  the  mangrove  trees,  with  their  interlacing  aerial  roots,  so 
break  the  force  of  the  waves  that  they  cannot  wash  away  even 
fine  mud. 
The  only  protection  afforded  by  animals  that  requires  mention 


ORGANIC  AGENCIES 


is  in  the  case  of  coral  reefs,  which,  thrown  up  along  or  parallel 
to  the  coast,  shield  it  from  the  heaviest  surf. 

(2)  The  Destructive  Effects  of  the  organic  agencies  are  decidedly 
subordinate  to  those  of  the  other  classes  which  have  so  far  been 
considered,  but  they  are  not  without  importance.  The  products 
of  vegetable  decomposition  in  bogs  and  in  beds  of  clay,  muds  on 
the  sea-bottom,  etc.,  are  efficient  means  of  chemical  change,  and 


FIG.  82.  —  Soil  destruction  due  to  removal  of  forest ;    Mitchell  Co.,  N.C. 
(U.S.  Bureau  of  Forestry) 

observations  show  that  the  decay  of  animals  in  the  deep  sea  is  an 
agent  of  no  mean  importance  in  promoting  the  chemical  changes 
which  there  take  place.  But  even  living  animals  and  plants  do 
an  important  work  in  disintegrating  rocks,  that  should  not  be  over- 
looked. Bacteria  play  a  considerable,  but  not  yet  fully  known,  part 
in  the  surface  decomposition  of  rocks  and  soils.  Certain  of  these 
microscopic  plants  have  the  power  of  fixing  the  atmospheric  nitro- 
gen and  converting  it  into  nitric  acid,  while  others  are  the  indispen- 


ORGANIC  AGENCIES  179 

sable  agents  of  organic  decomposition.  Seeds  germinating  in  the 
crevices  of  rocks,  or  the  roots  of  trees  which  invade  such  crevices 
from  above,  wedge  the  rocks  apart  with  the  same  irresistible  power 
as  is  displayed  by  frost,  and  often  large  areas  of  rock  are  thus  most, 
effectively  broken  up.  The  roots  of  living  plants  also  secrete  an 
acid,  which  dissolves  out  some  of  the  soluble  constituents  of  rock, 
thus  adding  a  chemical  activity  to  the  wedge-like  mechanical 
effects  of  growth. 

Many  marine  animals  bore  into  rocks,  even  the  -hardest,  and 
cause  them  to  crumble,  and  on  the  land  great  numbers  of  animals 
continually  bore  and  tunnel  through  the  soil,  allowing  a  freer 
access  of  air  and  water.  In  the  tropics  the  soil  is  fairly  alive  with 
the  multitude  of  burrowers.  Earthworms  are  among  the  most 
important  agents  in  work  of  this  kind,  and  the  last  of  Mr.  Dar- 
win's books  was  a  most  interesting  one  upon  the  geological  work 
of  worms.  The  worms  swallow  quantities  of  earth,  for  the  sake 
of  the  organic  matter  which  it  contains,  and  grind  it  exceedingly 
fine  in  their  muscular  gizzards.  This  ground-up  soil  is  always 
deposited  on  the  surface,  in  the  form  of  the  coiled  "  worm-cast- 
ings," so  abundant  in  grassy  places.  Worms  are  thus  continually 
undermining  the  soil,  bringing  up  material  from  below  and  depos- 
iting it  on  the  surface,  while,  by  the  collapse  of  the  old  burrows, 
the  first  surface  gradually  sinks.  In  England  the  material  thus 
yearly  brought  to  the  surface  varies  from  seven  to  eighteen  tons 
per  acre,  which  means  an  average  annual  addition  of  one-tenth 
to  one-sixth  of  an  inch.  By  this  means  the  surface  of  the  ground 
is  constantly  changed,  and  substances  spread  over  the  ground 
in  the  course  of  years  make  their  way  down  into  it,  forming  well- 
defined  layers  beneath  the  surface.  In  the  tropics  ants  and 
termites  (so-called  white  ants)  are  even  more  active  than  worms  in 
tunnelling  the  soil,  and  in  many  semi-arid  plains  burrowing 
mammals  in  incredible  multitudes  are  continually  working  over 
the  soil  to  great  depths,  as  in  the  prairie-dog  villages  of  our  western 
plains.  The  occasional  heavy  rains  thus  penetrate  to  depths  which 
could  not  otherwise  be  reached. 


ISO  .    SUMMARY  OF  DESTRUCTIVE  ACTION 

Summary  of  Destructive  Action.  — The  surface  of  the  land  is 
everywhere  attacked  by  the  universally  present  atmosphere  at  a  rate 
which  differs  much  in  different  regions,  depending  upon  climate, 
elevation  above  sea-level,  and  the  resistant  power  of  the  rocks.  The 
rain  chemically  decomposes  the  rocks,  converting  them  into 
soil,  and  mechanically  washing  this  soil  to  lower  levels  and  into 
the  streams.  Frost  shatters  the  rocks  into  smaller  and  smaller 
fragments.  In  arid  regions  the  extreme  changes  of  temperature 
break  up  the-  rocks  much  as  does  the  expansive  force  of  freezing 
water,  while  the  wind  transports  immense  volumes  of  sand  and 
dust,  which  cut  and  carve  and  wear  away  the  exposed  rocks. 
Underground  waters,  especially  when  heated,  do  an  important 
work  of  solution  and  decomposition,  and,  under  favourable  cir- 
cumstances, cause  the  dislodgment  of  great  masses  of  earth  and 
rock  in  land-slips  and  rock-slides.  Rivers  excaVate  valleys  and 
serve  as  the  great  agents  of  transportation,  bearing  the  waste  of  the 
land  to  the  sea,  and  glaciers  do  similar  work  in  a  highly  charac- 
teristic manner.  The  sea  cuts  into  its  coasts  by  the  action  of 
waves,  deepening  its  bed  in  shallow  places  by  tidal  currents,  and  in 
the  case  of  a  slowly  sinking  land  may  plane  down  great  areas  to  a 
flat,  gently  sloping  surface.  Animals  and  plants  add  an  important 
quota  to  the  general  work  of  destruction. 

The  annual  waste  of  the  land  at  the  present  time  is  estimated  at 
20  cubic  kilometers  (Penck),  and,  in  past  times,  an  incalculably 
great  amount  of  material  has  been  removed  from  the  land.  The 
Appalachian  Mountain  system  has  thus  lost  thicknesses  of  rock 
which  vary  in  different  regions  from  8000  to  20,000  feet,  and  it  is 
altogether  probable  that  the  average  waste  of  all  the  continents 
amounts  to  several  thousands  of  feet.  The  figures  given  for  the 
basins  of  the  Mississippi  and  Ganges  show  that  such  waste  implies 
enormously  long  periods  of  time. 


CHAPTER   VII 

RECONSTRUCTIVE   PROCESSES.  —  CONTINENTAL    DE- 
POSITS, LAND,  SWAMP,  AND   RIVER 

WE  have  now  to  inquire  what  becomes  of  the  material  which  is 
derived  from  the  decomposition  and  disintegration  of  the  rocks. 
At  the  present  time,  it  is  estimated,  about  one-half  of  the  waste 
of  the  land  is  carried  directly  into  the  sea,  while  the 
remainder  is  arrested  in  its  journey  and  deposited  upon  the 
land.  It  must  be  remembered,  however,  that  when  the  sea  ad- 
vances over  the  land,  these  deposits  are,  to  a  large  extent,  rapidly 
worked  over  by  the  waves  and  converted  into  marine  deposits. 
The  accessible  rocks  of  the  earth's  crust  are  more  largely  composed 
of  marine  deposits  than  of  those  laid  down  in  other  ways,  yet  the 
non-marine  sedimentary  rocks  are  also  extensively  represented. 
It  is  only  quite  lately  that  the  importance  of  this  latter  class  of 
rocks  Has  been  appreciated.  Furthermore,  their  importance  is 
not  merely  quantitative,  but  lies  also  in  the  help  which  they  give 
in  the  determination  of  ancient  land  surfaces,  lake  beds,  river 
channels,  ice-fields,  and  the  like.  It  is  therefore  necessary  to 
study  all  the  methods  by  which  rock  reconstruction  is  effected,  on 
however  small  a  scale. 

The  most  natural  primary  division  of  the  sedimentary  accumu- 
lations is  into  the  marine  and  the  continental,  including  in  the 
latter  the  deposits  which  are  made  upon  the  land,  or  in  such  bodies 
of  water  as  are  not  parts  of  the  sea.  Between  these  two  principal 
classes  there  is  a  transitional  series,  consisting  of  deposits  laid 
down  in  bodies  of  salt  water  which  are  in  tidal  connection  with  the 
sea,  such  as  estuaries,  almost  closed  bays  and  sounds,  or  seas,  like 
the  Baltic ,  which  are  partly  brackish,  as  well  as  the  littoral,  or 

181 


1 82  RECONSTRUCTIVE   PROCESSES 

seashore,  which  by  the  movement  of  the  tides  is  alternately  a 
land  surface  and  a  sea-bottom.  These  distinctions  are  sufficiently 
obvious,  yet  they  are  not  always  easy  to  apply,  especially  in  the 
absence  of  fossils ;  hence  great  differences  *of  opinion  continually 
arise  concerning  the  interpretation  of  certain  rock  masses. 

Stratification.  —  It  is  an  almost  universal  characteristic  of  sedi- 
mentary accumulations,  whether  modern  deposits  or  ancient  rocks, 
that  they  are  stratified,  that  is,  divided  into  more  or  less  parallel 
layers  or  beds.  Indeed,  the  terms  secondary,  derivative,  sedi- 
mentary, and  stratified  rocks  are  but  different  names  for  the 
same  thing.  Stratification  is  due  to  the  sorting  power  of  water,  or 
of  the  wind,  by  which,  so  long  as  conditions  remain  the  same,  par- 
ticles or  fragments  of  similar  size  and  weight  are  thrown  down  at 
the  same  spot.  If  sand,  gravel,  mud,  and  clay  are  shaken  together 
in  a  jar  of  water  and  then  allowed  to  stand,  the  various  materials 
will  settle  to  the  bottom  in  the  order  of  their  coarseness,  the  finest 
coming  down  last.  Yet  the  change  from  one  kind  of  material 
to  another  will  be  so  gradual  that  no  well-defined  layers  will 
appear,  and  thus  no  true  stratification  results.  Layers  clearly  de- 
marcated from  one  another  may  be  produced  in  either  one  of  two 
ways:  (i)  by  such  a  change  of  conditions  that  the  material  depos- 
ited changes  abruptly,  though  perhaps  only  as  a  mere  film  of  a 
different  substance,  or  (2)  by  a  pause,  however  brief,  in  the  process 
of  deposition.  In  the  latter  case,  each  layer  represents  a  time 
of  deposition  broken  by  an  interval  which  allows  the  surface  par- 
ticles to  arrange  themselves  somewhat  differently  from  the  position 
they  would  take  were  the  deposition  continuous.  The  planes  of 
contact  between  the  successive  layers,  which  may  be  indistinct  or 
very  sharply  defined,  are  called  the  bedding  or  stratification  planes, 
and  each  one  of  these  formed  the  surface  of  the  lithosphere,  either 
as  a  land  surface  or  the  bottom  of  some  body  of  water,  for  a  short 
time.  The  thickness  of  each  layer  indicates  the  length  of  time 
during  which  the  deposition  of  similar  material  went  on  without 
interruption,  and  varies  from  hundreds  of  feet  to  a  small  frac- 
tion of  an  inch. 


STRATIFICATION  183 

The  power  of  ordinary  winds  to  transport  material  is  much  less 
than  that  of  water,  and  wind-borne  debris  is,  on  the  average,  much 
finer  than  water-borne  sediment,  and  furthermore  the  winds  are  less 
constant  in  direction  and  subject  to  greater  and  more  sudden 
changes  of  velocity.  Consequently,  stratification  by  the  wind  is, 
as  a  rule,  less  even  and  regular  than  that  which  is  due  to  water; 
but  still  wind-made  deposits  are  stratified,  and  it  is  not  always 
practicable  to  distinguish  with  certainty  between  the  two  classes. 
Fine  volcanic  ash  and  dust  may  be  spread  by  the  winds  over 
immense  areas  and  in  very  regular  beds  or  strata. 

The  sorting  power  of  water  or  wind  results  in  the  concentration 
of  similar  material,  so  that,  as  a  rule,  each  bed  is  made  up  of  some 
predominant  substance  in  a  state  of  greater  or  less  purity,  such  as 
gravel,  sand,  clay,  etc.,  and  thus  heterogeneous  material  is  sepa- 
rated into  its  constituent  parts,  though  the  separation  is  rarely 
quite  complete,  and  sometimes  there  is  hardly  any  separation  at  all. 
On  examining  a  thick  series  of  deposits,  we  find  that  the  materials 
are  apt  to  change  both  vertically  and  horizontally.  Changes  in  the 
vertical  direction  imply  changes  of  conditions,  in  accordance  with 
which  different  kinds  of  material  are  successively  laid  down  over 
the  same  area,  so  that  gravel  is  deposited  on  sand,  sand  on  mud,  or 
vice  versa.  Such  changes  are  usually  abrupt,  so  that  each  stratum 
is  sharply  demarcated  from  the  one  above  and  the  one  below  it. 
On  the  other  hand,  changes  in  material  in  the  horizontal  direction 
are  usually  gradual,  and  a  bed  of  sand  may  pass  by  imperceptible 
transitions  into  one  of  gravel  or  of  mud.  This  is  because  of  the 
gradual  change  in  the  velocity  of  the  transporting  agent  and  there- 
fore of  its  carrying  power.  In  the  sea  or  a  large  lake  the  material 
on  the  bottom  grows  finer  outward  from  the  shore,  while  a  river, 
whose  velocity  diminishes  from  head  waters  to  mouth,  lays  down 
material  of  decreasing  coarseness,  from  the  boulders  and  cobbles 
of  the  head  waters  to  the  fine  silt  of  the  lower  course. 

Each  agent  of  reconstruction,  or  deposition,  has  its  own  charac- 
teristic manner  of  accumulating  material,  and,  in  typical  instances, 
it  is  easy  to  distinguish  between  them,  but  there  are  also  many 


1 84  CONTINENTAL  DEPOSITS 

similarities  and,  as  we  have  already  learned,  it  is  sometimes  ex- 
ceedingly difficult  to  determine  which  of  several  possible  agents 
was  the  actual  means  of  forming  a  given  series  of  deposits.  If 
no  fossils  (i.e.  recognizable  traces  of  animals  or  plants)  are  present, 
it  is  not  always  easy  to  determine,  for  example,  whether  a  given 
sandstone  was  laid  down  in  the  sea,  or  in  a  lake,  or  heaped  up  by 
the  winds  in  a  desert.  This  uncertainty  is,  however,  largely  due 
to  our  ignorance  concerning  all  the  minute  details  of  structure 
which  characterize  the  work  of  each  agent,  and  may  be  expected  to 
disappear  as  knowledge  of  these  details  advances.  Of  late  years 
great  progress  has  been  made  in  these  matters,  and  systematic  study, 
it  may  reasonably  be  hoped,  will  remove  an  ignorance  which  is 
owing  chiefly  to  a  neglect  of  the  subject  and  to  certain  precon- 
ceptions inherited  from  the  early  days  of  geology. 

A.  CONTINENTAL  DEPOSITS 

The  continental  deposits  may  be  classified  in  several  different 
ways,  each  one  of  which  has  its  advantages  according  to  the  object 
aimed  at.  Our  present  purpose  will  best  be  served  by  arranging 
these  accumulations,  in  general,  in  accordance  with  the  agency  by 
which  they  are  made.  However,  it  is  not  feasible,  nor  even  de- 
sirable, '.o  carry  out  this  scheme  with  rigid  consistency,  for  so  many 
deposits  are  formed  by  two  or  more  agents  acting  together,  wind 
and  rain,  ice  and  water,  rivers  and  the  sea,  etc.  Then,  too,  the 
various  agents  so  often  have  shifting  boundaries :  on  the  seashore 
the  tides,  especially  the  spring  and  neap  tides,  make  the  limits 
of  land  and  sea  somewhat  indefinite,  while  rivers,  now  con- 
fined to  their  channels,  again  are  flooded  so  as  to  form  great 
temporary  lakes;  the  rare  but  violent  rains  of  the  desert  may 
cover  with  a  sheet  of  shallow  water  great  areas  which  are  ordi- 
narily baked  and  cracked  by  the  blazing  sun.  Owing  to  this  shift- 
ing of  limits  and  the  alternation  of  agencies,  continental  deposits 
seldom  display  such  uniformity  over  wide  areas  as  obtains  on  the 
sea-bottom,  where  the  conditions  are  so  much  more  constant. 


CONTINENTAL  DEPOSITS 


I85 


The  land  is  the  scene  both  of  denudation  and  deposition,  and 
which  of  these  two  processes  shall  prevail  in  any  area  depends 
upon  the  topography  and  the  climate  of  that  area.  As  is  shown 
in  the  diagram,  Fig.  83,  only  about  one-fifth  of  the  land  surface 
is  raised  more  than  1 200  meters  (about  4000  feet)  above  the  level 
of  the  sea,  and  this  fifth  includes  the  regions  of  most  active  denuda- 
tion; three-fifths,  at  successively  lower  levels,  are  areas  of  progres- 


..*& 


FIG.  83.  — Diagram  showing  the  relation  between  height  and  area  of  land  above 
sea-level  and  of  water  in  ocean  basins.  Vertical  columns  of  figures  indicate 
heights  and  depths  in  meters ;  on  horizontal  line,  millions  of  square  kilometers. 
(Penck) 

sively  less  effective  erosion,  as  we  descend  from  higher  to  lower 
ground,  while  the  remaining  fifth  receives  deposits  upon  it.  Nearly 
a  fifth  of  the  land  of  the  globe  is  comprised  in  interior  continental 
basins  which  have  an  arid  climate  and  are  without  an  outlet  to  the 
sea;  some,  indeed,  like  the  lower  Jordan  valley  and  the  Dead  Sea, 
are  far  below  the  ocean  level.  Probably  one-half  of  this  desert  and 
semi-desert  area  is  the  seat  of  extensive  deposition.  The  areas  of 


1 86  TERRESTRIAL   DEPOSITS 

denudation  and  those  of  deposition  are  thus  determined  by  climate 
and  topography,  and  shift  as  those  factors  change  or  are  modified 
by  diastrophic  movements  of  the  earth's  crust. 

i.  TERRESTRIAL  DEPOSITS 

Under  this  head  are  included  those  accumulations  of  the  me- 
chanical and  chemical  waste  of  preexistent  rocks  which  are  formed 
on  land  surfaces  and  not  in  permanent  bodies  of  water.  Deposits 
made  by  ice  are  considered  in  a  separate  section.  The  principal 
agencies  which  form  deposits  of  this  class  are  rain  and  wind  and 
springs,  and  the  great  variety  of  them  is  due  to  climatic  factors,  the 
velocity  and  constancy  of  the  winds,  the  quantity  and  seasonal 
distribution  of  the  rainfall,  the  amount  and  rapidity  of  temperature 
changes.  Hence  we  find  different  kinds  of  deposits  in  deserts  and 
humid  regions,  in  cold,  temperate,  and  tropical  climates,  near  the 
seashore,  and  in  the  interior  of  the  continents.  The  necessity  of 
considering  and  emphasizing  these  differences  lies  in  their  value 
for  historical  studies.  Every  rock  bears  within  it  a  record  of  its 
history,  could  we  only  decipher  it. 

Residual  Accumulations;  Soil.  —  As  we  have  already  seen,  the 
disintegration  and  decay  of  rock  results  in  the  formation  of  soil, 
which  is  the  residuum  after  the  removal  of  more  or  less  of  the  par- 
'ent  roclr.  In  humid  climates'  there  is  usually  a  distinct  subsoil 
which  is  less  thoroughly  oxidized  and  hydrated  and  i«  lighter  in 
colour  and  much  less  fertile  than  the  top  soil,  which  is  largely  due 
to  the  washing  downward  of  the  fine  clay  particles  by  percolating 
rainwater.  In  arid  climates  there  "is  less  kaolinization  of  the 
aluminous  silicates,  a  much  deeper  top  soil,  and  little  or  no  distinct 
subsoil.  Under  the  influence  of  wind,  rain,  and  other  agencies, 
the  soil  travels  down  the  slopes  and  accumulates,  often  to  great 
depths,  in  valleys  and  depressions,  and  is  carried  in  enormous 
volume  by  the  rivers.  Very  little  soil,  as  such,  is  built  into  the 
rocks  of  the  earth's  crust,  but  sometimes  it  is  buried  under  a  lava 
stream  or  depressed  beneath  the  sea  or  a  lake  in  such  a  manner 


CHEMICAL  AND   MECHANICAL  DEPOSITS  187 

as  not  to  be  washed  away,  but  immediately  covered  by  other  de- 
posits. Such  an  old  soil,  or  "  dirt  bed,"  may  be  recognized  by  its 
texture  and  appearance  and  by  the  fossil  roots  and  stems  of  land 
plants  with  which  it  is  apt  to  be  filled. 

Laterite  is  a  peculiar  soil  very  widely  spread  in  the  tropics  and  of 
a  deep  red  colour,  caused  by  the  presence  of  Fe2O3.  It  differs  from 
ordinary  soils  in  the  fact  that  much  of  the  alumina  is  present  as  an 
oxide,  instead  of  the  silicate,  and  is  frequently  characterized  by 
lumps  and  nodules  of  Fe2O3,  which  are  produced  by  a  chemical 
process  of  concentration. 

Chemical  Deposits.  —  In  the  tropics,  which  so  largely  have  a 
regular  alternation  of  rainy  and  dry  seasons,  and  in  arid  regions, 
where  the  rain  often  falls  in  torrential  showers,  followed  by  long 
periods  of  drought,  the  movement  of  water  through  the  soil  is 
frequently  reversed  in  direction.  During  the  rains  the  movement 
is  downward ;  in  the  dry  period  evaporation  from  the  surface  and 
capillarity  cause  a  slow  ascent  of  the  water  through  the  soil.  Often 
this  ascending  water  is  charged  with  material  in  solution  and  this 
material  is  deposited  on  or  near  the  surface  as  the  water  evaporates. 
In  deserts  and  semi-deserts  the  surface  is  often  white  with  salt,  the 
sulphate  or  carbonate  of  soda,  borax,  and  other  soluble  com- 
pounds. The  iron  nodules  of  laterite  are  produced  in  this  manner, 
and  sometimes  these  nodules  are  cemented  into  continuous  sheets 
of  crude  haematite.  Where  the  soil  and  underlying  rocks  contain  the 
carbonate  of  lime  abundantly,  the  water  concentrates  them  at 
the  surface,  it  may  be,  as  in.  South  Africa,  in  very  extensive  sheets 
of  hard  limestone.  These  terrestrial  chemical  deposits  may  cover 
very  wide  areas,  but  never  in  any  great  thickness. 

Mechanical  Deposits  are  made  on  land  surfaces  by  various  agen- 
cies and  form  quantitatively  much  the  most  important  series  of  the' 
class. 

Talus  and  Breccia.  —  As  has  been  pointed  out  (pp.  114  and  1 18), 
great  masses  of  angular  blocks  of  all  sizes  accumulate  at  the  foot 
of  cliffs  and  on  mountain  slopes  as  talus,  which  shows  an  imper- 
fect division  into  layers  and  is  slowly  but  continually  creeping 


1 88 


TERRESTRIAL  DEPOSITS 


downward.     By  the  deposition  of  some  cementing  material  (usu- 
ally CaCO3)  in  the  interstices  of  the  talus  the  blocks  may  be  bound 


FIG.  84.  —  Loess  deposits  ;  North  China.     (Photograph  by  Willis) 

into  a  solid  mass,  called  breccia,  of  which  the  peculiarity  is  that  the 
fragments  composing  it  are  angular,  not  rounded. 

Loess.  —  In  arid  regions  the  wind  often  carries  the  finer  parts  of 


LOESS  — BLOWN   SAND 


189 


the  soil  to  immense  distances  and  deposits  them  where  they  are  less 
exposed  to  the  wind,  and  where  there  is  vegetation  enough  to  hold 
them.  In  Central  Asia  the  sun  is  often  darkened  for  days  by  these 
dust-storms,  and  after  they  are  past,  a  fine  deposit  of  yellow  dust 
is  found  over  everything.  Loess  is  a  deposit  formed  in  this  way, 
and  it  is  found  in  many  lands.  One  of  the  largest  known  accumu- 
lations of  it  is  in  northern  China,  where  it  covers  an  immense  area, 


FIG.  85.  — Sand  dune  with  wind-ripples,  River  Terraces  in  distance ;  Biggs,  Oregon, 

(U.  S.  G.  S.) 

to  depths  of  1000  to  1500  feet.  It  is  not  stratified,  but  cleaves 
vertically,  and  thus  the  ravines  and  valleys  excavated  in  it  have 
very  abrupt  sides.  Loess  also  occurs  in  Europe,  and  the  Pampas 
of  the  Argentine  Republic  are  covered  with  a  great  thickness  of  it. 
The  loess  of  the  Mississippi  valley,  though  of  rather  exceptional 
character,  is  yet  probably  of  aeolian  origin. 

Blown  Sand.  —  Wherever  a  sandy  soil  occurs  unprotected  by 
vegetation,  as  in  deserts  or  along  the  seacoast,  the  wind  drifts  the 


190 


TERRESTRIAL   DEPOSITS 


sand  and  piles  it  up  into  hills  or  sand  dunes.  The  dunes  are 
roughly  divided  into  layers,  the  thickness  and  inclination  of  which 
depend  upon  the  force  and  direction  of  the  wind,  and  often  imi- 
tate the  confused  arrangement  of  sands  piled  up  by  waves  and 
currents  under  water.  The  sand-grains  of  the  dunes  are,  however, 
more  rounded  by  the  abrasion  which  they  have  undergone  and, 
especially  in  deserts,  they  are  apt  to  be  smaller.  When  the  sands 


FIG.  86.  —  Sand  dune;  Beaufort  Harbor,  N.C.     (U.  S.  G.  S.) 


are  mixed  with  pieces  of  shells  and  other  calcareous  material,  per- 
colating waters,  by  dissolving  and  redepositing  the  CaCOa,  may 
cement  the  sands  into  firm  rock.  This  is  the  more  conspicuous 
when  the  whole  material  is  calcareous,  as  in  the  shell  sands  of 
Bermuda.  This  substance,  ground  up  by  the  surf,  is  transported 
inland  by  the  wind  and  piled  up  into  dunes.  Rain-water  cements 
the  loose  grains  together,  and  by  the  alternate  accumulation  by 


SPRING  DEPOSITS  IQI 

wind  and  cementing  by  rain  is  formed  the  stratified  aeolian  or 
drift-sand  rock. 

Spring  Deposits.  —  As  our  knowledge  of  microscopic  plants 
increases,  many  processes  which  were  believed  to  be  purely  chemi- 
cal, are  found  to  be  dependent  upon  the  activity  of  minute  plants. 
At  present,  it  is  not  possible  to  distinguish  accurately,  in  all  cases, 
between  the  two  kinds  of  processes. 

Many  springs  precipitate  carbonate  of  lime,  on  coming  to  the 
surface.  The  quantity  of  CaCO3  which  a  given  volume  of  water 
will  dissolve  depends  upon  the  amount  of  CO2  contained  in  that 
water,  and  the  quantity  of  dissolved  gas,  again,  is  determined  by 
the  pressure  to  which  it  is  subjected.  When  the  spring-waters 


FIG.  87.  — Ideal  section  through  Mammoth  Hot  Springs,  showing  the  water  rising 
through  limestone.     (Hayden) 

reach  the  surface,  the  pressure  is  relieved,  much  of  the  CO2  im- 
mediately escapes,  and  more  or  less  of  the  CaCO3  is  deposited  as 
travertine  in  the  neighbourhood  of  the  spring,  often  in  masses  of 
considerable  extent  and  thickness.  The  process  is  not  always 
entirely  chemical.  The  beautiful  calcareous  terrace  formed  by 
the  Mammoth  Hot  Springs,  in  the  Yellowstone  Park,  is,  in  part 
at  least,  due  to  the  separation  of  the  lime  salt  from  the  water  by  a 
jelly-like  plant,  which  grows  in  the  hot  water  and  is  spread  in 
bright  coloured  layers  over  the  surface  of  the  terrace.  The  parts 
of  the  terrace  .where  deposition  is  no  longer  in  progress  can  be  at 
once  distinguished  by  their  white  colour. 

Siliceous  deposits  are  much  less  common  than  the  calcareous, 
because  of  the  rare  conditions  under  which  silica  is  dissolved 


192 


TERRESTRIAL   DEPOSITS 


in  any  considerable  quantity,  hot  solutions  of  alkaline  carbon- 
ates being  necessary  for  this  purpose.  In  the  Yellowstone  Park, 
especially  on  the  Firehole  River,  are  great  terraces  and  flats  of 
hard  white  siliceous  sinter,  or  geyserite,  which  have  been  formed 
and  are  still  being  added  to  by  the  innumerable  hot  springs  and 
geysers.  The  silica  is  deposited  partly  by  the  evaporation  of  the 
water  and  partly  by  the  action  of  Algce  (minute  plants)  which 


FlG.  88.  —  Travertine  terrace  of  the  Mammoth  Hot  Springs,  Yellowstone  Park 


flourish  in  hot-water  pools.     Similar  deposits  are  found  in  the 
geyser  regions  of  Iceland  and  New  Zealand. 

Iron  deposits  are  formed  by  the  springs  known  as  chalybeate, 
which  contain  the  carbonate  of  iron  (FeCO8)  in  solution.  Con- 
tact with  the  air  speedily  converts  the  soluble  carbonate  into  the 
insoluble  Fe2O3,  which  forms  brown  stains  and  patches  on  the 
channels  leading  from  such  springs,  and  considerable  quantities  of 
it  collect  in  pools.  Here  again,  organic  agency  may  supplement 


TERRESTRIAL   DEPOSITS 


193 


IQ4  TERRESTRIAL  DEPOSITS 

the  chemical  work,  for  certain  diatoms  extract  iron  from  the  water, 
as  other  Algae  extract  lime  and  silica. 

Certain  mineral  springs  are  of  importance,  as  indicating  a 
way  in  which  mineral  veins  may  have  been  formed  (see  p.  430). 
The  Sulphur  Bank  Springs  in  the  Coast  Range  of  California  are  an 
especially  instructive  example  of  this  activity.  Below  the  depths 
to  which  the  atmospheric  influences  penetrate,  the  fissures  in  the 
rocks  are  filled  with  hydrated  silica,  which  is  as  soft  as  cheese  and 
contains  more  or  less  cinnabar  (sulphide  of  mercury).  In  other 
places  the  silica  is  hardened  to  chalcedony,  and  deposits  of  cin- 
nabar mixed  with  iron  pyrites  fill  up  the  crevices.  The  hot  waters 
which  build  up  these  deposits  are  alkaline,  charged  with  certain 
acids  and  alkaline  sulphides.  Near  Virginia  City,  Nevada,  hot 
alkaline  springs  rise  through  a  series  of  fissures,  in  which  they  have 
deposited  linings  of  silica,  amorphous  and  chalcedonic,  with  some 
quartz,  containing  minute  crystals  of  iron  pyrites  and  traces  of 
copper  and  gold.  On  the  surface  the  springs  have  formed  a  thick 
layer  of  geyserite. 

Phosphate  Deposits  are  the  only  strictly  terrestrial  organic  for- 
mations which  require  notice.  These  are  principally  derived  from 
guano,  which  is  the  accumulated  excrement  of  birds  (in  caves,  of 
bats),  and  contains  phosphates  in  large  quantity.  In  rainless 
regions,  such  as  the  Peruvian  coasts  and  islands,  the  guano  may 
accumulate  to  great  thickness  without  loss  of  its  soluble  matters, 
but  in  rainy  districts  these  are  largely  carried  away  by  percolating 
waters.  Should  the  underlying  rock  be  a  limestone,  it  will  be 
gradually  converted  from  a  carbonate  into  a  phosphate  of  lime. 
Such  is  believed  to  be  the  mode  of  origin  of  the  phosphatic  rock  of 
Florida  and  the  West  Indies.  On  the  other  hand,  the  phosphatic 
nodules  of  South  Carolina  are  regarded  as  due  to  the  action  of 
swamp  water  upon  underlying  shell  rocks,  though  the  source  of 
phosphoric  acid  is  not  well  understood. 

Cave  Deposits.  — The  chemically  formed  cave  deposits  are  due 
to  the  solution  and  redeposition  of  carbonate  of  lime.  Caves  are 
very  generally  found  in  limestones,  and  the  percolating  waters 


CAVE   DEPOSITS  IQ5 

which  make  their  way  through  the  roof  of  a  limestone  cavern 
always  have  more  or  less  CaCO3  in  solution.  A  drop  of  such 
water,  hanging  from  the  cavern  roof,  will  lose  some  of  its  CO2,  upon 
the  presence  of  which  the  solubility  of  the  CaCO3  depends,  and 
deposit  a  little  ring  of  the  lime  salt.  Successive  depositions  will 
lengthen  the  ring  to  a  tube,  and  then  the  tube  will  be  built  up 
by  layers  on  the  inner  side,  until  it  becomes  a  cone.  At  first,  the 
deposit  is  white,  opaque,  and  very  friable,  crumbling  at  a  touch, 
but  repeated  depositions  fill  up  the  interstices  of  the  porous  mass 
and  convert  it  into  a  hard,  translucent  stone,  which  assumes  a 
crystalline  structure  through  the  development  of  calcite  or  ara- 
gonite  crystals.  The  masses,  thus  formed,  that  depend  from  the 
roof  of  the  cavern,  are  called  stalactites.  After  hanging  for  a  time 
from  the  roof,  the  drop  of  water  falls  to  the  floor  of  the  cave,  and 
there,  in  similar  fashion,  deposits  a  little  layer  of  CaCO3,  which 
gradually  grows  upward  into  a  cone.  This  is  a  stalagmite,  and 
differs  from  the  stalactite  only  in  the  fact  that  it  grows  upward 
from  the  floor,  instead  of  downward  from  the  roof.  The  stalag- 
mite is,  of  course,  exactly  beneath  the  stalactite,  and  as  long  as 
the  water  continues  to  follow  the  same  path,  the  two  cones  are 
steadily,  though  very  slowly,  increased  both  in  height  and  thick- 
ness, until  they  meet,  unite,  and  form  a  pillar  extending  from  floor 
to  roof  of  the  cavern. 

These  deposits  form  the  most  curious  and  beautiful  features  of 
limestone  caverns.  The  stalactites  assume  all  manner  of  shapes, 
determined  by  the  way  in  which  the  water  trickles  over  them,  and 
the  abundance  or  scantiness  of  the  water  supply.  Fantastic  and 
beautiful  shapes  of  every  description,  fringes  of  crystal  spar,  and 
curtain-like  draperies  hang  from  the  roof  and  cover  the  walls  of 
the  chambers,  while  grotesque  shapes  rise  from  the  floor,  which  is 
itself  often  a  solid  mass  of  the  same  deposit,  and  the  pillars,  once 
formed,  are  ornamented  with  every  variety  of  fringe  and  sculpture. 
The  constancy  of  the  paths  by  which  the  water  descends  through 
the  roof  of  the  cavern,  insures  that  the  process  shall  continue 
uninterruptedly  for  very  long  periods  of  time.  The  Luray  Caverns 


196  PALUSTRINE  DEPOSITS 

of  Virginia  are  famous  for  the  bizarre  beauty  of  their  formations, 
but  limestone  caves  everywhere  have  more  or  less  of  the  same 
deposit  to  show. 

This  process  may  be  readily  observed  in  any  masonry  arch, 
through  which  rain-water  percolates,  as  a  bridge,  for  example. 
The  lime  of  the  mortar  is  converted,  in  course  of  time,  by  contact 
with  moist  air,  into  CaCO3,  and  this  again  is  partially  dissolved  by 
the  rain.  When  the  rain-water  trickles  through  the  arch,  it  leaves 
icicle-like  deposits,  or  thin  sheets  of  calcareous  matter,  fringing 
the  under  side. 

In  a  cave,  it  frequently  happens  that  angular  fragments  fall 
from  the  roof  and  are  cemented  into  a  breccia  by  deposits  of 
stalagmite.  In  caves  connected  with  the  surface  by  openings, 
sand  and  gravel,  or  fine  soil  and  loam,  are  washed  in  by  streams, 
or  by  the  rain,  and  form  the  characteristic  deposit  known  as  cave 
earth.  In  ancient  caverns,  no  longer  subject  to  this  wash,  the 
whole  deposit  of  earth  may  be  sealed  in  by  a  covering  of  stalag- 
mite. Cave  earth  has,  in  many  instances,  yielded  great  quantities 
of  bones,  which  were  washed  in  with  the  earth,  or  dragged  in  by 
the  carnivorous  animals  which  inhabited  the  cavern.  The  Port 
Kennedy  cave  in  Pennsylvania  is  almost  filled  up  by  the  bones  of 
extinct  animals  which  were  washed  into  it,  and  many  such  cases 
are  known,  especially  in  Europe. 

II.   PALUSTRINE  OR  SWAMP  DEPOSITS 

The  most  important  of  the  swamp  and  bog  deposits  are  the 
vegetable  accumulations,  for  the  preservation  of  which  a  certain 
amount  of  water  is  necessary.  The  vast  quantities  of  coal  which 
occur  in  so  many  parts  of  the  world,  testify  to  the  significance  of 
the  part  which  bog  and  swamp  accumulations  of  vegetable  matter 
have  played  in  the  earth's  history.  The  nearest  approach  to  coal 
that  we  have  in  process  of  formation  at  the  present  day,  we  find 
in  the  peat-bogs,  which  are  especially  abundant  and  extensive  in 
cool,  damp  climates,  as  in  Ireland,  Scandinavia,  and  the  northern 


PALUSTRINE  DEPOSITS  1 97 

parts  of  North  America.  In  northern  regions  the  peat  is  formed 
principally  by  mosses,  and  especially  by  the  bog  moss,  Sphagnum; 
elsewhere,  as  in  the  Great  Dismal  Swamp  of  Virginia,  the  leaves 
of  trees  and  various  aquatic  plants  are  the  sources  of  supply. 

The  processes  of  organic  decomposition  depend  upon  the 
activities  of  bacteria,  but,  for  the  sake  of  simplicity,  we  may 
treat  the  subject  as  if  the  processes  were  chemical  only.  Vege- 
table matter  consists  of  carbon,  hydrogen,  oxygen,  and  nitrogen, 
with  a  certain  proportion  of  mineral  matter,  or  ash.  When  de- 
caying on  the  ground,  exposed  to  the  air,  the  plant  tissues  are  com- 
pletely oxidized,  and  form  such  simple  and  stable  compounds  as 
CO2,  H2O,  NH3,  and  the  more  complex  humous  acids,  and  thus 
hardly  any  solid  residue  is  left.  In  forests  the  accumulation  of 
leaves  for  many  centuries  results  only  in  a  shallow  layer  of  vege- 
table mould.  Under  water,  where  the  supply  of  oxygen  is  very 
limited,  vegetable  decomposition  is  much  less  complete.  Some 
CO2,  H2O,  and  CH3  (marsh  gas)  are  formed,  but  much  of  the 
hydrogen  and  nearly  all  of  the  carbon  remain;  the  farther  decom- 
position proceeds,  the  higher  does  the  percentage  of  carbon  rise, 
and  the  darker  does  the  colour  of  the  mass  become.  Peat  fre- 
quently forms  in  small  lakes  and  ponds,  aquatic  plants  growing 
out  from  the  edges  and  on  the  surface,  until  they  gradually  fill  up 
the  basin  and  convert  the  pond  into  a  bog. 

The  Great  Dismal  Swamp  of  Virginia  and  North  Carolina  prob- 
ably more  nearly  reproduces  than  do  most  existing  peat-bogs  the 
conditions  of  the  ancient  coal  swamps.  The  swamp,  which  meas- 
ures thirty  miles  by  ten,  is  a  dense  growth  of  vegetation  upon  a 
water-covered  soil  of  pure  peat  about  fifteen  feet  deep  and  with  no 
admixture  of  sediment.  The  swamp  cypress  grows  abundantly  in 
the  bog,  and  prevents,  by  its  dense  shade,  the  evaporation  which 
would  take  place  in  summer,  could  the  sun's  rays  penetrate  to  the 
wet  soil.  The  shallow  layer  of  water  which  covers  the  ground 
receives  the  fallen  leaves,  twigs,  and  branches,  and  sometimes 
even  the  trunks  of  fallen  trees,  preventing  their  complete  decom- 
position, while  the  dense  covering  of  mosses,  reeds,  and  ferns 


198 


PALUSTRINE  DEPOSITS 


which  carpet  the  ground,  add  their  quota  to  the  mass  of  decaying 
vegetable  matter.  At  the  bottom  of  the  bog,  it  is  of  interest  to 
observe,  is  a  layer  of  fire-clay,  which,  by  its  imperviousness,  tends 
to  hold  the  water  and  prevent  its  draining  away.  Peat  swamps, 
formed  in  a  similar  manner,  also  occur  at  the  mouths  of  great 
rivers,  such  as  the  Mississippi. 


FIG.  90. — Great  Dismal  Swamp,  Virginia.     (U.  S.  G.  S.) 

The  bogs  of  northern  latitudes  are  due  principally  to  the  bog 
moss  Sphagnum,  which  forms  dense  and  tangled  masses  of  vege- 
tation, dead  and  decaying  below,  green  and  flourishing  above. 
As  these  mosses  hold  water  like  a  sponge,  they  will  develop  bogs  in 
any  shallow  depression,  or  even  on  a  flat  surface,  where  they  may 
get  a  foothold.  The  depth  of  peat  is  sometimes  as  much  as  fifty 
feet,  and  its  density  and  fineness  of  grain  increase  with  the  depth 
and  the  length  of  time  it  has  been  macerating  in  water. 


FLUVIATILE   DEPOSITS  199 

Fire-clay  is  frequently  found  at  the  bottom  of  peat-bogs,  and  is 
directly  connected  with  the  processes  of  vegetable  decomposition, 
though  not  itself  of  organic  origin.  Fire-clay  contains  a  large 
admixture  of  siliceous  sand,  but  is  free  from  lime,  magnesia,  the 
alkalies,  and  any  high  percentage  of  iron;  it  is  thus  a  mixture  of 
nearly  pure  clay  and  sand,  which  may  be  heated  very  highly  with- 
out melting  or  crumbling.  The  iron,  alkalies,  and  alkaline  earths 
are  gradually  leached  out  of  the  clay  by  the  action  of  the  peaty 
water,  which  is  charged  with  organic  acids,  and  thus  an  ordinary 
clay  is  converted  into  a  fire-clay.  Fire-clay  occurs  frequently 
beneath  coal  seams;  as  the  percentage  of  silica  becomes  very  high, 
fire-clay  passes  over  into  gannister,  which  is  largely  used  for  the 
lining  of  iron  furnaces. 

Bog  Iron-ore  is  another  substance  which  is  indirectly  due  to  the 
decay  of  plants;  it  is  found  at  the  bottom  of  bogs,  or  lakes,  in 
deposits  which  are  sometimes  many  feet  thick.  Iron  is  a  very 
widely  disseminated  substance,  occurring  in  almost  all  rocks  and 
soils,  though  usually  in  very  small  quantities;  by  the  action  of  the 
bog  water  the  oxide  is  converted  into  the  soluble  carbonate  (FeCO3). 
Solutions  of  FeCO3  accumulate  under  peat-bogs  and  deposit  their 
mineral  by  concentration;  but  when  the  iron-bearing  waters 
evaporate  in  contact  with  the  air,  the  carbonate  is  reconverted  into 
the  red  oxide,  by  the  loss  of  CO2  and  absorption  of  O. 

III.  FLUVIATILE  OR  RIVER  DEPOSITS 

In  a  preceding  chapter  we  learned  that  the  power  of  a  stream  of 
water  to  transport  sediment  depends  upon  its  velocity,  which,  in  its 
turn,  is  determined  by  the  slope  of  the  ground  and  the  volume  of 
water.  Further,  we  discovered  the  very  surprising  fact  that,  for 
the  coarser  material  which  is  pushed  along  the  bottom,  the  trans- 
porting power  increases  as  the  sixth  power  of  the  velocity  (T=V&). 
It  follows  from  this  that  a  slight  decrease  in  the  swiftness  of  a  stream 
will  cause  it  to  throw  down  the  greater  part  of  its  load  of  sediment, 
while  a  slight  increase  will  cause  it  to  carry  off  what  it  had  before 


20O 


FLUVIATILE  DEPOSITS 


deposited.  Thus,  great  rivers,  like  the  Mississippi,  which  flow  in 
soft,  easily  moved  deposits,  are  preeminently  whimsical  and  treach- 
erous. As  the  volume  and  velocity  of  the  stream  are  much  sub- 
ject to  change,  there  will  obviously  be  corresponding  changes  in 
the  scour  and  deposition  at  any  given  point,  but  there  are  certain 
places  where  deposition  is  so  constant  that  extensive  accumulations 


FIG.  91.  —  Manti  Creek,  Utah ;  flood  of  August,  1901 

may  be  formed  there.  As  we  trace  a  river  downward  from  its 
source  in  a  mountain  region,  we  find  that  in  the  upper  stream, 
which  is  a  torrent  in  swiftness,  only  large  stones  remain  at  rest, 
everything  else  being  swept  along.  Farther  down  stream,  as  the 
slope  of  the  bed  diminishes,  the  coarse  gravel  is  thrown  down,  next 
the  coarse  sand  is  deposited,  and  in  the  lower  reaches  of  a  river, 
which,  like  the  Mississippi,  flows  over  land  that  has  a  very  gentle 


FLUVIATILE  DEPOSITS 


2OI 


slope,  and  is  raised  but  little  above  the  sea-level,  only  the  finest  silt 
gathers  on  the  bottom.  The  exact  limits  of  the  different  kinds  of 
deposit  will  vary  with  the  stage  of  water. 

At  points  where  the  velocity  of  the  stream  meets  a  constant  check, 
there  will  be  constant  deposition,  and  thus  bars  and  islands  are 
built  up  in  the  channel,  which  will  be  permanent  unless  some  change 
of  conditions  is  brought  about.  In  the  sand-bars  and  gravel-spits 


FIG.  92.  —  Effects  of  flood ;  Black  Hills,  S.D.     (U.  S.  G.  S.) 

the  up-stream  side  is  a  gentle  slope,  ending  abruptly  on  the  down- 
stream side,  the  bar  or  spit  advancing  by  having  sand  or  gravel 
pushed  up  the  gentle  slope  by  the  current  and  dropped  over  the 
steep  face,  where  it  forms  inclined  layers.  Flattened  and  elon- 
gated pebbles  arrange  themselves  so  as  to  offer  the  least  resistance 
to  the  current,  in  a  slanting  position,  with  their  tops  down  stream. 
When  the  stream  is  subsiding,  the  material  tends  to  assume  a 


202 


FLUVIATILE  DEPOSITS 


more    horizontal    direction,   giving    an    irregular    and    confused 
stratification  to  these  deposits. 

Alluvial  Cones  or  Fans.  —  Where  a  swift  torrent,  descending  a 
steep  slope,  debouches  on  a  plain  or  wide  valley,  its  velocity  is 
greatly  diminished,  and  a  large  part  of  the  material  which  it  carries 
is  thrown  down  and  spread  in  a  fan  shape  from  the  opening  of  the 
ravine  in  which  the  torrent  flows.  The  thickness  of  the  cone  is 


FIG.  93.  — Sand  deposits,  North  Platte  River,  Nebraska.     (U.  S.  G.  S.) 

greatest  at  the  mouth  of  the  ravine,  while  its  breadth  increases  out- 
ward from  that  point.  Where  several  such  torrents  open  on  the 
plain  near  together,  their  fans  may  coalesce  and  form  afcontinuous 
fringe  along  the  base  of  the  mountain.  The  slope  of  the  cone's 
surface  diminishes  with  the  size  of  the  stream;  in  small  streams  it 
may  be  as  steep  as  10°.  These  cones  are  formed  on  much  the 
same  principle  as  deltas,  and  might,  with  propriety,  be  called  ter- 
restrial deltas.  Very  large  alluvial  cones  are  found  in  the  Rocky 


ALLUVIAL  CONES 


203 


Mountain  and  Great  Basin  regions,  generally  in  the  forelands 
which  front  high  mountains.  In  the  western  part  of  the  Argentine 
Republic,  along  the  front  of  the  Andes,  the  temporary  rivers  formed 
by  the  melting  snow  bring  down  enormous  quantities  of  mud  and 
fine  sand  and  spread  it  out  over  the  plain.  Where  the  rivers 
discharge  into  the  sea,  this  process  of  upbuilding  is  limited,  except 
where  there  is  slow  downwarping  of  the  foreland,  but  in  interior 


FIG.  94.  —  Alluvial  cone,  trenched  by  stream,  with  secondary  cone  below. 
(U.  S.  G.  S.) 

arid  basins,  without  outlet,  it  may  accumulate  very  great  thick- 
nesses of  river-made  sediments. 

Flood  Plains.  —  Rivers,  as  is  well  known,  are  subject  to  floods 
when  the  volume  of  water  is  enormously  increased  and  can  no  longer 
be  contained  in  the  ordinary  channel,  but  spreads  out  over  the 
level  ground  on  each  side.  By  this  spreading,  which  may  be 
for  many  miles  in  both  directions,  the  velocity  of  the  water  is  much 


204 


FLUVIATILE  DEPOSITS 


FLOOD   PLAINS 

diminished,  and  over  the  flooded  area  (flood  plain)  large  quan- 
tities of  material  are  thrown  down,  while  the  unchecked  velocity 
in  the  channel  may  cause  a  scouring  and  deepening  there  or, 
under  other  conditions,  the  channel  and  flood  plain  may  both  be 
built  up,  especially  if  the  river  flow  through  a  slowly  subsiding  re- 
gion. The  nature  of  the  material  deposited  over  the  flood  plain  will 
depend  on  the  character  and  swiftness  of  the  flooded  stream,  and 
varies  from  the  coarsest  gravel  to  the  finest  silt.  The  latter  is  more 
usual,  for  the  flood  plain  is  widest  along  the  lower  course  of  the 
river.  Flood-plain  deposits  attain  great  importance  in  interior 
basins  which  have  no  drainage  outlet  and  consequently  retain  all 
the  material  which  is  washed  into  them  from  the  surrounding  moun- 
tains by  the  rain  or  rivers.  In  such  basins  the  rivers  end  in  salt 
lakes  or  die  out  in  the  sands,  but  at  intervals,  it  may  be  only 
rarely  or  during  an  annual  wet  season,  the  rivers  are  flooded  and 
immense  areas  of  the  desert  are  converted  into  shallow  seas.  Near 
the  mountains  is  formed  a  fringe  of  alluvial  cones,  which  may 
coalesce  into  a  continuous  belt,  and  over  the  central  parts  of  the 
basin  is  spread  the  finer  material  in  even  and  regular  stratification. 
The  wind  may  carry  away  all  this  material  and  remove  it  beyond  the 
limits  of  the  basin,  leaving  only  stony  wastes,  or  the  deposits  may 
accumulate  to  a  great  thickness,  according  to  circumstances. 

Even  in  climates  of  heavy  rainfall  great  interior  basins,  due  to 
downwarping,  are  found,  and  though  they  drain  to  the  sea,  they 
may  become  filled  to  great  depths  with  river  deposits.  The  interior 
of  South  America,  drained  by  the  Orinoco,  Amazon,  Paraguay, 
etc.,  is  an  example  of  such  basins  where  river  deposition  is  actively 
progressing  on  a  very  large  scale. 

In  climates  with  abundant  rainfall  (pluvial  climates)  the  flood 
plains  of  rivers  are  covered  with  vegetation  which  protects  the 
flood  deposits,  but  in  arid  climates  the  flood  plains  are  bare  of 
vegetation  for  the  whole  or  most  of  the  year,  and  the  river  deposits 
are  exposed  to  the  sun.  Great  areas  of  mud  and  silt,  thus  ex- 
posed, shrink  in  drying  and  crack  in  deep  fissures,  which  enclose 
polygonal  areas?  as  may  be  seen  in  any  mud  puddle  which  is  drying 


206 


FLUVIATILE  DEPOSITS 


in  the  sun.  The  cracks  thus  formed  are  called  sun  or  mud  cracks 
and  may  be  preserved  indefinitely  in  rocks  which  are  formed  from 
flood-plain  accumulations.  In  such  deposits  there  is  apt  to  be  a 
difference  in  the  material  thrown  down  in  the  earlier  and  later  stages 
of  the  flood,  because  of  the  difference  in  the  velocity  with  which  the 
waters  move.  After  the  river  ceases  to  rise,  the  water  over  the 
flood  plain  becomes  almost  stagnant  and  lays  down  very  fine 


FlG.  96.  —  Sun  cracks  in  Newark  shale,  about  14  natural  size 

material,  which  thus  forms  the  cracked  surface.  When  the  next 
flood  arrives,  it  carries  coarser  material,  frequently  sand,  which  fills 
up  the  cracks  and  thus  preserves  them.  Mud  cracks  are  formed 
under  other  conditions,  as  will  be  seen  in  the  following  pages,  but 
probably  nowhere  on  such  an  extensive  scale  as  on  the  flood 
plains  of  rivers  which  flow  through  arid  regions.  Footprints  of 


RIVER  TERRACES  2O? 

land  animals  may  be  preserved  in  the  same  manner  by  being  baked 
hard  in  the  hot  sun  and  then  buried  under  the  deposits  of  the  next 
flood.  Such  cracks  and  footprints,  and  even  the  impression  of  rain- 
drops, are  frequently  found  in  the  rocks  and  give  valuable  assistance 
in  determining  the  conditions  under  which  those  rocks  were  formed. 

In  ancient  flood-plain  deposits,  which  the  rivers  that  made  them 
have  long  since  deserted,  the  old  channels  are  indicated  by  coarser 
material,  sands  and  gravel  cemented  into  sandstone  and  conglom- 
erate. When  these  old  channels  are  plotted  on  a  map,  their  sinuous 
course,  great  length  in  proportion  to  width,  and  their  ramifying 
tributaries  clearly  mark  them  out  as  the  records  of  an  ancient 
system  of  drainage.  Such  channels  and  the  accompanying  broad 
and  regularly  stratified  flood-plain  deposits  cover  very  extensive 
areas  in  South  Dakota,  Nebraska,  and  others  of  the  Western  States. 

River  Terraces  and  Old  Gravels.  — The  lower  courses  of  many 
rivers,  including  most  of  those  in  the  northern  United  States,  and 
some  in  the  southern,  are  bordered  by  a  succession  of  terraces  that 
rise  symmetrically  on  the  two  sides  of  the  stream.  Sometimes,  as 
in  many  English  rivers,  the  terraces  are  at  different  levels  on  oppo- 
site sides.  The  formation  of  these  terraces  is  due  to  a  twofold 
activity  of  the  river;  the  combined  deepening  of  the  channel  and 
building  up  of  the  flood  plain  at  length  make  the  trough  of  the  river 
so  deep  that  floods  no  longer  fill  it,  especially  if  the  velocity  of  the 
current  be  maintained  or  increased  by  an  elevation  of  the  region 
drained  by  the  river.  Then  the  energy  of  the  current  is  partly 
employed  in  widening  the  channel  and  forming  a  new  flood  plain, 
cutting  back  the  edges  of  the  old  flood  plain,  which  it  can  no  longer 
overflow,  thus  converting  it  into  a  terrace,  which  is  the  remnant 
of  an  old  flood  plain.  The  process  may  be  repeated  many  times, 
and  thus  successive  terraces  rise,  one  above  another,  as  we  recede 
from  the  river. 

It  necessarily  follows  from  this  account  that  the  highest  terrace 
is  the  oldest,  and  the  lowest  is  the  last  formed.  This  seems  to  be 
a  violation  of  the  rule  that,  in  any  series  of  sedimentary  deposits, 
the  oldest  must  be  at  the  bottom  and  the  newest  at  the  top;  but 


208 


FLUVIATILE  DEPOSITS 


the  violation  is  only  apparent,  not  real.  Were  the  river  to  flow  at 
a  constant  level,  no  terraces  could  be  formed,  and  the  deposits 
would  follow  the  rule,  just  as  they  do  now  in  each  successive  flood 
plain  and  terrace.  Because,  however,  the  stream  flows  at  suc- 
cessively lower  levels,  the  lower  flood  plain  is  made  up  of  the 
newer  deposits.  It  should  further  be  observed  that  the  older 
gravels  do  not  actually  overlie  the  newer  ones,  but  are  merely  at 
higher  levels. 


FIG.  97.  —  River  terraces ;  Chelan  River,  Wash.     (U.  S.  G.  S.) 

Unsymmetrical  terraces,  which  are  either  confined  to  one  side 
of  the  river,  or  if  present  on  both  sides,  are  on  different  levels,  are 
formed  when  a  stream  is  widening  its  valley  by  steadily  cutting 
away  the  bank  on  one  side,  shifting  the  channel  toward  that  side, 
and  at  the  same  time  deepening  it.  This  will  result  in  the  forma- 
tion of  terraces  representing  the  former  positions  of  the  stream. 
If  the  lateral  movement  be  all  in  one  direction,  the  terraces  wil] 


RIVER  TERRACES 


209 


all  be  on  the  side  away  from  which  the  channel  is  shifting ;  if  it 
be  alternately  in  opposite  directions,  terraces  will  be  formed  on 
both  sides,  but  at  different  levels. 

Another  method  of  terrace  formation  should  be  mentioned. 
If  a  river  which  has  excavated  a  deep  valley,  have  its  velocity 
checked  by  a  slow  subsidence  of  the  country,  it  will  commence  to 
fill  up  its  valley  with  gravel  or  other  sediment,  and  may  thus 


FIG.  98.  — Terrace  on  deserted  channel;  central  New  York.     (U.  S.  G.  S.) 

accumulate  material  of  great  thickness  and  extent.  Should  a  re- 
elevation  of  the  country  now  occur,  the  river  will  acquire  new 
destructive  power  and  cut  a  terraced  channel  down  through  its 
own  deposits.  In  such  a  case  the  material  is  a  continuous  mass, 
and  the  gravels  of  the  higher  terraces  are  newer  (not  older)  than 
those  of  the  lower.  The  rivers  Mersey  and  Irwell  in  England  are 
believed  to  be  examples  of  this  mode  of  terrace  formation, 
p 


2IO  FLU VI ATILE   DEPOSITS 

Rock  terraces  in  river  valleys  are  the  result  of  erosion,  not  of 
deposition,  and  are  due  to  harder  ledges  of  rock  which  are  exposed 
by  the  cutting  of  the  river.  Rock  terraces  may  likewise  be  occa- 
sioned by  diastrophic  movements  and  by  long-period  fluctuations 
in  the  volume  of  the  stream.  In  all  cases  terraces  indicate  the 
successive  levels  at  which  the  river  has  flowed,  but  they  do  not 
imply,  as  would  seem  at  first  sight  to  be  the  case,  that  the  river 
was  once  of  sufficient  volume  to  fill  up  the  space  between  and  below 
the  terraces. 

Deltas  are  accumulations  of  river  deposits  at  the  mouths  of 
streams,  land  areas  which  the  rivers  have  recovered  from  the  body 
of  water  into  which  they  flow.  The  factors  which  determine  the 
formation  of  a  delta  are  not  altogether  clear.  The  presence  or 
absence  of  a  strong  tide  is  evidently  one  of  these  factors,  for  in 
lakes  and  in  seas  with  little  or  no  tide,  almost  all  streams  form 
deltas,  while  those  rivers  which  empty  into  the  open  ocean  almost 
invariably  do  so  by  means  of  estuaries,  in  which  the  sea  encroaches 
on  the  land.  In  North  America  the  rivers  which  discharge  into 
the  Gulf  of  Mexico  form  deltas,  while  the  Atlantic  streams  nearly 
all  have  estuaries.  In  Europe  the  delta-forming  rivers  empty  into 
the  Mediterranean,  the  Baltic,  and  the  Black  and  Caspian  seas. 
Neverttieless,  the  tide  is  evidently  not  the  sole  factor  in  deter- 
mining the  presence  of  a  delta.  The  Ganges  and  Brahmapootra 
have  formed  a  vast  delta  in  spite  of  the  powerful  tide  of  the  Bay  of 
Bengal;  the  Thames  and  the  Rhine  discharge  into  opposite  sides 
of  the  North  Sea,  yet,  while  the  latter  has  built  up  a  delta,  the 
former  opens  into  a  wide  estuary.  If  the  sea-bottom  is  subsiding 
faster  than  the  river  deposit  is  built  up,  no  delta  will  be  formed,  but 
an  estuary;  while,  on  the  other  hand,  slow  and  moderate  subsi- 
dence is  favorable  to  delta  formation. 

When  a  stream  loaded  with  sediment  flows  into  the  relatively 
stationary  waters  of  a  lake  or  .sea,  its  velocity  is  checked  and  the 
greater  part  of  its  load  very  rapidly  thrown  down.  Deposition 
takes  place  much  more  rapidly  in  salt  water  than  in  fresh,  because 
the  dissolved  salts  reduce  the  cohesion  of  the  water,  and  hence 


DELTAS 


211 


diminish  the  friction  which  retards  the  settling  of  silt.  The  exces- 
sively fine  particles  of  clay,  which  in  fresh  water  remain  suspended 
for  weeks,  are  thrown  down  in  salt  water  in  a  few  hours;  hence 
the  great  mass  of  the  sediment  falls  to  the  bottom  in  the  vicinity 
of  the  stream's  mouth.  Such  rapid  accumulation  obstructs  the 
flow  of  the  river  and  causes  it  to  divide  and  seek  new  channels, 
especially  in  time  of  flood,  and  form  a  network  of  sluggish  streams 


FIG.  99. —  Delta  of  Rondout  Creek  in  the  Hudson  River;  Rondout,  N.Y. 
(Photograph  by  van  Ingen) 

meandering  across  the  low  flats.  The  height  of  the  delta  is  in- 
creased by  the  spreading  waters  of  the  river,  when  in  flood,  and  the 
growth  of  vegetation  assists  in  raising  the  land.  Though  the 
Mississippi  delta  is  an  area  of  subsidence,  two-thirds  of  its  surface 
is  above  water,  when  the  river  is  in  its  ordinary  stages.  But  for 
the  levees,  however,  most  of  it  would  be  inundated  in  times  of 
flood,  when  the  unconfined  waters  of  the  river  would  form  a  lake 


212 


FLUVIATILE  DEPOSITS 


600  miles  long,  60  miles  wide,  and  with  an  average  depth  of 
12-  feet. 


FIG.  zoo.  —  Settling  of  clay  in  salt  and  fresh  water,  after  24  hours;  the  jar  on  the 
left  contains  salt  water  and  is  clear,  while  the  fresh  water  is  still  turbid. 

The  nature  of  the  materials  of  which  deltas  consist  varies  ac- 
cording to  circumstances.     When  mountains  are  near  the  coast, 


DELTAS  213 

the  streams  flowing  from  them  may  descend  into  the  sea  with 
sufficient  velocity  to  build  a  delta  of  cobblestones  and  coarse 
gravel.  Usually,  however,  deltas  formed  in  seas  are  composed  of 
very  fine  materials,  because  the  lower  course  of  most  rivers  is 
through  flat  plains,  and  the  stream  can  carry  only  very  fine  silt. 
Even  in  such  cases,  there  will  be  differences  in  the  coarseness  and 
fineness  of  the  material,  corresponding  to  the  seasons  of  high  and 
low  water  in  the  river. 

The  material  composing  a  delta  is  stratified  in  characteristic 
ways,  which  vary  according  to  circumstances.  Three  different 
kinds  of  beds  may  be  distinguished:  (i)  the  bottom-set  beds,  which 
consist  of  fine  material,  spread  out  in  regular,  nearly  horizontal 
layers  over  the  sea-bottom;  (2)  the  foreset  beds,  which  are  made 
up  of  slightly  coarser  sediment  in  layers  which  have  a  decided  sea- 
ward inclination,  the  steepness  of  which  depends 'upon  the  depths 
of  water  in  which  the  debris  is  thrown  down;  (3)  the  topset  beds, 
horizontal  layers,  which  are  deposited  by  the  river  upon  the  ad- 
vancing foreset  strata,  as  the  latter  shoal  the  water,  and  are 
usually,  for  the  most  part,  of  subaerial  origin.  As  a  rule,  the  topset 
and  bottom-set  beds  cover  the  wider  area,  but  the  foreset  make  up 
the  greater  volume  of  the  delta. 

When  a  rapid  subsidence  is  going  on,  the  whole  delta  may  be 
submarine,  and  the  same  result  may  be  effected  by  powerful 
wave  and  tidal  action,  as  in  the  case  of  the  Amazon,  which  has  a  sub- 
merged delta  extending  about  125  miles  to  sea  and  covered  with 
less  than  10  fathoms  of  water.  When  the  rate  of  sinking  is  less 
than  that  of  accumulation  and  the  power  of  waves  and  tides  is  rela- 
tively small,  most  of  the  area  of  the  delta,  neglecting  the  bottom- 
set  beds,  belongs  to  the  region  of  continental  sedimentation. 

The  rate  of  delta  growth  depends  upon  the  quantity  of  sediment 
supplied  by  the  river,  the  depth  of  the  sea  or  lake  into  which  it 
discharges,  the  power  of  the  waves,  tides,  and  currents  which  dis- 
tribute the  sediment,  and  the  stationary  or  subsiding  character  of 
the  sea-bottom.  The  Mississippi  delta  is  advancing  into  the  Gulf 
at  the  rate  of  a  mile  in  16  years,  and  that  of  the  Rhone  has  been 


214  FLUVIATILE  DEPOSITS 

built  out  more  than  14  miles  into  the  Mediterranean  since  the  be- 
ginning of  our  era.  The  coast  of  the  upper  Adriatic  is  fringed  with 
delta  deposits  which  have  widened  from  2  to  20  miles  since  Roman 
times.  The  combined  delta  of  the  Ganges  and  Brahmapootra 
measures  about  50,000  square  miles  and  is  still  gaining,  despite  the 
powerful  tides  and  waves  of  the  Bay  of  Bengal.  On  the  other 
hand,  there  seems  to  be  a  limit  to  delta  growth  in  the  sea;  the 
Nile  delta  has  advanced  very  little  in  the  last  2000  years,  for  a  strong 
current  sweeps  along  the  sea-front  and  carries  away  the  sediment. 
Debris  from  the  Indus  extends  out  800  miles  from  the  mouth  of  the 
river,  covering  an  area  of  more  than  700,000  square  miles. 


CHAPTER  VIII 

RECONSTRUCTIVE   PROCESSES.  —  CONTINENTAL 
DEPOSITS,    LAKE   AND    ICE 

IV.   LACUSTRINE  OR  LAKE  DEPOSITS 

THE  term  lake  is  a  comprehensive  one  and  includes  all  continen- 
tal bodies  of  water,  not  in  tidal  communication  with  the  sea,  in 
which  the  water  is  relatively  stationary  and  not  actively  running 
like  a  stream.  In  lakes  which  have  an  outlet  there  is  a  movement 
of  the  water  toward  the  outlet,  but  this  movement  is  extremely 
slow,  and  in  the  Lake  of  Geneva  a  given  particle  of  water  requires 
more  than  n  years  to  pass  through  the  length  of  the  basin. 
(Forel.) 

Lakes  are  formed  in  a  great  variety  of  ways,  of  which  it  is  neces- 
sary to  mention  here  only  a  few  of  the  more  important  ones.  We 
have:  (i)  Tectonic  lakes,  due  to  movements  of  the  earth's  crust, 
whether  warping  or  faulting,  by  which  basins,  subsequently  rilled 
with  water,  are  formed.  In  this  class  might  be  included  volcanic 
lakes,  which  occupy  ancient  craters.  (2)  Erosion  lakes,  in  basins 
which  have  been  excavated  by  one  or  other  of  the  erosive  processes. 
In  both  of  these  classes  the  lakes  occupy  basins  which  are  below 
the  general  drainage  level  of  the  country.  (3)  Barrier  lakes,  in 
which  the  water  is  retained  by  a  built-up  dam  or  barrier,  such  as  a 
landslip,  a  lava  stream,  glacial  moraines,  a  delta  formed  by  a  tribu- 
tary in  the  main  stream,  etc.  Such  lakes  are  frequently  above  the 
general  drainage  level. 

As  compared  with  the  sea,  lake  basins  are  but  small  and  shallow, 
and,  from  the  geological  point  of  view,  they  are  short-lived'  and 
ephemera],  because  in  course  of  time  they  are  either  drained  by 


216  LACUSTRINE  DEPOSITS 

the  outlet's  cutting  through  the  retaining  barrier,  or  by  filling  up 
with  the  sediment  brought  in  by  tributary  streams.  The  material 
transported  by  the  Mississippi,  which  would  require  11,000,000 
years  to  fill  the  Gulf  of  Mexico,  would  fill  the  basin  of  Lake  Su- 
perior in  66,000  years.  (Barrell.) 

From  another  point  of  view  we  may  speak  of  temporary  and  per- 
manent lakes,  the  former  usually  in  arid  climates.  Such  tempo- 
rary lakes  are  called  playas  and  are  formed  occasionally  or  periodi- 
cally, according  to  the  amount  and  distribution  of  the  rainfall, 
but  the  distinction  between  playas  and  river  floods  is  rather  arbi- 
trary, and  hence  playa  deposits  have  already  been  described  in  con- 
nection with  flood  plains. 

Lakes  are  important  places  of  sedimentary  accumulation,  for 
they  act  as  settling  basins  and  retain  all  the  sediment  brought  in 
by  streams.  However  turbid  the  inflowing  streams  may  be,  the 
outlet  is  beautifully  clear,  as  is  exemplified  very  strikingly  by  the 
Rhone,  which  enters  the  Lake  of  Geneva  a  muddy  stream  and 
leaves  it  in  a  state  of  exquisite  clearness  and  brilliancy.  The 
Yellowstone  and  Niagara  rivers  are  other  examples  of  the  same 
kind.  Occasional  exceptions  to  this  rule  may  occur  when  a 
shore  current  washes  some  sediment  into  the  outlet,  as  happens 
in  Lake  Huron  and  the  St.  Clair  River. 

i.  Fresh-water  Lakes,  a.  Mechanical  Deposits.  The  mechani- 
cal sediment  which  accumulates  in  a  lake  basin  is  of  two  kinds, 
(i)  that  which  is  brought  in  by  tributary  streams,  and  (2)  that  which 
the  lake  itself  acquires  by  cutting  back  its  shores;  of  these  the 
former  is  much  the  greater  in  volume.  Almost  without  exception, 
rivers  entering  lakes  form  deltas,  which  spread  out  fan-like  from 
the  stream  mouths,  and,  if  sufficiently  numerous,  may  fringe  the 
entire  lake  shore  with  delta  deposits.  Part  of  the  materials  will 
be  distributed  by  waves  and  currents,  but  the  coarser  material 
remains  to  form  the  foreset  beds,  the  inclination  of  which  depends 
upon  the  depth  of  the  lake  at  the  stream  mouth  and  upon  the 
coarseness  of  the  debris.  If  deposited  in  deep  water,  the  beds 
may  be  inclined  at  a  considerable  angle;  if  in  shallow  water, 


FRESH-WATER   LAKES 


they  form  a  very  gradual  slope.  In  small  lakes  the  coales- 
cence of  deltas,  or  the  advance  of  a  single  one,  will  eventually  fill 
up  the  basin,  forming  first  swamps  and  then  smooth,  grassy  mead- 
ows, through  which  flow  the  streams,  keeping  their  own  channels 
clear.  Such  filled-up  lakes  are  common  in  many  mountain  valleys. 
In  large  lakes  the  process  is,  of  course,  much  slower. 


FIG.  ioi.  — Gravel  beach,  Lake  Ontario.     (U.  S.  G.  S.) 

Away  from  the  deltas  the  combined  action  of  the  waves  and 
currents  fringes  the  lake  with  coarse  deposits  of  boulders,  gravel, 
and  sand,  which  form  the  beach,  the  sand  extending  some  dis- 
tance out  into  shallow  •  water.  In  large  lakes  the  heavy  surf 
cuts  a  terrace  on  the  shore  and  the  debris  thus  obtained  builds 
out  the  terrace,  which  is  therefore  said  to  be  "  cut  and  built." 
A  succession  of  terraces  indicates  the  various  levels  of  the  water, 
for  lakes  are  often  subject  to  great  fluctuations  of  level.  The  finer 
materials  are  carried  out  into  deeper  water  and  deposited  in  succes- 
sive layers  over  the  whole  lake  bottom,  the  finest  materials  in  the 


218 


LACUSTRINE  DEPOSITS 


centre.  The  coarse  and  fine  sediments  grade  into  each  other, 
dovetail  and  overlap,  because  in  heavy  storms,  or  when  the  streams 
are  in  flood,  the  coarser  sediments  are  carried  farther  out  and  de- 
posited on  the  fine,  and  these  changes  of  material  in  any  given 
vertical  section,  not  too  far  from  shore,  may  be  often  repeated. 
Special  lines  of  accumulation  for  the  coarse  substances  also  occur 
in  the  form  of  shoals,  spits,  embankments,  and  the  like.  If  the 
lake  is  subject  to  fluctuations  of  its  level,  with  the  water  much 


FIG.  102.  — Outlet  of  Lake  Bonneville,  Utah.     (U.  S.  G.  3.) 

higher  at  one  time  than  another,  even  more  wide-spread  changes 
in  the  character  of  the  deposits  will  occur.  The  deposits  now  form- 
ing in  the  great  Laurentian  lakes,  which -occupy  a  relatively  small 
drainage  basin  and  receive  no  large  tributaries,  are  principally 
blue  muds  and  clays,  partly  made  up  of  kaolirtite  and  partly  of  the 
debris  of  other  minerals  in  an  extremely  fine  state  of  subdivision, 
but  not  decomposed  chemically.  In  Lake  Superior  the  clay  has 
generally  a  pinkish  tinge. 
Owing  to  the  way  in  which  the  materials  are  arranged,  lake 


FRESH-WATER  LAKES 


219 


deposits  betray  the  form  of  the  basin  in  which  they  were  laid 
down.  .Around  the  old  shore  line  are  masses  of  coarse  materials, 
with  deltas  interspersed,  to  mark  the  mouths  of  streams,  while 
towards  the  middle  of  the  basin,  quantities  of  fine  mud  and  clay 
have  accumulated.  An  excellent  example  of  such  a  deserted  lake 
basin  is  that  known  as  Lake  Bonneville  in  Utah,  of  which  Salt 
Lake  is  the  shrunken  remnant.  The  drying  up  of  this  lake,  which 
was  once  fresh  and  had  an  outlet  northward  to  the  Snake  River, 


FiG.  103.  — Terraces  of  Lake  Bonneville,  Utah.     (U.  S.  G.  S.) 

is  an  event  geologically  so  recent,  that  its  form  and  size,  its  shores 
and  islands,  its  high  and  low  stages,  in  short,  its  history,  can  be 
made  out  with  great  clearness,  as  has  been  admirably  done  by 
Mr.  Gilbert  of  the  United  States  Geological  Survey.  At  its  time 
of  greatest  extension,  Lake  Bonneville  had  an  area  of  19,750 
square  miles  and  a  maximum  depth  of  1050  feet,  while  Salt  Lake 
(which  is  variable)  had  in  1869  an  area  of  2170  miles  and  an 
extreme  depth  of  46  feet.  Around  the  ancient  shores  are  beauti- 


22O  LACUSTRINE  DEPOSITS 

fully  preserved  the  terraces,  embankments,  and  deltas  of  the  various 
stages  of  water,  with  the  gravels  and  sands  appropriate  to  the  shal- 
low water.  The  principal  part  of  the  basin  is  a  level  plain,  filled  to 
a  great  but  unknown  depth  with  beds  of  clay  and  marl  (Fig.  104). 
In  still  more  ancient  lakes  the  terraces,  embankments,  and  other 
shore  features  have  been  swept  away  by  the  processes  of  denuda- 
tion, but  the  outline  of  the  lake  may  frequently  be  reconstructed 
from  the  character  of  the  deposits. 

b.  Chemical  Deposits  are  not  common,  nor  of  much  importance 
in  fresh  lakes.     In  a  few,  chemically  precipitated  carbonate  of 
lime  is  found,  and  more  abundant  is  limonite  (2  Fe.2O3,3  H2O).  This 
is  carried  into  the  lake  by  streams  that  contain  dissolved  ferrous 
carbonate  (FeCO3),  which,  becoming  oxidized  and  hydrated,  is  no 
longer  soluble,  and  accumulates  on  the  bottom.     In  Sweden  ores 
of  this  kind  are  dredged  out  of  the  lakes  and  employed  as  a  source 
of  iron. 

c.  Organic  Deposits  are  seldom,  important  in  large  lakes,  but 
often  decidedly  so  in  small  ones.     As  we  have  already  seen,  peat 
often  forms  to  such  an  extent  as  to  choke  up  the  lake  and  convert 
it  into  a  bog.     Siliceous  accumulations  are  made  on  an  extensive 
scale  by  the  minute  plants,  diatoms,  which  though  of  microscopic 
size,  yet  multiply  with  extraordinary  rapidity;  their  tests  of  trans- 
parent flint  gather  on  many  lake  bottoms  in  a  fine  deposit,  as 
white  as  flour,  and  variously  called  Tripoli  or  polishing  powder,  or 
infusorial  earth.    Calcareous  accumulations  are  formed  by  the 
shells  of  fresh-water  molluscs,  often  in  masses  of  considerable 
thickness.    The  lower  layers  of  this  shell  marl  have  generally  been 
so  much  disintegrated  by  the  water  as  to  be  without  any  obvious 
organic  structure.     Such  marls  are  frequently  found  under  peat 
bogs  and  indicate  that  the  latter  were  originally  lakes,  and  in  the 
marl  often  occur  the  bones  of  extinct  animals. 

2.  Salt  Lakes  are  especially  characteristic  of  arid  climates,  in 
which  the  rainfall  is  light  and  evaporation  great.  They  may  be 
formed  in  either  of  two  ways:  (i)  through  the  separation  of  bodies 
of  water  from  the  sea.  This  is  exemplified  by  the  Salton  Sink  in 


SALT  LAKES 


221 


222  LACUSTRINE  DEPOSITS 

the  Colorado  desert  of  southeastern  California;  the  bottom  of  the 
Sink  is  considerably  below  sea  level  and  has  recently  been  con- 
verted into  a  lake  by  the  influx  of  the  Colorado  River.  Originally 
the  Salton  Sink  was  the  head  of  the  Gulf  of  California  and  old 
beaches,  shell  banks,  etc.,  still  remain  to  indicate  this.  The  Colo- 
rado River,  which  enters  the  Gulf  from  the  east  some  distance 
below  the  head,  built  its  delta  across  the  Gulf  and  thus  cut  off  the 
apper  portion  into  a  salt  lake,  which  subsequently  disappeared  by 
evaporation.  Beds  of  salt,  which  demonstrate  the  lacustrine 
stage,  occur  in  the  Sink.  (2)  By  the  long-continued  concentration 
of  river  water  in  basins  that  have  no  outlet,  where  the  influx  of 
water  is  disposed  of  by  evaporation  from  the  surface  of  the  lake. 
In  either  case  an  arid  climate  is  requisite  to  maintain  the  salinity; 
in  a  moist  region  the  large  rainfall  and  slower  evaporation  would 
cause  the  lake  to  rise  until  it  found  an  outlet,  and  then  the  water, 
if  originally  salt,  would  become  fresh.  The  history  of  Lake  Bonne- 
ville  exemplifies  the  change  from  fresh  to  saline  conditions.  As  long 
as  the  water  level  was  maintained  above  the  outlet,  the  lake  was 
fresh,  but  when  the  advancing  aridity  of  the  climate  diminished 
the  rainfall  and  increased  the  rate  of  evaporation,  the  water  level 
sank  until  it  fell  below  the  outlet.  Then  the  lake  became  saline, 
reaching  its  maximum  salinity  in  the  intensely  bitter  waters  of 
Salt  Lake,  which  is  the  remnant  of  the  large  lake. 

All  river  water  contains  greater  or  less  quantities  of  dissolved  sub- 
stances, and  of  these  one  of  the  commonest  is  ordinary  So.lt  (NaCl). 
When  such  waters  are  evaporated,  the  solids  remain  behind,  and 
thus  the  water  becomes  more  and  more  saline  till  it  reaches  satu- 
ration. The  salt  is,  however,  not  entirely  derived  from  the  rivers, 
for  salt  is  a  very  wide-spread  constituent  of  desert  soils  and  surfaces, 
and  in  the  Pampas  of  Argentina,  salt  crusts  form  repeatedly.  Wind 
and  rain  thus  bring  into  the  lake  quantities  of  salt  in  addition  to 
that  carried  by  the  tributary  streams.  Other  substances  occur 
also,  as  will  be  seen  below. 

The  mechanical  deposits  formed  in  salt  lakes  do  not  differ  in 
any  very  important  manner  from  those  of  fresh  lakes.  The  finer 


SALT  LAKES  223 

clays  settle  more  rapidly  in  brine  than  in  fresh  water,  which  makes 
strongly  saline  lakes  extraordinarily  clear  and  limpid.  The  or- 
ganic deposits  of  salt  lakes  are  practically  nothing,  for  brackish 
water  is  not  favourable  to  many  organisms  and  in  dense  brines  very 
few  animals  or  plants  can  exist,  and  those  that  can  are  not  the 
kinds  which  give  rise  to  peat,  or  to  siliceous  or  calcareous  deposits. 
For  the  same  reason,  the  deposits  of  whatever  kind,  laid  down  in 
salt  lakes,  are  almost  barren  of  fossils,  except  of  land  animals  and 
plants,  such  as  are  washed  into  the  lake  by  flooded  streams. 

The  chemical  deposits  are  much  the  most  interesting  and  char- 
acteristic of  the  accumulations  gathered  in  salt  lakes.  These 
chemical  precipitates  differ  much  in  the  various  lakes,  according 
to  the  nature  of  the  rocks  which  form  the  drainage  basins,  but 
while  some  of  the  substances  are  rare  and  restricted  in  extent, 
others  are  extremely  common  and  wide-spread.  Several  changing 
factors  combine  to  vary  the  order  of  precipitation  of  the  salts, 
which  is  a  highly  complex  process,  but,  in  general,  it  follows  the 
inverse  order  of  solubility,  the  least  soluble  material  being  deposited 
first  and  the  most  soluble  last.  Comparatively  little  chemical 
reaction  appears  to  take  place  in  these  lakes;  the  substances  arej 
for  the  most  part,  thrown  down  merely  by  the  evaporation  of  satu- 
rated solutions  and  are  the  same  as  those  carried  in  very  dilute  solu- 
tions by  the  tributary  springs  and  streams.  If  the  precipitation  of 
salts  is  slow  and  occasional,  the  chemically  and  mechanically 
formed  deposits  are  mingled  together;  but  if  such  precipitation  be 
rapid,  then  thick  and  nearly  pure  masses  of  the  salts  are  thrown 
down  in  their  proper  order,  as  the  concentration  by  evaporation 
proceeds. 

The  first  substances  to  be  deposited  from  solution  are  the  car- 
bonate of  lime  and  oxide  of  iron  (CaCO3  and  Fe2O3),  and  in 
moderately  saline  lakes  this  is  about  the  limit  of  precipitation. 
These  same  materials  are  thrown  down  in  fresh  lakes  also,  and 
their  deposition  is  principally  due  to  the  loss  of  the  solvent  CO2. 
The  ancient  Lake  Lahontan,  which  formerly  occupied  part  of 
northwestern  Nevada,  was  the  seat  of  calcareous  deposition  on  a 


224 


LACUSTRINE  DEPOSITS 


magnificent  scale,  and  every  crag  and  island  which  its  waters 
touched  is  sheathed  in  thick  masses  of  calcareous  sinter.  Pyramid 
Lake,  a  remnant  of  Lahontan,  has  a  remarkable  island  of  cal- 
careous tufa;  and  Mono  Lake,  California,  is  famous  for  similar 
deposits,  which  have  assumed  curious  and  whimsical  shapes. 

As  the  concentration  of   the  lake  waters  proceeds,  the  next 
substance  to  be  precipitated  is  gypsum  (CaSO4,  2  H2O),  which, 


FIG.  105.  —  Island  of  calcareous  tufa,  Pyramid  Lake,  Nevada.     (U.  S.  G.  S.) 

though  much  more  soluble  than  the  carbonate  of  lime,  is  yet  only 
sparingly  so.  After  all  the  gypsum  in  solution  has  been  thrown 
down,  there  follows  a  pause  in  the  deposition,  until  a  further  stage 
of  concentration  has  been  reached,  and  then  common  salt  is  pre- 
cipitated, which  deposition  continues  steadily  as  concentration 
proceeds,  but  at  an  advanced  stage  the  salt  is  mingled  with  the 
sulphate  of  magnesia  (MgSO4),  should  that  be  present.  The 


SALT   LAKES 


225 


highly  soluble  salts,  such  as  the  chlorides  of  magnesium  and  cal- 
cium (MgCl2,  CaCy,  remain  in  solution  until  the  water  is  com- 
pletely evaporated  to  dryness,  hence  they  are  rarely  found  in  beds 
of  rock  salt ;  yet  when  they  do  occur,  as  at  Stassf  urt  in  Prussia, 
they  are  mingled  with  salt,  which  is  thus  precipitated  till  the  very 
end  of  the  process. 


FIG.  106.  — Salt  deposit,  El  Paso,  Texas.     (U.  S.  G.  S.) 

Various  circumstances  may  change  the  order  of  precipitation 
just  given.  In  seasons  of  high  water  the  flooded  rivers  dilute  the 
waters  of  the  lake,  checking  the  chemical  precipitation  and,  at  the 
same  time,  increasing  the  mechanical  deposition;  thus  beds  of 
sand  and  mud  are  thrown  down  upon  the  beds  of  gypsum  and 
salt,  alternating  with  them,  as  the  influx  of  fresh  water  or  evapora- 
tion predominates.  Changes  of  temperature  also  have  an  effect 
upon  the  order  of  precipitation.  Thus,  in  cold  weather,  Salt  Lake 

Q 


226  LACUSTRINE  DEPOSITS 

washes  up  on  its  shores  quantities  of  sulphate  of  soda  (Na2SO4), 
which  is  formed  at  low  temperatures  by  the  double  decomposition 
of  NaCl  and  MgSO4. 

Besides  the  chemical  deposits  already  mentioned,  others  occur 
on  a  smaller  scale.  On  the  western  side  of  the  Great  Basin,  in 
Nevada,  California,  and  Oregon,  are  several  lakes  which  contain 
large  proportions  of  carbonate. of  soda,  and  in  some  of  them  the 


FIG.  107.  —  Deposits  in  an  alkali  lake.     (U.  S.  G.  S.) 

concentration    is    sufficiently    advanced    to    cause   precipitation, 
while  others  form  deposits  of  borax. 

Much  the  most  abundant  of  the  chemical  deposits  made  in  salt 
lakes  are  gypsum  and  rock  salt,  and  the  enormous  scale  on  which 
the  latter  was  formed  in  past  ages  of  the  world's  history  is  demon- 
strated by  the  vast  bodies  of  rock  salt  which  are  found  embedded 
in  the  rocks  in  so  many  parts  of  the  world.  Near  Berlin,  at  Speren- 


GLACIAL  DEPOSITS  22; 

berg,  an  artesian  well  was  sunk  through  such  a  deposit  for  nearly 
4000  feet,  without  reaching  the  bottom.  In  various  regions  of 
the  United  States,  notably  in  New  York  and  Kansas,  large  bodies 
of  salt  are  found,  but  not  on  such  a  scale  as  in  Europe. 

Salt  bodies  of  such  immense  extent  and  thickness,  with  little  or 
no  interstratified,  mechanically  deposited  material,  are  not  ex- 
plained by  the  usual  operations  of  a  salt  lake  as  above  described. 
The  key  to  the  understanding  of  these  enormous  deposits  of  salt 
appears  to  be  given  by  a  gulf  from  the  eastern  part  of  the  Caspian 
Sea,  known  as  the  Karibogas.  This  gulf,  which  is  connected  with 
the  Caspian  only  by  a  very  narrow  channel,  is  situated  in  an  ex- 
tremely hot  and  almost  rainless  region,  and  evaporation  is  very 
rapid.  On  the  bottom  masses  of  salt  have  been  deposited  unin- 
terruptedly for  a  long  time  past,  and  in  a  very  deep  or  a  slowly 
subsiding  basin  the  process  might  continue  almost  without  a  limit. 

It  should  be  noted  that  the  chemical  deposits  made  in  salt  lakes 
are  crystalline  and  at  the  same  time  stratified.  This  association 
is  not  the  usual  one,  as  stratified  rocks  are  ordinarily  not  crystal- 
line, and  crystalline  rocks  are  mostly  Unstratified. 

V.    ICE  DEPOSITS 

Deposits  made,  directly  or  indirectly,  by  the  agency  of  ice  are 
very  characteristic,  and  though  some  are  formed  on  land  and  some 
under  water,  it  is  desirable  to  consider  them  together  in  a  single 
section.  The  peculiar  features  of  ice  formations  may  be  much 
obscured  by  the  action  of  water,  either  at  the  time  of  their  deposi- 
tion or  at  some  subsequent  period.  Ice  deposits  play  but  a  small 
part  quantitatively  in  the  construction  of  the  earth's  crust,  but  the 
light  which  they  throw  upon  changes  of  climate  and  similar  ques- 
tions, lends  them  an  unusual  degree  of  interest.  Only  very  re- 
cently has  the  great  importance  of  the  part  played  by  glaciers  in 
former  ages  of  the  earth's  history  been  appreciated. 

Glacial  Deposits.  —  We  have  already  learned  that  glaciers  carry 
with  them  great  masses  of  debris,  either  in  the  form  of  lateral 


228 


ICE  DEPOSITS 


and  medial  moraines  upon  their  upper  surfaces,  or  frozen  in  the 
interior  of  the  ice,  or  pushed  along  beneath  it.  When  the  ice 
reaches  the  end  of  the  glacier,  where  the  rates  of  motion  and 
melting  balance  each  other,  all  the  burden  which  it  is  transporting 
is  deposited  in  a  great  mound  or  ridge,  the  terminal  moraine.  The 
terminal  moraine  is  composed  of  material  which  was  carried  upon 
and  within  the  ice  and  that  which  was  pushed  along  beneath  the 


FIG.  108.  — Fluted  ground  moraine ;  Columbia  Glacier,  Alaska.     (U.  S.  G.  S.) 

glacier.  It  is  gradually  built  up  around  the  end  of  the  glacier  and 
extends  up  along  its  sides  so  far  as  the  conformation  of  the  ground 
will  permit  the  material  to  gather,  and  is  thus  more  or  less  crescentic, 
with  concave  side  directed  upward.  Moving  ice  does  not  sort  the 
material  which  it  carries,  as  flowing  water  does,  because  in  a  glacier 
there  is  no  such  definite  relation  between  velocity  and  transporting 
power.  Hence,  the  terminal  moraine  is  unstratified  and  is  com- 
posed of  materials  of  all  sizes,  from  dust  and  sand  up  to  great 


GLACIAL  DEPOSITS 


229 


boulders  weighing  hundreds  of  tons,  all  mingled  together  in  con- 
fusion. In  the  case  of  a  glacier  which  carries  the  principal  part 
of  its  burden  upon  its  upper  surface,  the  terminal  moraine  is  chiefly 
made  up  of  angular  blocks  that  have  undergone  little  or  no  abrasion, 
together  with  earth,  sand,  gravel,  and  whatever  kind  of  material 
the  overhanging  cliffs  may  have  delivered  to  the  moving  ice. 
Mingled  with  these  materials,  however,  will  be  found  more  or  fewer 


FIG.  109. —  Glacial  moraine,  Montauk  Point,  L.I.      (Photograph  by  B.  N.  Mitchill) 


of  the  characteristically  worn  and  grooved  glacial  pebbles  and 
boulders,  which  have  been  dragged  along  under  the  ice,  and  scored 
and  polished  by  the  rocky  bed.  There  will  also  be  found  some,  at 
least,  of  the  sand  and  fine  rock  flour  which  the  glacier's  own  move- 
ment produces  and  which  have  escaped  the  washing  of  the  sub- 
glacial  stream. 

When  a  glacier  is  retreating,  it  may  build  up  a  new  terminal 


230 


ICE  DEPOSITS 


moraine  at  each  point  of  arrested  withdrawal,  or  if  the  retreat  is 
gradual  and  steady,  the  ground  in  front  of  the  ice  will  be  covered 
with  moraine  material,  spread  out  in  a  sheet,  not  heaped  up  in  a 
moraine  or  mound.  The  retreat  of  the  glacier  may  leave  behind 
it  isolated  masses  of  ice  deeply  buried  in  the  debris  of  the  terminal 
moraine;  when  such  masses  melt,  they  form  depressions  in  the 
mound  and  give  rise  to  the  "  kettle  moraines."  A  shrinking  glacier 


FIG.  no.  —  Glacial  pebble.     (U.  S.  G.  S.) 

will  contract  laterally  and  in  depth,  as  well  a_s  longitudinally,  and 
in  this  way  the  blocks  of  the  lateral  moraine  will  be  left  stranded  at 
intervals  over  the  former  glacial  bed.  Such  blocks  and  boulders 
are  known  as  erratics,  or  perched  blocks,  and  when  their  parent 
ledge  can  be  discovered,  it  is  easy  to  determine  the  distance  to 
which  they  have  been  carried.  Sometimes  a  great  boulder  is 
lowered  so  gradually  and  gently  by  the  retreating  ice,  that  it  is 


GLACIAL  DEPOSITS 


FIG.  in.  — Striated  glacial  boulders  from  Permian  of  South  Africa 


FIG.  112.  —  Kettle  moraine,  Alaska.     (U.  S.  G.  S.) 


232 


ICE  DEPOSITS 


exactly  balanced,  and  may  be  moved  backward  and  forward  by  the 
hand.  This  is  a  "  rocking-stone,"  though  it  must  not  be  supposed 
that  all  rocking-stones  are  glacial  (see  p.  122). 

The  ground  moraine  consists  of  the  debris  which  is  deposited 
underneath  moving  ice  and  that  which  is  left  from  the  bottom  of  a 
retreating  glacier.  Deposition  beneath  moving  ice  is  much  less 
in  amount  than  erosion,  but  may  occur  in  areas  sheltered  from  ice 


FIG.  113.  — Perched  block  near  the  Yellowstone  Canon,  National  Park.     (U.  S.  G.  S.) 

pressure,  or  where  the  ice  thins  rapidly  from  melting.  The  ground 
moraine  is  not  stratified,  or  but  imperfectly  so  when  deposited 
partly  from  water,  and  is  of  very  different  thickness  in  different 
places;  it  consists  of  fine  material  containing  boulders  and  peb- 
bles, many  of  which  show  the  characteristic  facetting  and  striae, 
and  is  called  till,  or  boulder  clay.  The  general  term  for  a  sheet 
of  glacial  deposits  is  drift. 


GLACIAL  DEPOSITS 


FIG.  114.  — Glacial  drift,  Bangor,  Pa.     (U.  S.  G.  S.) 


FIG.  115.  —  Glacial  drift  in  Permian  of  South  Africa.     (Photograph  by  Rogers) 


234 


ICE  DEPOSITS 


The  water  deposits  which  are  made  in  the  neighbourhood  of  and  in 
association  with  a  glacier,  are  also  characteristic  and  should  be  no- 
ticed in  this  connection.  The  streams  which  flow  beneath  and  from 
the  foot  of  the  glacier  are  loaded  to  their  utmost  capacity  with  debris, 
and  usually  build  up  their  beds  by  rapid  deposition  of  the  coarser 
sediment.  From  an  alpine  glacier  this  deposit,  which  is  stratified, 
forms  a  valley  train,  but  when  the  glacier  ends,  as  do  some  of  those  in 
Iceland,  upon  a  more  or  less  flat  surface,  a  broad  overwash  plain  is  the 


FIG.  116.  —  Glacio-fluvial  deposits,  Yahtse  River,  Alaska.     (U.  S.  G.  S.) 

result.  These  accumulations  may  usually  be  distinguished  from 
ordinary  river  and  flood  plain  deposits  by  their  upward  extension 
into  moraines  and  by  the  evidently  glacial  origin  of  their  materials. 
Eskers  (or  Asar)  are  long,  winding  ridges  of  gravel,  which  often 
ramify  like  the  branches  of  a  stream  and  were  formed  by  streams 
which  flowed  in  channels  upon  or  in  tunnels  beneath  the  ice. 
Kames  are  hillocks,  or  short  ridges  of  stratified  gravel  and  sand 
heaped  up  by  subglacial  streams,  as  they  escape  from  the  margin 


GLACIAL  DEPOSITS 


235 


of  the  ice,  and  drumlins  are  elliptical  mounds  of  ground  moraine, 
sometimes  with  some  stratification,  formed  near  the  margin  of  the 
ice. 

Very  instructive  examples  of  this  combined  action  may  be  ob- 
served about  the  great  Malaspina  Glacier  in  Alaska.  This  is  an 
immense  ice-sheet,  with  an  area  of  1500  square  miles,  which  is 
formed  at  the  foot  of  the  St.  Elias  Alps  by  the  confluence  of  several 


FIG.  117.  —  Gravel  flood  plain  of  glacial  stream,  Alaska.     (U.  S.  G.  S.) 

great  glaciers.  All  the  outer  borders  of  the  glacier  are  covered 
with  sheets  of  moraine  matter,  and  upon  the  stagnant  portion  of  this 
is  a  luxuriant  growth  of  bushes,  beneath  which  is  a  thickness  of  not 
less  than  1000  feet  of  ice.  About  the  margin  of  the  ice-sheet,  small 
lakes. are  formed,  the  water  being  held  in  place  by  the  ice  barrier, 
but  these  lakes  are  subject  to  great  fluctuations,  and  often  their 


ICE  DEPOSITS 


FIG.  1 1 8.  —  Esker,  central  New  York.     (Photograph  by  Fairchild) 


FiG.  up.  —  Kame  moraine,  central  New  York.     (U.  S.  G.  S.) 


GLACIAL  DEPOSITS 


237 


waters  escape  through  tunnels  in  the  ice.  In  some  of  these  lakes 
stratified  deposits  are  made  by  the  inflowing  streams.  Innumer- 
able streams,  some  of  them  quite  large,  rise  from  under  the  glacier, 
and  many  others  flowing  from  the  north  pass  under  the  free  margin 
of  the  ice  by  means  of  long  tunnels.  All  of  these  streams  are  loaded 
to  their  utmost  capacity  with  sediment,  gravel,  and  boulders;  by 


FIG.  120.  —  Drumlin  near  Newark,  N.Y.     (U.  S.  G.  S.) 


blocking  up  their  own  openings  from  the  ice,  they  likewise  cause 
the  deposition  of  sand,  gravel,  and  boulders  within  their  tunnels, 
which,  when  the  glacier  retreats,  will  be  left  standing  as  eskers, 
while  conical  mounds,  or  kames,  are  built  up  where  the  streams 
burst  from  under  the  ice,  and  sometimes,  owing  to  the  great  press- 
ure, rise  like  fountains.  This  kind  of  deposition  is  characteristic 
of  retreating  ice-sheets,  such  as  theMalaspina;  in  advancing  gla- 


238 


ICEBERG  DEPOSITS 


ciers  denudation  will  prevail  over  deposition,  or,  if  the  advance 
be  not  so  great  as  to  sweep  away  all  the  previous  accumulations, 
purely  glacial  deposits  may  be  laid  down  upon  the  stratified  fluvio- 
glacial  material. 

Iceberg  Deposits.  —  When  a  glacier  flows  into  the  sea  great 
masses  are  broken  off  from  the  foot  and  float  away  as  icebergs. 
Icebergs  are  thus  seen  to  be,  as  indeed  they  always  are,  derived 
from  land  ice  and  not  from  the  freezing  of  sea-water.  The  ice- 


FIG.  121.  —  Drift-covered  surface  of  the  Malaspina  Glacier,  Alaska.     (U.  S.  G.  S.) 

berg  will,  of  course,  carry  with  it  whatever  parts  of  the  glacial 
debris  are  contained  within  or  upon  that  particular  fragment  of  the 
glacier,  and  drops  this  load  over  the  sea-bottom,  as  the  berg  gradu- 
ally melts.  As  the  Greenland  icebergs  sometimes  drift  as  far  south 
as  the  Azores,  glacial  boulders  are  scattered  all  over  the  bed  of  the 
North  Atlantic,  and  thus  we  see  how  large  blocks  may  be  em- 
bedded in  stratified  deposits  very  far  from  the  place  where  they 
were  torn  from  their  parent  ledges. 


ICE  DEPOSITS 


239 


Coast  Ice  Deposits.  —  In  high  latitudes  with  intensely  cold 
winters,  great  fields  of  ice  (the  ice-foot)  are  formed  by  the  freezing 
of  sea-water  .along  the  shore.  The  ice-foot  becomes  loaded  with 
great  masses  of  rock,  part  of  which  is  thrown  down  from  over- 
hanging cliffs  by  the  action  of  frost,  part  picked  up  from  the  shore- 
line by  the  ice  forming  around  it.  In  summer  the  coast  ice  breaks 
up  and  floats  away  with  its  load  of  blocks  and  boulders,  distributing 


FIG.  122.  —  River  issuing  from  the  Malaspina  Glacier,  Alaska. 
(U.  S.  G.  S.) 

them  over  the  sea-bottom  just  as  icebergs  do.  In  storms  great 
masses  of  coast  ice  are  often  driven  on  the  shore,  where  they  may 
pile  up  to  heights  of  fifty  feet  or  more,  carrying  some  of  the  boulders 
above  the  levels  at  which  they  were  picked  up.  The  coast  of 
Labrador  is  covered  for  long  distances  with  boulders  thus  trans- 
ported, as  are  many  other  Arctic  shores.  Great  masses  of  rock 
are  thus  transported  in  the  Baltic,  and  the  divers  report  that  in 


240 


CONTINENTAL  DEPOSITS 


the  Copenhagen  Sound  the  sunken  wrecks  of  vessels  are  covered 
with  ice-borne  blocks. 


CLIMATIC  RELATIONS  OF  CONTINENTAL  DEPOSITS 

This  subject,  though  of  great  importance  in  deciphering  the  earth's 
history,  is  extremely  complex  and  can  be  but  briefly  outlined  here. 


FIG.  123.  —  Deposit  partly  made  by  strandad  ice,  west  coast  of  Greenland. 
(Photograph  by  Libbey) 

i.  Polar  Regions.  —The  activity  of  frost  destruction  in  polar 
lands  results  in  the  accumulation  of  great  talus  masses  of  sharply 
angular  blocks  and  fragments,  while  the  low  temperature,  even  of 
summer,  is  unfavourable  to  chemical  decomposition  of  the  rocks 
and  the  consequent  formation  of  soil.  River  deposits  are  not  ex- 
tensively formed,  for  rivers  in  the  true  polar  lands  are  comparatively 
tew  and  small,  while  glacial  accumulations,  on  the  other  hand, 


CLIMATIC   RELATIONS  241 

assume  great  importance.  Small  lakes  are  frequently  found  among 
the  moraines,  but  their  deposits  are  not  made  on  a  large  scale.  On 
the  other  hand,  peat  is  very  largely  accumulated  in  the  illimitable 
"  tundras,"  or  swamps,  of  the  lower  polar  latitudes,  the  low  tem- 
peratures retarding  the  work  of  decomposition. 

2.  Temperate  Regions. — The  temperate  regions   are   distin- 
guished from  the  tropics  on  the  one  hand,  and  the  polar  lands 
on  the  other,  by  their  frequent  and  wide  changes  of  temperature 
and  by  the  very  great  variety  of  conditions  which  obtain.     In 
temperate  climates  with  normal  rainfall,  river  deposits,  and  some- 
times lake  deposits,  assume  the  most  important  place,  while  glacial 
drift  and  moraines  are  local  only  and  confined  to  mountain  areas 
sufficiently  high  to  extend  above  the  snow-line.     Such  mountains 
are,  in  fact,  extensions  of  polar  lands  into  the  temperate  regions, 
and  the  deposits  characteristic  of  the  former  accompany  them. 
The  temperate  regions  of  pluvial  climate  are  densely  covered  with 
vegetation,  so  that,  except  on  sandy  coasts,  wind  deposits  are  of 
small  importance  and  the  material  laid  down  in  the  flood  plains  of 
rivers  is  not  extensively  sun-cracked,  while  vegetable  accumula- 
tions are  very  extensively  formed  in  swamps  and  bogs,  especially 
in  the  cooler  and  moister  parts.      Pluvial  climates,  again,  are 
highly  favourable  to  the  decomposition  of  the  rocks  and  the  for- 
mation of  deep  soils  which  are  prevailingly  brown  in  colour  from 
the  presence  of  hydrated  iron  oxide,  or  limonite. 

In  the  semi-arid  parts  of  the  temperate  regions  wind-made 
accumulations  of  loess  are  developed  very  extensively,  and  in  the 
more  arid  parts,  river  and  playa  and  salt  lake  deposits  become  con- 
spicuous. Sun  cracks  are  highly  characteristic  of  playa  muds 
and  around  the  margins  of  fluctuating  salt  lakes. 

3.  The  Desert  Zones.  —  In  both  the  northern  and  southern 
hemispheres,  between  the  temperate  and  tropical  regions,  is  an 
irregular  belt  of  desert,  encircling  the  earth.    The  deserts  are  not 
entirely  rainless,  but  so  nearly  so  that  they  have  no  drainage  out- 
let.    The  Nile  and  the  Colorado  flow  across  deserts,  but  their 
sources  lie  outside  of  the  desert  zone,  and  water  plays  but  a  subor- 

R 


242  CONTINENTAL  DEPOSITS 

dinate  part  in  desert  accumulations.  Talus  masses,  due  to  the 
rapid  changes  of  temperature,  are  very  common  in  true  deserts, 
where  "  all  the  mountains  rise  like  islands  out  of  an  almost  flat 
sea  of  talus  "  (Walther).  Wind-bloWn  sand  is,  of  course,  very 
characteristic  of  deserts,  which  lies  in  chains  of  dunes,  instead  of 
with  a  level  surface;  the  sand  grains  are  small,  rounded  and 
polished  by  attrition,  often  fractured  by  the  sun's  heat,  and  pre- 
vailingly of  a  more  or  less  reddish  colour.  Salt  lakes  can  exist  only 
in  arid  climates,  so  that  salt  and  gypsum  are  among  the  most  char- 
acteristic of  desert  deposits. 

4.  The  Tropics  have  a  constantly  high  temperature  with  but 
small  daily  or  seasonal  range,  and  a  very  heavy  rainfall,  which, 
however,  is  usually  confined  to  a  part  of  the  year.  Chemical 
decomposition  is  thus  very  active  and  complete,  and  the  soil  accu- 
mulates to  great  depths.  Especially  characteristic  of  the  tropics 
is  laterite  (see  p.  104),  the  red  colour  of  which  tinges  the  river  silts- 
Where  the  topography  is  favourable  to  such  accumulations,  enor- 
mous masses  of  river  deposits  are  formed,  as  in  the  interior  basins 
of  South  America,  while  the  alternating  wet  and  dry  seasons,  by 
reversing  the  movement  of  water  in  the  soil,  occasion  the  concen- 
tration and  deposition  at  the  surface  of  iron  and  limestone. 
Despite  the  great  luxuriance  of  tropical  vegetation,  the  climate  is 
much  less  favourable  to  the  formation  of  peat  than  is  that  of  cooler 
latitudes,  because  of  the  rapidity  and  completeness  of  decomposi- 
tion. It  is  almost  needless  to  say  that  the  climatic  zores  pass  into 
one  another  by  gradual  transitions. 


CHAPTER    IX 

RECONSTRUCTIVE    PROCESSES.  —  MARINE   AND 
ESTUARINE   DEPOSITS 

B.     MARINE   DEPOSITS 

THE  sea  is  the  great  theatre  of  sedimentary  accumulation,  and 
rocks  of  marine  origin  form  the  larger  part  of  the  present  land  sur- 
faces. Important  as  other  classes  of  deposits  may  be,  they  are 
less  so  than  those  laid  down  in  the  ocean  and  the  waters  imme- 
diately connected  with  it.  There  is  great  variety  in  the  sedimen- 
tary deposits  made  in  the  sea,  which  change  in  accordance  with 
climate,  the  depth  of  water,  the  nature  of  the  coast  rocks,  the  force 
of  winds  and  tides,  and  the  nearness  or  remoteness  of  the  mouths 
of  rivers  and,  in  a  very  important  degree,  with  the  elevation  and 
relief  of  the  adjoining  land  masses;  very  different  materials  are 
supplied  by  bold  and  rocky  shores  from  those  derived  from  flat, 
sandy  coasts.  Large  land-locked  seas,  like  the  Gulf  of  Mexico 
and  the  Mediterranean,  again,  have  deposits  more  or  less  different 
from  those  of  the  open  ocean,  a  difference  which  is  largely  due  to 
the  absence  or  insignificance  of  the  tide,  and  the  reduced  force  of 
the  waves. 

It  is  important  to  remember  that  the  actual  line  of  meeting  of 
sea  and  land  is  not  the  structural  margin  of  the  continent,  for  the 
water  may  cover  a  broad  submerged  shelf  of  the  latter.  For  100 
miles  east  from  the  coast  of  New  Jersey  the  water  deepens  very 
gradually  to  the  loo-fathom  line,  whence  it  shelves  very  steeply 
to  the  profound  oceanic  abyss.  Shoal  water,  less  than  100  fathoms 
deep,  surrounds  all  coasts,  sometimes  as  a  narrow  belt,  again  as  a 
very  broad  zone.  The  transition  from  shoal  water  to  the  deep  sea 

243 


244 


MARINE  DEPOSITS 


is  by  steep  slopes,  with  well-defined  upper  margin  or  edge;  as  a 
rule,  these  steep  slopes  of  the  continental  mass  begin  at  the  100- 
fathom  line,  but  there  are  frequent  departures  from  this  rule. 
For  example,  in  the  Gulf  of  Guinea  the  steep  descent  begins  at  the 
4o-fathom  line,  while  off  the  west  coast  of  Ireland  the  descent  is 
gradual  almost  to  a  depth  of  200  fathoms  and  then  becomes  steep. 
The  bed  of  the  Atlantic  off  the  Carolinas  displays  no  well-marked 


FIG.  124.  —  Basin  of  the  Gulf  of  Mexico,  showing  the  submerged  margin  of 
the  continental  platform  and  the  steep  descent  of  the  botton.  at  the  100- 
fathom  line.  Vertical  scale  much  exaggerated.  (From  a  model  by  the 
U.  S.  Coast  Survey) 

edge  of  the  continental  shelf.  The  ocean  thus  fills  its  own  basin 
and  overflows  the  margins  of  the  continental  platforms  to  a  greater 
or  less  extent;  -this  submerged  shelf  constitutes  the  shallow  sea 
(Fig.  124). 

Marine  deposits  may  be  classified  primarily  in  accordance  with 
the  depth  of  water  in  which  they  were  laid  down,  one  of  the  most 
valuable  guides  to  the  history  of  ancient  rocks,  and  secondarily  in 
accordance  with  the  nature  of  the  material  of  which  they  are  com- 


MARINE   DEPOSITS 


245 


posed,  and  the  processes  by  which  they  were  accumulated.  The 
following  table  gives,  in  a  somewhat  modified  form,  the  classifica- 
tion of  Murray  and  Renard,  founded  upon  the  great  collections  of 
modern  marine  deposits  made  by  the  "  Challenger  "  expedition. 


1.  Littoral   Deposits, 

between  high  and 
low  water  marks. 

2.  Shoal -water  Deposits, 

between  low-water 
mark  and  100  fath- 
oms. 

3.  Aktian  Deposits,  laid 

down  on  the  conti- 
nental slope. 


Abysmal  Deposits, 
laid  down  on  the 
ocean  floor. 


MARINE  DEPOSITS 

Sands,  Gravels,  Muds, 
etc. 

Sands,  Gravels,  Muds, 
Cal  careous  Accumu- 
lations, etc. 

Coral  Mud. 
Volcanic  Mud. 
Green  Mud  and  Sand. 
Blue  Mud. 
Red  Mud. 
Calcareous  Deposits. 

Foraminiferal  Ooze. 
Pteropod  Ooze. 
Diatom  Ooze. 
Radiolarian  Ooze. 
Oceanic  Red  Clay. 


I.  Terrigenous  De- 
posits, material 
derived  from  the 
land  in  sus- 
pension (except 
the  calcareous 
masses). 


II.  Pelagic  Deposits, 
formed  in  deep 
water  far  re- 
moved from  land. 


The  material  brought  into  the  sea  by  rivers,  or  washed  from  the 
shore  by  waves,  is  partly  mechanically  suspended  and  partly  in  a 
state  of  solution;  the  former  is  deposited  when  the  water  is  no 
longer  able  to  transport  it,  while  some  of  the  latter  is  extracted 
by  animals  and  plants,  and  some  remains  permanently  dissolved. 
The  sorting  power  of  the  water  arranges  the  mechanically  borne 
sediments  according  to  the  coarseness  and  fineness  of  their  con- 
stituent particles,  at  the  same  time  separating  them  according  to 
their  mineralogical  composition,  a  separation  which  is  usually 
imperfect,  but  sometimes  very  complete.  Marine  deposits  are 
thus  typically  stratified,  though  when  deposition  continues  long 


246 


MARINE   DEPOSITS 


and  uninterruptedly,  thick  masses,  not  obviously  divided  into  layers, 
may  be  accumulated,  but  this  is  exceptional  in  those  parts  of  the 
sea  where  deposition  is  most  rapid. 

i.  Littoral  Deposits  are  laid  down  between  low  and  high-water 
marks,  and  by  heavy  storms  and  exceptional  tides,  somewhat 
above  the  former.  Thus,  the  accumulations  of  this  class  grade  into 
continental  deposits  on  one  side  and  into  those  of  the  shallow  sea, 


FIG.  125.  —  Gravel  beach  and  wall,  Conception  Bay,  Newfoundland.     (U.  S.  G.  S.) 

on  the  other,  and  are  themselves  alternately  covered  with  water 
and  exposed  to  the  sun  and  wind.  The  material  of  the  littoral 
beds  varies  much  on  different  coasts;  on  rocky  shores  boulders, 
coarse  shingle  and  gravel  form  the  beach,  but  gravel  and  more  par- 
ticularly sand  are  the  most  widely  distributed.  Boulders  and  shingle 
may  be  composed  of  any  kind  of  hard  rock,  but  as  the  process  of 
attrition  continues,  the  greater  hardness  of  quartz  has  its  effect, 
so  that  gravel  and  sand  generally  consist  of  that  mineral,  the  softer 


LITTORAL  DEPOSITS 


247 


minerals  being  ground  into  fine  particles  and  swept  out  into  deeper 
water.  Locally,  sand  of  other  composition  occurs,  as,  for  exam- 
ple, around  the  Bay  of  Naples  sands  of  olivine,  felspar  and  other 
volcanic  minerals,  are  found. 

The  waves  cast  material  upon  the  beach,  throwing  the  coarsest 
parts  up  in  storms  as  a  beach  wall  or  embankment,  above  their 
ordinary  reach,  while  the  undertow  carries  back  the  finer  particles, 


FIG.  126.  —  Gravel  beach,  Long  Island,  N.Y.     (U.  S.  G.  S.) 

thus  washing  the  sands  and  gravels  clean  of  other  minerals.  Even 
in  the  littoral  belt,  fine  sand  and  mud  may  gather  in  sheltered 
spots,  but  the  material  is  preponderatingly  coarse.  At  any  given 
time  the  littoral  is  a  narrow  belt,  measuring  at  present  about  62,000 
square  miles,  but  its  breadth  at  a  particular  place  depends  upon  the 
amount  of  tidal  rise  and  fall,  and  the  slope  of  the  bottom.  On  a 
stationary  coast,  or  one  where  accumulation  is  more  rapid  than 


248 


MARINE   DEPOSITS 


sinking,  littoral  deposits  may  form  a  broad  area  by  building  out  the 
land  at  the  expense  of  the  sea.  When  sinking  and  building  are 
about  equal,  great  thicknesses  of  littoral  beds  may  be  formed. 

Aside  from  the  coarseness  of  the  material,  littoral  deposits  are 
apt  to  retain  certain  characteristic  marks  of  their  exceptional  mode 
of  formation.  Ripple  Marks  are  formed  by  the  wind,  or  by  the 
rippling  movement  of  water,  and  may  be  seen  on  any  sandy  beach; 


FIG.  127.  —  Ripple- marked  sands,  low  tide;  Mont  St.  Michel,  France 

they  occur  especially  in  sands,  in  shoal-water  deposits,  as  well  as 
in  those  made  on  flood  plains  and  in  lakes  and  on  sand  dunes. 
They  are  found  in  rocks  of  all  geological  periods,  and  though 
most  frequent  in  sandstones,  occur  in  other  kinds  of  rocks.  Wave 
Marks  are  formed  by  waves  washing  up  on  the  beach  after  they 
have  broken,  and  are  preserved  by  the  deposition  of  thin  layers 
of  sand  on  the  edges  of  the  waves;  they  are  confined  to  the  littoral 


LITTORAL  DEPOSITS 


249 


zone.  Rill  Marks  also  are  peculiar  to  littoral  deposits  and  are 
made  by  the  excavating  action  of  rills  of  water  trickling  over  the 
sand  or  mud,  as  the  tide  ebbs.  Sun  Cracks  (also  called  mud  cracks 
and  shrinkage  cracks)  are  formed  where  flats  of  fine  mud  or  silt 
exposed  to  the  drying  action  of  the  sun,  harden  and  crack  in 


FIG.  128.  —  Ripple-marked  sandstone.     (U.  S.  G.  S.) 

more  or  less  regular  patterns.  As  we  have  already  learned,  such 
cracks  form  over  vast  areas  of  flood-plain  and  playa  deposits, 
and  the  littoral  are  the  only  truly  marine  deposits  which  display 
them,  though  on  a  limited  scale.  They  do  not  form  in  the  clean 
sands  and  gravels  which  make  up  the  greater  part  of  littoral  sedi- 
ment, but  only  in  fine  silt,  and  in  pluvial  climates  probably  only  in 


250 


MARINE  DEPOSITS 


such  areas  as  are  not  reached  by  the  ordinary  tides.  In  very  hot 
and  dry  climates,  as  on  parts  of  the  Red  Sea  coast,  cracks  may 
develop  in  the  course  of  a  few  hours. 


FIG.  129.  —  Steeply  inclined  beds  of  ripple-marked  shale ;  near  Altoona,  Pa. 
(U.  S.  G.  S.) 


LITTORAL   DEPOSITS 


251 


Rain  Prints  are  little  pit-like  marks  made  by  light  showers; 
the  prints  are  circular  where  the  raindrops  fall  vertically,  or  oval 
and  with  edge  raised  on  one  side  where  the  rain  falls  obliquely 
before  the  wind.  Tracks  of  Land  Animals  are  made  by  the  ani- 
mals walking  upon  the  soft  sediment,  which  is  yet  fine  enough  to 
retain  the  footprints  and  is  afterwards  hardened  by  exposure  to  the 
sun  and  air.  Rain  prints  and  footprints  are  not  so  common  in 


FlG.  130.  —  Wave  mark  and  rain  prints,  modern  sandy  beach.     (U.  S.  G.  S.) 

littoral  deposits  as  in  those  of  flood  plains  and  playas,  but  where 
they  do  occur  in  rocks  which  contain  marine  fossils,  they  prove 
the  littoral  origin  of  those  rocks,  for  only  in  the  littoral  zone  can 
such  marks  be  made. 

It  might  seem  incredible  that  such  slight  marks  could  be  pre- 
served for  ages  in  the  solid  rocks,  were  it  not  for  the  fact  that 
we  actually  find  them  so  often.  The  explanation  is  that  surfaces 
which  are  capable  of  preserving  these  marks  are  those  of  accu- 


2$2  MARINE  DEPOSITS 

mulation  and  that  each  layer  with  its  marks  is  hardened  by  the  sun 
and  wind  before  the  next  layer  is  deposited  upon  it. 

Climatic  differences  are  not  well  marked  in  the  littoral  zone,  the 
character  of  which  is  chiefly  determined  by  the  elevation  and  topog- 
raphy of  the  adjoining  land.  Only  in  the  polar  regions  is  a  special 
character  given  to  the  littoral  by  the  activity  of  frost  and  coast  ice, 
so  that  block  and  boulder  beaches  are  more  common,  and  sandy 
beaches  less  frequent  than  elsewhere. 


FIG.  131.  —  Rill  marks  on  modern  sandy  beach.     (U.  S.  G.  S.) 


2.  Shoal-water  Deposits.  —The  material  of  the  littoral  zone  is 
continued  out  beyond  low-water  mark  to  distances  which  vary 
according  to  several  circumstances.  Where,  for  long  distances,  no 
large  rivers  enter  the  sea  and  the  material  is  all  derived  from  the 
wear  of  the  coast,  the  arrangement  of  coarse  and  fine  deposits  is 
quite  regular,  and  gravel  beds  may  extend  as  far  as  ten  miles  from 
land.  Waves  and  currents  sweep  sediment  not  only  toward  the 


SHOAL-WATER   DEPOSITS 


253 


shore,  but  parallel  with  it,  and  tend  to  simplify  the  coast-line  by 
building  barriers  and  spits  across  the  mouths  of  bays,  which  the 


FlG.  132.  — Sun  cracks  in  limestone,  Rondout,  N.Y.     ( Photograph  by  van  Ingen) 

/ 

waves  may  pile  up  above  high  tide,  as  is  seen  all  along  the  eastern 
coast  of  the  United  States.     Behind  these  barriers  streams  bring 


254 


MARINE   DEPOSITS 


in  sediments,  filling  up  the  bays  and  converting  them  into  salt 
marshes  and  eventually  into  land. 

While  gravel  and,  in  sheltered  or  deeper  spots,  mud  are  found 
in  the  shallow  sea,  the  most  abundant  and  characteristic  material 
of  this  zone  is  quartz  sand.  If  the  bottom  shelves  very  gradually 
and  the  continental  margin  is  far  from  land,  the  sand  will  extend 
100  to  150  miles  out,  growing  finer  and  finer  with  the  increasing 


FIG.  133.  —  Cross-bedded  sands,  Bennett,  Nebraska.     (U.  S.  G.  S.) 

depth  of  water.  Further,  the  sand  travels  along  the  shore  for  long 
distances  from  its  place  of  origin,  as  on  the  Atlantic  coast  of  Florida, 
where  there  is  a  belt  of  siliceous  sand  that  cannot  have  been  de- 
rived from  the  peninsula. 

Throughout  the  whole  of  the  shoal-water  zone  wave  action  is 
exerted  upon  the  bottom,  though  to  a  very  insignificant  extent  in 
the  deeper  parts.  Very  near  shore  currents  produce  irregularities 
of  stratification,  especially  the  structure  known  as  cross  bedding 


SHOAL- WATER   DEPOSITS 


255 


(also  called  current  or  false  bedding)  in  which  the  separate  layers 
are  inclined  at  a  considerable  angle  to  the  horizontal.  (See  Figs. 
133-4  and  Frontispie'ce.)  This  structure  is  due  to  the  heaping  up 
of  bars  and  ridges  on  the  sheltered  side  of  which  sand  or  gravel  is 
dropped  in  inclined  layers.  Frequently  we  find  horizontal  strata 
built  up  of  inclined  layers,  the  latter  all  truncated  by  a  horizontal 
bedding  plane.  Cross  bedding  occurs  in  shoal  water  of  all  kinds, 
the  sea,  lakes,  and  rivers,  wherever  the  bottom  is  frequently  stirred 


FIG.  134.  —  Cross-bedded  sandstones,  Arizona.     (U.  S.'G.  S.) 


up  by  currents,  and  the  foreset  beds  of  deltas  are  a  typical  exam- 
ple of  it.  In  the  rocks  it  is  most  frequently  observed  in  consoli- 
dated sands  and  gravels,  which  are  called  respectively  sandstones 
and  conglomerates.  Ripple  marks  are  also  extremely  common  in 
deposits  of  the  shoal- water  zone  and  tracks  of  marine  animals; 
tracks  of  land  animals  and  sun  cracks  are  of  course  wanting. 

Much  less  widespread  than  sand  or  gravel  on  the  bottom  of  the 
shallow  sea  is  mud  or  clay.  When  these  occur,  their  presence  may 
be  due  either  to  holes  and  depressions,  where  the  bottom  water 


256 


MARINE  DEPOSITS 


is  less  disturbed  and  therefore  deposits  finer  material,  or  to  a  large 
supply  from  the  neighbouring  land.  For  example,  a  large  trian- 
gular patch  of  clay  invades  the  sand  area  south  of  Block  Island, 
and  mud-holes  are  found  along  the  New  Jersey  coast  near  the 
entrance  to  New  York  Bay. 


FIG.  135.  — Markings  by  marine  worms,  modern 


It  is  manifest  that  a  great  thickness  of  shoal-water  deposits 
can  be  formed  only  upon  a  sinking  sea-bottom,  for  otherwise  the 
water  would  be  filled  up  and  the  coast-line  pushed  out  to  sea. 
If  the  subsidence  be  very  slow,  deposition  may  shoal  the  water 
and  thus  extend  the  coarse  materials  seaward ;  if  it  be  rapid,  deepen- 


SHOAL-WATER  DEPOSITS 


257 


ing  the  water,  fine  sediment  will  be  thrown  down  upon  coarse, 
while,  if  the  rate  of  deposition  and  subsidence  be  nearly  equal, 
the  coarser  material  will  form  long,  narrow  bands,  running  parallel 
with  the  coast.  Thus,  in  the  same  vertical  line.may  be  accumulated 
many  different  kinds  of  sediment,  corresponding  to  the  different 
depths  of  water  at  the  same  spot.  When  traced  laterally,  beds 
of  any  given  kind  of  material  will  eventually  give  way  to  those  of 
another  kind,  either  by  gradual  transition,  or  by  thinning  to  an  edge 
and  dovetailing  with  the  thin  edges  of  the  other  beds.  The  dove- 
tailed structure  is  caused  by  shifting  conditions,  a  succession  of 
heavy  storms  sweeping  coarser  material  out  to  unusual  depths, 
and  long  periods  of  calm  occasioning  the  deposition  of  fine  sedi- 
ment unusually  near  shore. 


FIG.  136.  —  Diagram  showing  dove-tailed  deposition  on  the  sea-floor 

Organic  deposits  are  much  less  common  in  shallow  water  than 
are  the  terrigenous,  and  yet  under  favourable  conditions  they  are 
developed  on  a  very  extensive  scale.  The  most  important  of  such 
conditions  is  an  abundant  supply  of  food.  Even  in  the  far  North 
limestone  accumulations  are  formed,  but  this  work  is  most  exten- 
sively done  in  the  warm  waters  of  tropical  and  subtropical  seas. 
The  sea  is  constantly  receiving  from  the  land  materials  in  solution, 
of  which  the  most  important  are  the  carbonate  and  sulphate  of 
lime.  Many  classes  of  marine  animals  extract  the  CaCO3  from  the 
sea- water  and  form  it  into  hard  parts,  either  as  external  shells  and 
tests,  or  as  internal  skeletons.  There  is  also  good  reason  to  believe 
that  some,  at  least,  of  these  organisms  are  able  to  convert  the  sul- 
phate into  the  carbonate. 


MARINE  DEPOSITS 


The  classes  of  marine  organisms  which  at  present  or  in  times 
past  have  played  the  most  important  part  in  the  accumulation  of 
calcareous  material  are:  the  Foraminifera,  Corals,  Echinoderms, 
and  Molluscs;  but  other  groups,  such  as  Bryozoa,  worms  and 
calcareous  seaweeds,  contribute  extensively  to  the  same  result. 
The  Foraminifera  do  not  accumulate  with  sufficient  rapidity  to  add 
largely  to  the  calcareous  deposits  of  shallow  water,  and  will  there- 
fore be  considered  in  connection  with  the  deep-sea  formations. 

Mollusca.  —  The  ordinary  shell-fish  (Mollusca)  supply  a  very 
large  amount  of  calcareous  material  for  the  formation  of  shallow- 


FlG.  137.  —  Modern  shell  limestone  (coquina),  Florida 

water  limestones,  especially  in  the  neighbourhood  of  the  coasts, 
and  are  found  in  warm,  temperate,  and  even  in  Arctic  seas.  The 
shells  accumulate  in  great  banks,  frequently,  though  not  always, 
mingled  with  more  or  less  sand  and  mud,  and  when  gathered  below 
the  limit  of  violent  wave  action,  they  are  entire,  embedded  in  finer 
material,  which  may  be  calcareous  or  not.  More  commonly  the 
shells  are  ground  by  the  waves  into  fragments,  making  shell  sand 
and  mud,  which  is  then  cemented  into  a  more  or  less  compact  mass. 
The  coquina  rock  of  Florida  is  an  example  of  a  recently  made 
shell  limestone  (though  it  is  forming  no  longer),  and  among  the 


SHOAI^- WATER   DEPOSITS  259 

rocks  of  the  earth's  crust  are  many  immense  limestones  which  were 
accumulated  in  this  way.  In  the  formation  of  shell-banks  car- 
nivorous Crustacea  and  fishes  play  an  important  part,  for  they  grind 
up  even  quite  thick  shells  and  produce  an  angular  calcareous  sand, 
which  may  be  deposited  by  itself,  or  constitutes  the  finer  material 
in  which  the  entire  shells  are  embedded.  The  shell-banks  thus 
form  lens-shaped  limestone  masses  of  greater  or  less  extent  and 
thickness,  which  are  intercalated  in  areas  of  terrigenous  sediment, 
more  particularly  of  sand. 

Echinodermata.  —  This  group  of  marine  animals,  which  includes 
the  starfishes,  sea-urchins,  crinoids  or  sea-lilies,  etc.,  is  made  up 
of  forms  which  all  secrete  skeletons  of  calcareous  plates,  and 
which  contribute  largely  to  the  formation  of  marine  limestones. 
At  the  present  day,  however,  they  seldom  build  up  any  extensive 
masses  unassisted,  but  in  former  ages  of  the  world's  history  they 
did  so  on  a  great  scale.  This  is  particularly  true  of  the  crinoids 
'(sea-lilies  or  feather-stars),  which  have  now  become  comparatively 
rare,  but  many  ancient  limestones  are  composed  almost  entirely  of 
their  remains,  and  especially  of  their  hard  and  heavy  stems. 

Limestone  Banks.  —  In  favourable  situations  immense  subma- 
rine plateaus  or  banks  are  built  up  in  shallow  waters  by  the  accu- 
mulated remains  of  all  sorts  of  lime-secreting  animals,  corals, 
echinoderms,  molluscs,  worms,  and  Foraminifera.  These  are  well 
exemplified  in  the  Gulf  of  Mexico  and  the  Caribbean  Sea  by  the 
great  banks  along  the  west  coast  of  Florida,  the  Yucatan  Bank,  and 
the  plateau  which  extends  from  the  coast  of  Nicaragua  almost  to 
Jamaica.  On  these  banks  the  luxuriance  and  fulness  of  life  are 
astonishing,  myriads  of  animals  flourishing  in  the  warm  waters, 
and  abundantly  supplied  with  food  by  the  great  ocean  currents 
which  sweep  over  the  banks.  Innumerable  molluscs,  echinoderms, 
and  calcareous  worms  are  continually  dying  and  adding  their  hard 
parts  to  the  sea-floor;  the  waves  and  tides  sweep  calcareous  sand 
and  mud  from  the  coral  reefs  over  the  flats,  and  all  of  these  masses 
are  rapidly  consolidated  into  rock. 

An  example  of  a  limestone  bank  in  moderately  deep  water  is 


260 


MARINE  DEPOSITS 


the  Pourtales  Plateau,  which  extends  southward  from  the  Florida 
Keys,  and  is  covered  by  90  to  300  fathoms  of  water.  "  The  bot- 
tom is  rocky,  rather  rough,  and  consists  of  a  recent  limestone, 
continually,  though  slowly  increasing  from  the  accumulation  of  the 
calcareous  debris  of  the  numerous  small  corals,  echinoderms,  and 
molluscs,  living  on  its  surface.  These  debris  are  consolidated  by 
tubes  of  serpulae;  the  interstices  are  filled  up  by  Foraminifera  and 
further  smoothed  over  by  nullipores.  —  The  region  of  this  recent 
limestone  ceases  at  a  depth  varying  from  250  to  350  fathoms, 
and  beyond  it  comes  the  trough  of  the  straits."  (A.  Agassiz.) 


FIG.  138.  —  Rock  from  the  Pourtales  Plateau.     (A.  Agassi^ 

It  is  not  known  how  thick  these  modern  limestone  banks  are, 
but  some  indication  of  their  thickness  is  given  by 'the  raised  terrace 
of  modern  limestone  in  northern  Yucatan,  composed  of  the  same 
species  of  animals  as  still  abound  in  the  adjoining  seas.  In  this 
rock  are  caverns  more  than  400  feet  deep,  which  do  not  reach  the 
bottom  of  the  mass. 

Corals.  —  The  animals  of  this  group  show  great  variety  of  form, 
size,  and  habit  of  growth,  and  by  no  means  all  of  them  are  impor- 
tant as  rock-makers.  The  solitary  corals,  which  are  widely  dis- 


SHOAL-WATER   DEPOSITS  26 1 

tributed,  even  in  the  deep  sea,  are  never  sufficiently  abundant  to 
form  deposits  by  themselves.  The  corals  which  do  accumulate 
in  great  masses  and  are  called  "  reef -builders."  form  compound 
colonies  or  stocks,  consisting  of  hundreds  and  thousands  of  indi- 
viduals. The  adult  corals  are  sedentary,  but  the  newly  hatched 
young  are  worm-like,  free-swimming  larvae.  When  the  young 
animal  has  established  itself  in  a  suitable  place,  preferably  upon  a 
rock  or  other  fixed  foundation,  it  develops  into  a  polyp,  or  fleshy 
sack,  with  rows  of  tentacles  around  the  mouth,  and  then  by  budding 
or  partial  division  (fission)  gives  rise  to  great  numbers  of  other 
polyps,  which  are  connected  by  a  tissue  common  to  them 
all.  In  this  compound  mass  is  secreted  a  skeleton  of  carbonate 
of  lime,  which  reproduces  the  form  of  the  colony  and,  in  most  cases, 
displays  cells  for  the  individual  polyps.  The  great  variety  of  form 
shown  by  these  compound  colonies  is  determined  by  the  mode  of 
budding  or  fission  and  the  relative  position  of  the  newer  to  the 
older  polyps.  Thus,  some  are  like  trees,  others  like  bushes; 
some  form  flat,  irregular  plates,  while  others  grow  into  great 
dome-like  masses. 

The  reef  corals  have,  at  present,  a  restricted  distribution,  and 
can  flourish  only  where  several  favourable  conditions  are  found 
united.  They  are  preeminently  shallow-water  animals  and  can  live 
only  in  depths  of  less  than  twenty  fathoms.  They  also  require  a 
high  temperature,  and  they  cease  wherever  the  average  tempera- 
ture of  the  water  for  the  coldest  month  is  below  68°  F.;  this  is  the 
minimum,  and  for  full  luxuriance  a  higher  temperature  is  neces- 
sary. Another  requisite  is  sea-water  of  full  salinity  and  uncontami- 
nated  with  mud;  hence,  few  corals  can  live  at  the  mouth  of  a  river, 
which,  even  if  it  brings  down  no  sediment,  freshens  the  water  and 
is  thus  fatal  to  the  polyps.  Another  condition  favourable  to  the 
growth  of  corals  is  the  presence  of  ocean  currents,  not  too  rapid, 
which  bring  abundant  supplies  of  food,  and  they  flourish  best  in 
the  broken  waters  of  heavy  surf,  which  gives  the  necessary  oxygen 
and  prevents  the  smothering  of  the  polyps  in  the  calcareous  silt 
and  debris  of  the  reef.  In  short,  the  reef  corals  are  tropical, 


262 


MARINE   DEPOSITS 


marine,  shallow-water  animals,  and  their  reefs  are  widely  spread 
throughout  the  warmer  seas  of  the  globe,  but  they  do  not  always 
occur  where  we  should  naturally  expect  to  find  them. 

A  coral  reef  is  not  built,  as  many  people  imagine,  by  the  indus- 
try of  the  polyps  —  these  furnish  the  material  by  extracting  lime 
salts  from  the  water  and  forming  solid  skeletons;  the  actual 
construction  is  largely  the  work  of  the  waves  and  other  agencies. 


FIG.  139.  —  Corals  on  the  Great  Barrier  Reef  of  Australia.     (Savile  Kent) 

The  coral  colonies  are  scattered  over  the  sea-bottom,  much  like 
vegetation  on  the  land,  scantily  in  some  places,  thickly  in  others, 
and  in  still  others  they  are  absent.  The  waves,  especially  in  storms, 
break  up  the  masses  of  coral,  which  are  much  weakened  by  the 
borings  of  many  kinds  of  marine  animals,  and  the  surf  grinds  them 
down  to  fragments  of  all  sizes,  from  large  blocks  to  the  finest  and 
most  impalpable  mud.  The  process  is  the  same  as  with  the  ordi- 


SHOAL-WATER  DEPOSITS  263 

nary  rocks  of  the  coast,  only  the  material  differs,  and  thus  are 
formed  boulders,  pebbles,  sand  and  mud,  all  of  coral  fragments. 
The  many  animals  which  feed  upon  coral  greatly  facilitate  this 
work,  partly  by  boring  into  the  masses,  partly  by  grinding  the 


FIG.  140.  —  Various  forms  of  modern  coral  limestone.     (Savile  Kent) 

smaller  fragments  into  fine  powder.  Considerable  masses  of  cal- 
careous debris  are  added  by  the  shells  and  tests  of  the  various 
animals  which  live  on  and  about  the  reef,  and  the  coral-like  sea- 
weeds, called  Nullipores,  contribute  an  important  quota,  while 


264  MARINE  DEPOSITS 

shell  sand  often  makes  up  as  much  as  one-half  of  the  volume  of  a 
reef.  All  of  this  material  is  ceaselessly  ground  up  by  the  waves, 
distributed  by  tides  and  currents,  and  brought  to  rest  in  quiet 
waters.  A  single  deposit  of  two  or  three  inches  in  thickness  has 
been  observed  to  form  between  tides  after  a  gale  along  the  Florida 
reefs,  and  in  storms  the  water  is  often  discoloured  and  turbid  for 
miles  around  the  reef.  The  sea-water  dissolves  and  redeposits 
CaCO8,  cementing  the  fragments  into  a  firm  rock,  which,  especially 
after  exposure  to  the  air,  may  become  very  hard. 

By  these  processes  several  varieties  of  rock  are  formed,  corre- 
sponding, in  all  but  the  material,  to  the  ordinary  marine  deposits. 
In  one  form  the  standing  and  unbroken  colonies  are  filled  up  with 
calcareous  debris  and  enclosed  in  solid  masses.  This  is  perhaps 
the  most  important  kind  of  rock,  at  all  events,  in  many  reefs,  for 
the  branching  corals  retain  the  shell  sand  and  other  calcareous  de*- 
bris  and  prevent  the  waves  from  washing  it  away.  Reefs  of  this 
kind  have  many  and  deep  holes  penetrating  them,  where  the  col- 
onies are  not  in  contact  and  the  sand  has  not  filled  up  the  inter- 
spaces. Coral  conglomerate  or  breccia  is  a  cemented  mass  of 
coral  pebbles  or  angular  pieces,  or  is  made  up  of  fragments  of  an 
older  coral  rock.  Reef  rock  is  the  dense  and  solid  mass  formed 
by  the  cementing  of  the  finer  debris  which  accumulates  in  quiet 
water.  It  is  important  to  notice  that  even  under  the  microscope 
reef  rock  frequently  shows  no  trace  of  organic  structure,  and  is  a 
definite  proof  that  the  absence  of  such  structure  is  not  a  sufficient 
reason  for  denying  the  organic  origin  of  a  rock.  The  interior  of 
growing  masses  which  are  still  alive  on  the  outside,  and  have  never 
been  broken  up,  may  be  so  crystallized  by  the  action  of  the  sea- 
water  that  the  organic  structure  is  obscured  or  destroyed.  On  the 
beach  is  formed  a  curious  rock  called  oolite,  which  is  made  up  of 
minute  spherules  of  CaCO3  cemented  into  a  mass  not  unlike  fish- 
roe  in  appearance.  This  is  due  to  the  deposition  of  CaCO8  from 
solution  around  tiny  grains  of  calcareous  sand,  until  the  spherules 
are  built  up  and  cemented  together. 

The  growth  of  coral  ceases  when  the  reef  extends  up  a  little  above 


SHOAL-WATER   DEPOSITS  265 

low-water  mark,  but  the  waves  continue  their  work  and  throw  up 
debris  and  build  up  a  platform,  upon  which  they  establish  a  beach 
of  calcareous  sand.  The  latter  may  be  further  piled  up  by  the 
winds  into  dunes  and  solidified  by  the  cementing  action  of  per- 
colating rain-water.  According  to  circumstances,  the  new  plat- 
form may  be  an  extension  of  the  shore  or  an  island  like  the  Florida 
Keys. 

Coral  reefs  are  classed  according  to  their  relation  to  the  shore, 
and  are  of  three  kinds,  (i)  Fringing  reefs  are  those  attached 
directly  to  the  land,  though  the  exposed  part  may  be  at  some  dis- 
tance out  from  the  shore  and  separated  from  it  by  a  shallow  channel 
with  coral  bottom.  The  width  of  a  fringing  reef  is  determined  by 
the  slope  of  the  sea-bottom,  being  narrower  on  a  steep  grade, 
broader  on  a  gentle  one.  (2)  Barrier  reefs  are  farther  out  from 
shore,  to  which  the  reef  is  parallel  in  a  general  way,  and  separated 
by  a  broad  and  often  quite  deep  channel.  The  distinction  between 
the  two  kinds  of  reefs  is  not  very  sharply  drawn,  for  the  same  reef 
may  be  fringing  in  parts  of  its  course  and  a  barrier  in  others.  Even 
at  the  present  time  barrier  reefs  are  sometimes  constructed  on  an 
enormous  scale.  A  great  barrier  reef  runs  parallel  to  nearly  the 
whole  north  shore  of  Cuba,  while  the  barrier  reef  of  Australia,  the 
largest  known,  extends,  with  some  breaks,  for  over  1200  miles 
along  the  northeast  coast  of  Australia,  from  which  it  is  distant 
20  to  80  miles;  its  breadth  varies  from  10  to  90  miles,  though  but 
little  of  this  width  is  exposed  above  water;  its  sea-face  is  in  some 
places  more  than  1800  feet  high  (i.e.  above  the  sea-bottom,  not  the 
surface).  (3)  Atolls  are  coral  islands  of  irregularly  circular  shape, 
which  usually  enclose  a  central  lagoon  and  frequently,  as  in  the 
Pacific,  rise  from  the  profoundest  depths.  The  way  in  which  such 
islands  have  been  built  up  is  still  a  subject  of  much  controversy. 
No  doubt,  atolls  have  been  formed  in  various  ways,  especially  those 
which  arise  from  small  depths,  but  probably  the  most  important 
method  is  by  a  slow  subsidence  of  the  sea-bottom,  with  which 
the  growth  of  the  reef  can  keep  pace.  Such  subsidence  is  the  only 
explanation  of  great  thicknesses  of  coral  rock,  as  of  any  other  kind 


266  MARINE  DEPOSITS 

of  shoal-water  deposits.  Borings  in  the  Hawaiian  Islands, 
and  especially  in  Funafuti,  an  island  of  the  South  Pacific,  have 
demonstrated  the  existence  of  immensely  thick  coral  limestones 
formed  in  the  modern  period. 

Coral  reefs  in  shoal  water  frequently  have  gentle  slopes,  but  those 
which  rise  from  the  deep  sea  have  very  steep  faces,  sometimes 
as  much  as  65°,  and  thus  reefs  may  occur  as  lens-shaped  areas,  or 
steep-sided  masses  of  limestone,  in  which  stratification  is  very 
obscure,  or  absent,  in  the  midst  of  well-stratified  fragmental  sedi- 
ments. 

Dolomitization.  —  A  process  has  been  observed  in  the  closed 
lagoons  of  certain  atolls  which  is  significant  as  throwing  light  upon 
a  very  difficult  problem,  that  of  the  formation  of  dolomite  or 
magnesian  limestone.  In  the  closed  lagoon,  shut  off  entirely 
from  the  sea,  the  isolated  body  of  sea-water  becomes  con- 
siderably concentrated  by  evaporation.  All  sea-water  contains 
chloride  of  magnesium  (MgCl2),  and  this  percolating  into  the  coral 
rock,  by  double  decomposition  with  CaCO3,  forms  MgCO3.  The 
change  occurs  more  readily  when  the  CaCO3  is  in  the  form  of 
aragonite,  as  is  the  case  in  many  shells  and  corals. 

Chemical  Deposits.  —  It  is  not  known  just  how  important  a  part 
is  played  by  chemical  precipitation  in  the  formation  of  marine 
deposits,  but  probably  a  greater  one  than  has  been  generally  sup- 
posed. Rivers  which  bring  in  quantities  of  CaCO3  in  solution 
may  so  overload  the  sea  with  this  substance  (for  sea-water  will 
dissolve  little  of  it)  that  more  or  less  is  precipitated  in  the  neigh- 
bourhood of  the  land.  On  the  coast  of  Asia  Minor,  for  example, 
are  large  areas  of  sandstone  and  conglomerate,  formed  within  re- 
cent times  by  the  precipitation  of  CaCO3  in  masses  of  sand  and 
gravel,  binding  them  into  hard  rock.  Similar  examples  are  known 
elsewhere.  There  is  also  some  reason  to  believe  that  the  decay  of 
marine  animals  evolves  sufficient  carbonate  of  ammonia  to  con- 
vert the  sulphate  of  lime  into  the  carbonate  by  double  decompo- 
sition, and  to  precipitate  the  latter  in  some  quantity. 

3.   Aktian  Deposits. — The  zoo-fathom  line  is  by  Murray  and 


AKTIAN   DEPOSITS  267 

Renard  regarded  as  the  boundary  between  shallow  and  deep 
water,  for  it  generally  marks  the  edge  of  the  continental  shelfj 
from  which  the  bottom  rises  very  gently  to  the  land,  but  slopes 
abruptly  down  to  the  oceanic  depression.  The  great  bulk  of 
the  material  derived  from  the  waste  of  the  land  is  thrown  down 
upon  the  continental  shelf,  within  the  ico-fathom  line,  but  the 
finer  particles  are  carried  farther  out  and  subside  in  deeper  and 
quieter  water.  A  considerable  quantity  of  the  finest  sedimentary 
particles  remains  long  suspended  in  sea-water,  especially  in  the 
cold  water  of  the  polar  seas.  On  the  continental  slopes,  extend- 
ing from  the  loo-fathom  line  to  the  bottom  of  the  great  oceanic 
abysses,  are  laid  down  most  of  the  very  fine  materials  derived  from 
the  land,  which  are  grouped  together  under  the  somewhat  indefi- 
nite term,  mud.  Mud  is  a  mixture  of  minerals  in  a  state  of  extremely 
fine  mechanical  subdivision,  but  not  chemically  decomposed,  thus 
differing  from  clay. 

(i)  Blue  Mud. — The  materials  of  this  deposit,  which  are 
principally,  though  not  altogether,  derived  from  the  land,  are  very 
heterogeneous.  Quartz  grains  in  an  excessively  fine  state  of  sub- 
division are  very  abundant;  clay  is  often  a  considerable  ingredient, 
and  then  the  mud  is  plastic  when  wet,  but  it  is  usually  more  earthy 
than  clay-like.  Minute  particles  of  other  terrigenous  minerals, 
like  felspar,  hornblende,  augite,  etc.,  are  common.  CaCO3  is  al- 
most always  present,  averaging  7%,  and  in  some  instances  rising  to 
25  %;  this  is  due  chiefly  to  the  foraminiferal  shells,  both  of  those 
species  which  live  at  the  surface  and  those  which  live  on  the  bottom. 
Siliceous  organisms  are  also  present  to  the  average  amount  of  3  %, 
and  are  principally  diatoms,  radiolarians,  and  spicules  of  sponges. 
Glauconite  is  found  in  nearly  all  the  samples.  The  blue  colour 
of  this  mud  is  due  to  the  sulphide  of  iron  and  the  organic  matter 
which  prevents  the  oxidation  of  the  sulphide.  Of  the  terrigenous 
deep-sea  deposits  blue  mud  is  the  most  extensively  developed; 
it  is  estimated  as  covering  14,500,000  square  miles  of  the  sea: 
bottom,  and  surrounds  almost  all  coasts,  and  fills  enclosed  basins 
like  the  Mediterranean  and  even  the  Arctic  Ocean.  The  depths 
at  which  blue  mud  is  found  range  from  125  to  2800  fathoms. 


268 


MARINE   DEPOSITS 


FIG.  141.  — Map  of  marine  deposits.  (Kayser  after  Murray  and  Renard)  Dotted  area 
=  terrigenous;  vertical  lined  =  foraminiferal  ooze;  horizontal  broken-lined  =  dia- 
tom ooze ;  crosses  =  oceanic  red  clay ;  white  =  radiolarian  ooze 


ABYSMAL  DEPOSITS  269 

(2)  Red  Mud  is  a  local  development,  which  occurs  principally 
upon  the  Atlantic  coast  of  Brazil,  and  in  the  Yellow  Sea  of  China. 
Silt  of  this  character,  the  red  colour  of  which  is  due  to  Fe2O3,  con- 
tained in  laterite,  is  brought  down  in  large  quantities  by  the  Ama- 
zon and  the  Orinoco.     Foraminiferal  shells  are  abundant;  radio- 
larians  very  rare.     Probably  a  more  minute  examination  of  the 
continental  slopes  will  show  that  red  mud  has  a  wider  distribution 
in  tropical  seas  than  is  here  indicated. 

(3)  Green  Mud  is  much  the  same  in  character  as  the  blue  mud, 
but  owes  its  green  colour  to  the  higher  percentage  of  glauconite 
which  it  contains. 

(4)  Green  Sand  is  granular  in  appearance,  and  is  made  up 
largely  of  grains  of  glauconite  and  casts  in  that  material  of  the 
interior  of  foraminiferal  shells,  together  with  nearly  50  %  of  CaCO3. 
The  green  sands  occur  in  shallower  water  than  the  muds,  and  often 
within  the  loo-fathom  line,  as  in  the  case  of  a  deposit  of  this  kind 
which  is  now  forming  off  the  coast  of  Georgia  and  the  Carolinas. 
The  estimated  area  of  the  green  muds  and  sands  is  1,000,000 
square  miles. 

(5)  Volcanic  Muds.  —  In  the  deeper  water  surrounding  vol- 
canic islands  are  deposits  of  fine  mud  made  from  the  disintegra- 
tion of  volcanic  rocks,  mixed  with  considerable  clay,  and  also 
calcareous  matter  derived  from  organisms. 

4.  The  Abysmal  Deposits  are  those  the  materials  of  which  are 
not  directly  derived  from  the  land,  but  consist  of  matters  carried 
to  the  sea  in  solution  and  extracted  from  the  sea-water  by  the 
agency  of  organisms,  together  with  volcanic  substances  in  a  more 
or  less  advanced  stage  of  decomposition.  Only  rarely  are  terrige- 
nous materials  found  in  these  deposits,  as,  for  example,  off  the 
west  coast  of  Africa,  where  fine  sand,  carried  by  the  wind  from  the 
Sahara,  is  found  in  deep  water,  and  ice-borne  fragments  are  com- 
mon in  high  latitudes.  The  pelagic  deposits  are  found  far  from 
land,  mostly  in  the  deepest  oceanic  abysses,  where  the  rate  of 
accumulation  is  almost  inconceivably  slow,  and  the  remains  of 
extinct  animals  still  lie  exposed  upon  the  ocean  floor. 


270 


MARINE   DEPOSITS 


(i)  Foraminiferal  Ooze. — The  Foraminifera  are  minute  ani- 
mals, each  one  a  tiny  speck  of  jelly,  most  of  which,  in  spite  of  their 
extreme  simplicity  of  structure,  have  the  power  of  secreting  very 
beautiful  and  complex  shells  of  CaCO3.  The  species  which  are 
of  importance  in  this  connection  are  those  which  live  in  infinite 
multitudes  at  the  surface  of  the  ocean,  and  the  most  abundant  at 
the  present  time  are  those  which  belong  to  the  genus  Globigerinn 
whence  this  deposit  is  frequently  called  Globigerina  ooze.  These 
surface  Foraminifera  flourish  best  in  warm  water  and  follow  the 
warm  currents,  often  into  quite  high  latitudes.  Their  shells,  which 


FIG.  142.  —  Foraminiferal  ooze,  X  20.     (Agassiz  after  Murray  and  Renard) 

drop  to  the  bottom  as  the  occupants  die,  are  present  in  almost  all 
marine  deposits,  but  near  land  the  terrigenous  materials  prepon- 
derate to  such  a  degree  that  the  Foraminifera  make  up  but  a  slight 
proportion  of  the  deposit.  In  deeper  water,  where  the  wash  from 
the  land  does  not  come,  the  foraminiferal  shells  become  relatively 
much  more  abundant,  and  when  30  %  or  more  of  a  given  sample 
of  the  bottom  consists  of  them,  it  is  classed  as  a  foraminiferal  ooze. 
Other  organisms  which  secrete  calcareous  shells  or  tests  always 
contribute  more  or  less  to  these  oozes  (coral  mud,  echinoderms, 
molluscs,  nullipores,  etc.).  The  deposit  is  purest  and[  most 


ABYSMAL  DEPOSITS  2?  I 

typical  in  the  medium  depths  of  the  ocean,  far  from  any  land;  in 
such  places  the  ooze  may  contain  as  much  as  90  %  CaCO3  and  is 
white,  while  nearer  land  the  slight  admixture  of  terrigenous  miner- 
als gives  a  pink,  gray,  brown,  or  other  colour  to  the  mass.  Below 
the  depth  of  2500  fathoms  the  proportion  of  CaCO3  becomes  much 
diminished,  owing  to  the  increasing  percentage  of  CO2  in  the  sea- 
water,  which  attacks  and  dissolves  these  delicate  shells. 

The  foraminiferal  oozes  have  a  vast  geographical  extent,  esti- 
mated at  49,520,000  square  miles,  and  are  especially  developed  in 
the  Atlantic,  though  they  are  largely  present  in  all  except  the  polar 
seas,  and  range  in  depth  from  400  to  2900  fathoms. 


£  iG.  143.  —  Pteropod  ooze,  x  4.     (Agassiz  after  Murray  and  Renard) 

(2)  Pteropod  Ooze.  —  The  thin  and  delicate  shells  of  the  mollus- 
can  groups  known  as  the  pteropods  and  heteropods  abound  at  the 
surface  of  the  warmer  parts  of  the  ocean,  but  their  dead  shells 
are  found  only  in  depths  of  less  than  2000  fathoms.  In  shallow 
water  (and  even  in  greater  depths  near  land)  the  shells  are  con- 
cealed by  other  kinds  of  material,  but  in  moderate  depths,  far  from 
any  land,  these  shells  sometimes  become  so  frequent  in  the  fo- 
raminiferal ooze  as  to  give  it  a  special  character.  In  its  typical 
development  this  pteropod  ooze  has  been  found  only  in  the  Atlan- 


2/2  MARINE   DEPOSITS 

tic,  where  it  covers  some  relatively  small  areas,  in  depths  of  404 
to  1500  fathoms. 

(3)  Radiolarian  Ooze.  —  The  organisms  which  we  have  so  far 
considered  secrete  only  shells  or  tests  of  CaCO3,  but  this  is  not  the 
only  substance  which  is  very  extensively  extracted  from  sea-water 
by  living  beings.     Silica  is  also  dissolved  in  sea-water,  and  various 
organisms  construct  their  tests  of  that  substance.    The  Radiolaria 
are,  like  the  Foraminifera,  a  group  of  microscopic,  unicellular 
animals,  which  secrete  siliceous  tests  of  the  most  exquisite  delicacy 
and  beauty;  they  live  both  at  the  surface  and  at  the  bottom  of  the 
sea.     Radiolarian  tests  may  be  detected  in  all  sorts  of  marine 
deposits  of  both  deep  and  shallow  water,  but  it  is  only  in  very  pro- 
found depths  that  they  occur  in  quantity  sufficient  to  give  character 
to  the  deposit.     When  20  %  or  more  of  a  bottom  deposit  consists 
of  radiolarian  tests,  it  is  called  a  radiolarian  ooze,  but  clay  and 
volcanic  minerals  make  up  most  of  the  materials.    This  ooze  has 
been  found  only  in  the  Pacific  and  Indian  oceans,  where,  it  is  esti- 
mated, it  covers  2,290,000  square  miles  of  the  bottom,  at  depths 
of  2350  to  4475  fathoms. 

(4)  Diatom  Ooze.  —  In  our  study  of  fresh-water  deposits  we 
learned  that  the  siliceous  cases  of  the  microscopic  plants  known  as 
diatoms  form  considerable  accumulations  in  lakes  and  ponds,  and 
they  also  flourish  abundantly  in  brackish  water  and  in  the  sea. 
Diatoms  are  found  in  many  marine  deposits,  but  in  relatively  small 
quantities.      In  the  Antarctic  Ocean,  however,  is    in   immense 
belt  of  ooze,  believed  to  cover  10,880,000  square  miles  and  extend- 
ing around  the  globe,  which  is  largely  made  up  of  their  frustules. 
Besides  the  great  Antarctic  zone,  an  area  of  some  40,000  square 
miles  is  known  in  the  North  Pacific.    The  diatom  ooze  entirely 
resembles  the  fresh-water  deposit,  but  may  be  distinguished  by  the 
presence  of  foraminiferal  and  radiolarian  shells  and  tests.    The 
depths  at  which  this  ooze  is  found  are  from  600  to  2000  fathoms. 

(5)  Red  Clay.  —  The  profoundest  abysses  of  the  ocean,  far 
from  any  land,  are  covered  with  a  deposit  of   red  clay,  which, 
though  varying  much  in  composition  and  colour,  is  yet  of  a  quite 


CLIMATIC   RELATIONS  OF  MARINE   DEPOSITS          273 

uniform  character.  In  these  vast  depths  the  foraminiferal  shells 
are  almost  all  dissolved  by  the  carbonated  sea-water,  but  some 
CaCO3  is  very  generally  present,  averaging  about  6%,  and  di- 
minishing in  quantity  as  the  depth  increases.  In  the  less  profound 
abysses  the  red  clay  passes  gradually  into  the  foraminiferal  oozes, 
the  number  of  shells  increasing  until  the  ooze-like  character  is 
attained.  The  clay  is  derived  from  the  disintegration  and  decay 
of  volcanic  substances,  especially  pumice,  which  floats  upon  water, 
often  for  months,  and  drifts  long  distances  in  the  ocean  currents. 
The  greater  part  of  these  volcanic  materials  is  believed  to  be  derived 
from  terrestrial  volcanoes,  but  the  submarine  vents  doubtless  con- 
tribute largely;  particles  of  undecomposed  volcanic  minerals  and 
glasses  are  also  common.  In  some  regions  the  clay  is  coloured 
chocolate-brown  by  the  oxide  of  manganese,  and  many  sepa- 
rate nodules  of  this  substance  are  found.  The  excessive  slowness 
with  which  this  abysmal  deposit  is  formed,  is  shown  by  the  occur- 
rence, in  recognizable  quantities,  of  meteoric  iron,  which  reaches 
the  earth  in  the  form  of  meteorites,  or  "  shooting  stars,"  and  by  the 
presence  of  the  remains  of  animals  which  have  long  been  extinct. 

Of  all  the  oceanic  deposits  the  red  clay  is  the  most  widely 
extended,  covering  51,500,000  square  miles  of  the  bottom.  Almost 
four-fifths  of  this  vast  area  are  in  the  great  depths  of  the  Pacific; 
the  shallower  Atlantic  has  much  more  of  the  foraminiferal  ooze  than 
of  the  red  clay.  The  observed  range  in  depth  is  from  2225  to 
3950  fathoms. 

Comparing  the  marine  deposits  nmv  accumulating  in  the  sea  with 
the  rocks  of  evidently  marine  origin  which  form  most  of  the  land, 
we  find  that  the  great  bulk  of  these  rocks,  the  sandstones,  slates,  and 
limestones,  are  such  as  are  formed  in  water  of  shallow  and  moderate 
depths,  while  only  rarely  do  we  discover  a  rock  that  implies  really 
deep  water. 

Climatic  Relations  of  Marine  Deposits. — The  shoal- water  de- 
posits of  cold  and  temperate  seas  are  much  alike,  both  having 
a  preponderance  of  sand  and  gravel,  with  occasional  limestones 
which  are  more  frequent  in  lower  latitudes,  while  in  the  deepei 
T 


2/4  ESTUARINE  DEPOSITS 

waters  of  the  continental  slopes,  blue  mud  is  laid  down.  On  the 
floor  of  the  oceanic  abysses,  however,  is  a  difference  in  that  the 
terrigenous  material  is  more  widely  distributed,  while  foraminiferal 
ooze  and  oceanic  red  clay  are  absent  from  the  polar  seas,  and  in  the 
Antarctic  Ocean  is  a  complete  zone  of  diatom  ooze,  encircling  the 
earth.  The  tropical  seas  are  characterized  by  red  muds,  and  by 
great  development  of  limestones,  especially  of  the  coral  reefs,  which 
are  almost  entirely  confined  to  the  tropical  waters.  The  deposits  of 
the  truly  deep  sea  are  essentially  the  same  in  the  tropical  and  tem- 
perate zones,  and  are  determined  almost  entirely  by  the  depth  of 
water;  foraminiferal  and  radiolarian  oozes  and  red  clay  cover 
almost  the  whole  floor  of  the  oceanic  basins  in  these  regions.  As 
conditions  are  more  uniform  in  the  sea  than  on  the  land,  climatic 
differences  are  less  clearly  marked  in  marine  than  in  continental 
deposits. 

ESTUARINE  DEPOSITS 

An  estuary  is  a  wide  opening  at  the  mouth  of  a  river  into  which 
the  sea  has  penetrated  by  the  depression  of  the  land.  In  such 
bodies  of  water  the  tide  often  scours  with  much  force.  Estuaries 
abound  along  our  Atlantic  coast,  Delaware  and  Chesapeake  Bays 
and  the  mouth  of  the  Hudson  being  excellent  examples  of  such. 
The  water  in  them  is  brackish,  and  unfavourable  to  abundant 
aquatic  life,  for  only  a  limited  number  of  marine  animals,  and 
fewer  fresh-water  ones,  flourish  in  brackish  water. 
•  Estuarine  deposits  are,  in  general,  much  like  those  of  the  sea, 
except  that  they  are  apt  to  be  of  a  finer  grain  for  a  given  depth  of 
water;  muds  are  abundantly  laid  down,  especially  in  the  more 
sheltered  nooks  and  bays,  with  fine  and  coarse  sands  and  gravels 
in  the  more  exposed  situations.  The  sands  are  apt  to  show  a 
confused  stratification  from  the  conflicting  currents  and  eddies  in 
which  they  are  deposited,  but  with  horizontal  layers  formed  at  slack 
water.  Extensive  mud-flats  often  surround  an  estuary,  especially 
if  the  rise  and  fall  of  the  tide  be  great.  On  these  flats,  exposed 
during  low  tide  to  the  sun  and  air,  in  dry,  hot  climates,  sun  cracks 


ESTUARINE  DEPOSITS  2/5 

are  formed  on  the  drying  surface,  and  these,  together  with  the  prints 
of  raindrops  and  the  tracks  of  land  animals,  will  be  preserved  when 
the  incoming  tide,  advancing  too  gently  to  scour  the  slightly  har- 
dened surface  of  the  flat,  deposits  a  fresh  layer  of  sediment  upon  it. 
In  pluvial  climates,  the  mud-flats  of  estuaries  do  not  dry  with  suffi- 
cient rapidity  to  permit  the  formation  of  shrinkage  cracks,  except 
when  the  flats  are  exposed  to  the  air  for  a  longer  time  than  the 
ordinary  interval  between  tides.  This  longer  exposure  occurs 
when  part  of  the  flats  is  covered  only  by  spring  tides,  or  the  general 
level  of  the  water  is  raised  by  a  storm.  If  the  estuary  be  the  open- 
ing of  a  large  river,  considerable  deposits  of  river  sediment  will, 
in  times  of  flood,  be  laid  down  upon  the  other  beds,  producing  an 
alternation  of  fresh  and  brackish  water  beds.  On  the  coast  of 
North  Carolina  the  low  sand-spits  thrown  up  by  the  waves  enclose 
extensive  shallow  sounds,  into  which  the  tide  enters  by  only  narrow 
openings,  but  which  have  numerous  streams  flowing  into  them. 
At  high  water  the  incoming  tide  acts  as  a  barrier,  damming  back 
the  river  waters,  checking  their  velocity,  and  causing  them  to  de- 
posit their  burdens  of  sediment.  In  course  of  time,  the  sounds 
must  be  silted  up  by  the  rivers,  first  converted  into  salt  marshes 
and  then  into  land.  The  great  areas  of  salt  marsh  along  our  Atlan- 
tic coast  have,  for  the  most  part,  been  formed  in  this  way. 

For  reasons  that  we  have  already  discussed,  estuaries  are  not 
favourable  to  either  fresh-water  or  marine  organisms,  and  hence 
estuarine  deposits  do  not  contain  any  great  variety  of  remains  of 
either  group.  These  remains  may,  however,  represent  numerous 
individuals,  sufficient  sometimes  to  form  limestone  layers,  as  is 
especially  true  of  oyster  banks.  Diatoms  may  also  accumulate 
in  great  quantities,  as  in  one  of  the  Baltic  harbours,  where  they 
form  18,000  cubic  feet  of  deposit  annually,  which  necessitates 
continual  dredging  to  keep  the  harbour  open.  On  the  other 
hand,  estuaries  are  often  favourably  situated  for  the  reception 
and  preservation  of  the  remains  of  land  animals  and  plants 
which  are  swept  into  them  by  streams  and  buried  in  the  soft  silt 
of  the  mud-flats. 


2/6  THE  CONSOLIDATION   OF  SEDIMENTS 


THE  CONSOLIDATION  OF  SEDIMENTS    (DIAGENESIS) 

The  processes  of  deposition  upon  the  land  and  beneath  the 
water,  which  we  have  so  far  been  studying,  result,  for  the  most 
part,  only  in  the  bringing  together  of  great  masses  of  loose  and 
incoherent  material,  which  in  the  case  of  marine  deposits  are  satu- 
rated with  salt  water.  If  such  masses  are  properly  to  be  compared 
with  the  hard  rocks  of  the  earth's  crust,  it  will  be  necessary  to  show 
that  loose  sediments  may  be  consolidated  and  rendered  hard  and 
firm,  like  the  latter.  This  is  not  difficult,  for  we  have  abundant 
evidence  to  prove  that  such  consolidation  actually  does  take 
place,  and  in  a  variety  of  ways. 

(1)  Consolidation  by  Weight  of  Sediment.  —  When  deposited 
on  a  sinking  sea-bottom,  sediments  often  accumulate  in  masses 
of  great  thickness,  and  in  such  cases  the  lower  portions  must 
tend  to  consolidate  from  the  weight  of  the  overlying  masses.     Of 
course,  such  a  process  cannot  be  directly  observed  in  modern 
accumulations,  because  only  the  surface  of  them  is  accessible,  but 
from  the  analogy  of  observed  facts  we  may  safely  infer  that  this 
weight  is  not  without  effect. 

(2)  Consolidation  by  Cement.  —  Sediment  is  often  penetrated  by 
percolating  waters,  which  carry  in  solution  various  cementing  sub- 
stances, such  as  SiO2,  CaCO3,  FeCO3,  etc.,  and  the  deposition  of 
these  materials  in  the  interstices  of  the  loose  sediment  will  bind 
the  particles  into  a  firm  rock.    This  process  we  have  already  had 
occasion  to  observe  in  several  instances,  as  in  the  coral  reefs,  the 
drift-sand  rock  of  Bermuda,  the  modern  sandstones  on  the  coast 
of  Asia  Minor  and  Brazil,  and  many  others.      In  all  of  these  cases 
the  cementing  substance  is  CaCO3,  but  other  modern  rocks  are 
known  in  which  Fe2O3,  formed  by  the  oxidation  of  FeCO3,  plays  the 
same  role,  as  in  Florida  where  the  waters  from  ferruginous  springs 
bind  the  grains  of  calcareous  sand  into  a  hard  rock,  the  modern 
date  of  which  is  proved  by  the  presence  in  such  rock  of  the  bones 
of  Indians.     Both  of  these  substances  are  very  common  as  cements 


THE  CONSOLIDATION  OF  SEDIMENTS  277 

among  the  ancient  rocks.  The  deposition  of  silica  in  the  interstices 
of  sand  has  also  been  observed,  where  the  original  sand  grains  can 
with  difficulty  be  detected  with  the  microscope,  the  rock  appearing 
to  be  a  mass  of  crystalline  quartz  grains.  A  cementing  effect  may 
also  be  produced  by  reactions  within  the  mass  of  the  sediment  itself, 
as  is  seen  in  the  solidification  of  volcanic  ash  mingled  with  water 
to  form  tuffs. 

(3)  Consolidation  through  Heat.  — This  may  be  local,  as  in  the 
neighbourhood  of  volcanoes,  or  general  and  due  to  the  internal 
heat  of  the  earth.     For  sediment  to  reach  great  thickness  it  must 
subside,  and  this  subsidence  brings  the  lower  parts  of  the  mass 
deep  down  into  the  crust,  where  they  are  invaded  by  the  earth's 
interior  heat,  and  baked  as  bricks  are  burnt  in  a  kiln.    This  pro- 
cess is  likewise  one  which  cannot  be  directly  observed,  but  the 
effects  of  molten  lava  upon  loose  sediments  may  be  watched,  and 
the  consolidating  power  of  heat  has  been  tested  experimentally. 

(4)  Consolidation  by  Lateral  Pressure.  —  This  is  probably  the 
most  widely  acting    and    important    agency    of    consolidation. 
Though  it  acts  so  gradually  and  at  such  depths  that  we  cannot 
see  it  in  operation,  yet  the  inference  is,  none  the  less,  a  safe  one. 
We  shall  see  later  that  very  many  of  the  stratified  rocks  are  no 
longer  in  the  nearly  horizontal  position  in  which  they  were  first  laid 
down,  but  have  been  folded  and  fractured  through  the  operation  of 
great  lateral  pressures.      The  more  intensely  folded  and   com- 
pressed any  rock  has  been,  the  harder  has  it  become,  not  only 
through  the  mechanical  pressure,  but  by  the  heat  and  the  chemical 
changes  which  such  compression  generates.     In  addition  to  this,  we 
know  from  experiment  that  loose  materials  may  be  consolidated  by 
powerful  compression.     Certain  exceptional  rocks  of  very  ancient 
date  are  known,  which  are  almost  as  incoherent  as  when  first  accu- 
mulated, but  these  all  retain  their  original  horizontal  position  and 
have  not  been  compressed.     It  must  not  be  supposed,  however, 
that  only  compressed  sediments  have  become  hard,  for  great  areas 
of  scarcely  disturbed  rocks  are  found,  which  are  perfectly  solid 
and  firm;  here  some  other  solidifying  agent  has  been  at  work. 


2/8  SUMMARY  OF  RECONSTRUCTIVE  PROCESSES 

There  are  certain  other  features  in  which  the  loose  modern 
sediments  differ  from  the  older  and  harder  rocks,  such  as  joints, 
and  cleavage  which  divides  many  rocks  into  thin  plates,  indepen- 
dently of  the  planes  of  stratification.  These  may  be  shown,  how- 
ever, to  be  structures  which  the  rocks  have  acquired  after  their 
formation,  and  therefore  need  not  be  discussed  here. 

The  parallel  is  now  complete  between  the  sediments  which  we 
may  observe  to-day  in  the  process  of  accumulation,  and  the  hard 
stratified  rocks  which  make  up  by  far  the  largest  part  of  the  dry 
land.  For  all  these  ancient  rocks  we  may  find  a  counterpart  in 
sediments  now  forming,  and  we  may  conclude  with  perfect  confi- 
dence that  the  ancient  rocks  were  formed  by  the  same  agencies  as 
the  modern  accumulations.  Every  rock  contains  a  more  or  less 
legible  record  of  its  own  history. 

Summary  of  the  Reconstructive  Processes.  —  The  destructive 
agencies  supply  a  great  mass  of  material,  of  which,  under  existing 
conditions  of  climate,  topography,  etc.,  about  one-half  is  arrested 
in  its  journey  to  the  sea  and  the  remaining  half  completes  that 
journey;  the  former  moiety  constitutes  the  continental  deposits, 
and  the  latter  moiety  the  terrigenous  marine  deposits. 

Continental  deposits  are  of  great  variety,  and  their  nature  is  de- 
termined chiefly  by  the  factors  of  climate  and  topography.  In  the 
arid  and  desert  regions  we  have  great  accumulations  of  drift-sands, 
of  angular  talus,  of  flood-plain  and  playa  sands  and  muds,  which 
are  characteristically  sun-cracked  and  more  or  less  impregnated 
with  various  salts.  Deposits  from  salt  lakes,  such  as  salt,  gypsum, 
soda,  borax,  etc.,  are  confined  to  arid  climates  and  are  not  formed 
in  humid  climates.  In  pluvial  climates  of  the  temperate  zones,  rain- 
wash,  deep  soils,  lacustrine  deposits  from  fresh-water  lakes,  and  river 
deposits  on  flood  plains  and  in  channels  are  characteristic.  In  such 
climates  sun  cracks  do  not  form  over  great  areas,  as  they  are  largely 
prevented  by  the  dense  covering  of  vegetation.  Peat  bogs  are  the 
seats  of  great  vegetable  accumulations,  especially  in  the  cooler  and 
moister  regions.  In  the  polar  regions,  glacial  deposits  and  frost 
talus  are  the  principal  modes  of  accumulation,  and  in  high  mountains 


SUMMARY  OF  RECONSTRUCTIVE   PROCESSES  2/9 

these  also  penetrate  deep  into  the  temperate  and  even  the  tropical 
zones.  In  the  tropics  we  find  extremely  deep  soils,  which  contain 
or  are  made  up  of  the  red  laterite,  and  surface  deposits  of  iron  oxide 
and  chemically  formed  limestone  are  extensively  made.  Immense 
masses  of  river  alluvium  gather  in  interior  basins,  but  vegetable  accu- 
mulations are  less  abundant  than  in  temperate  lands,  and  lakes  are 
not  common  in  the  tropics.  The  absence  of  lakes,  however,  is  not 
determined  by  temperature,  but  by  the  antiquity  of  land  surfaces. 
It  cannot  be  inferred  from  the  fact  that  only  half  of  the  annual 
land-waste  finds  its  way  to  the  sea,  that  such  should  be  the  propor- 
tion between  continental  and  marine  deposits  among  ancient  rocks, 
for  a  transgression  of  the  sea  over  an  ancient  land  surface,  deeply 
buried  under  continental  deposits,  would  rapidly  rework  the  latter 
into  marine  deposits.  At  present,  we  observe  that  material  de- 
rived from  the  land  and  in  mechanical  suspension  laid  down  in 
the  sea  is  distributed  by  the  waves  and  currents,  sorted  into  layers 
according  to  the  fineness  of  the  material  and,  more  or  less  incom- 
pletely, according  to  its  mineralogical  composition.  The  most 
important  factors  which  determine  the  character  of  the  deposit 
at  any  given  point  on  the  sea-floor  are  the  depth  of  water  and  the 
topography  and  elevation  of  the  adjoining  land.  The  coarser 
material,  gravel  and  sand,  are  laid  down  upon  the  beach  and  in 
shoal  water,  the  sand  generally  extending  to  the  ico-fathom 
line,  while  on  the  continental  slope  are  deposited  the  various 
muds,  and  on  the  floor  of  the  ocean  basins  the  organic  oozes  and 
the  oceanic  red  clay,  derived  chiefly  from  the  decay  of  volcanic 
minerals.  Limestone  banks  are  formed  by  the  extraction  of  the 
dissolved  lime-salts  through  organic  agencies,  a  process  which 
goes  on  most  extensively  in  warm  seas  of  shallow  and  moderate 
depth.  Climatic  differences  also  have  their  effect  upon  marine 
deposits,  but  less  markedly  than  in  the  case  of  the  continental 
accumulations.  The  loose  sediments  accumulated  on  land  or  under 
water  are,  under  favouring  conditions,  consolidated  into  hard  rocks, 
thus  making  the  parallel  with  the  ancient  sedimentary  rocks  com- 
plete, and  finishing  the  cycle  of  destruction  and  reconstruction  from 


280  SUMMARY  OF  RECONSTRUCTIVE  PROCESSES 

rock  back  to  rock.  All  these  various  kinds  of  deposits,  continental 
and  marine,  are  forming  simultaneously,  but  one  kind  of  deposit 
does  not  gather  indefinitely  at  one  point,  except  perhaps  on  the 
floor  of  the  deep  ocean-basins.  Conditions  shift  and  change, 
so  that  one  kind  of  material  is  laid  down  upon  another,  and  in  the 
same  vertical  section  we  may  discover  many  different  beds,  each 
one  recording  the  conditions  at  that  point,  for  the  time  during  which 
the  bed  formed  the  surface  of  the  lithosphere.  All  these  changes 
we  have  studied  in  order  to  obtain  a  key  to  the  record  contained 
in  the  rocks,  and  we  have  found  that  the  processes  now  at  work  do 
furnish  a  partial  key.  However,  before  a  systematic  history  of  the 
earth  can  be  attempted,  we  have  first  to  study  the  ways  in  which 
the  rocks  are  arranged  and  the  disturbances  which  they  have 
undergone;  this  constitutes  structural  or  tectonic  geology. 


PART  II 

STRUCTURAL  OR  TECTONIC  GEOLOGY 

CHAPTER  X 

THE   ROCKS   OF   THE   EARTH'S   CRUST  —  IGNEOUS 

ROCKS 

IN  the  first  section  of  this  book  we  made  a  study  of  the  processes 
and  agencies  which  are  still  at  work  upon  and  within  the  earth, 
tending  to  modify  it  in  one  or  other  particular.  We  there  found 
that  slow  but  ceaseless  cycles  of  change  take  place  and  that  a 
continual  circulation  of  material  is  going  on. 

We  have  now  to  take  up  the  second  branch  of  our  subject,  that 
of  structural  geology,  which  deals  with  the  materials  of  the  earth's 
crust,  their  mode  of  occurrence,  and  their  arrangement  into  great 
masses.  Structural  geology  is,  however,  not  merely  a  descriptive 
study;  hand  in  hand  with  the  examination  of  the  rock-masses 
must  go  the  attempt  to  explain  their  structure,  and  to  show  how 
they  have  come  to  be  as  we  find  them.  Dynamical  principles 
must  be  continually  called  in  to  interpret  the  facts  of  structure,  and 
many  of  the  observations  concerning  the  construction,  destruction, 
and  reconstruction  of  rocks  find  their  application  in  the  study  of 
structure. 

This  application  cannot,  in  all  cases,  be  made  with  confidence, 
both  because  a  given  structure  may  often  be  referred,  with  equal 
probability,  to  different  processes,  and  because  certain  of  the  great 
dynamical  agencies  are  so  slow  and  gradual  in  their  mode  of  opera- 
tion, that  no  one  has  ever  been  able  to  observe  them  at  work.  In 

281 


282  ROCKS 

this  latter  class  of  cases  the  agency  must  be  inferred,  not  from  any- 
thing  which  we  have  actually  seen  accomplished,  but  from  the 
traces  which  it  has  left  in  the  structure.  Under  such  circumstances, 
it  need  not  surprise  us  to  find  that  the  explanation  is  not  always 
obvious,  but  may  be  very  problematical,  and  that  great  differences 
of  opinion  may  arise  concerning  the  rightful  interpretation  of  a 
complex  region. 

Here,  as  in  all  other  provinces  of  geology,  the  historical  stand- 
point is  the  dominant  one.  Our  object  is  to  learn,  not  only  the 
agencies  which  have  produced  the  structures  and  the  way  in  which 
they  operated,  but  also  the  successive  steps  by  which  the  structures 
originated,  the  order  in  which  they  occurred,  and  their  geological 
date.  Thus  they  may  be  coordinated  into  the  great  history  of  the 
earth,  which  it  is  the  main  problem  of  geology  to  construct. 

ROCKS 

The  distinction  between  a  rock  and  a  mineral  is  not  always  an 
easy  one  for  the  beginner  to  grasp,  yet  it  is  essential  that  he  should 
do  so.  A  Rock  is  any  extensive  constituent  of  the  earth's  crust, 
which  may  consist,  though  rarely,  of  a  single  mineral,  but  in  the 
great  majority  of  cases  is  a  mechanical  mixture  of  two  or  more 
minerals.  A  rock  thus  has  seldom  a  definite  chemical  compo- 
sition, or  homogeneous  internal  structure.  An  examination 
with  the  microscope  almost  always  shows  that  a  rock  is  an  aggre- 
gate of  distinct  mineral  particles,  which  may  be  all  of  one  kind, 
or  of  many  different  kinds,  in  varying  proportions.  Rocks,  then, 
are  mechanical  mixtures,  and  their  properties  vary  in  proportion 
to  their  various  ingredients,  while  minerals  are  chemical  com- 
pounds (see  p.  6). 

In  ordinary  speech  the  term  rock  is  held  to  imply  a  certain 
degree  of  solidity  and  hardness,  but  in  geological  usage  the  word 
is  not  so  restricted.  Incoherent  masses  of  sand  and  clay  are  re- 
garded as  being  rocks,  quite  as  much  as  the  hardest  granites. 

The  classification  of  rocks  is  a  very  difficult  and  obscure  prob- 


IGNEOUS   ROCKS  283 

iem,  and  would  be  so,  even  were  our  knowledge  much  more  com- 
plete and  exhaustive  than  it  is.  There  are,  therefore,  great 
diversities  in  the  various  schemes  of  classification  which  have  been 
proposed  and  which  are  still  in  use,  and  all  such  schemes  require 
modification  to  meet  continually  advancing  knowledge. 

Bearing  in  mind  the  principle,  already  emphasized  so  often,  that 
geology  is  primarily  a  historical  study,  the  most  logical  scheme  of 
classification  is  obviously  one  that,  so  far  as  possible,  is  genetic, 
that  is  to  say,  one  which  expresses  in  brief  the  history  and  mode 
of  formation  of  the  rocks.  Other  criteria,  such  as  texture  and 
chemical  and  mineralogical  composition,  must  be  employed  for 
the  minor  subdivisions.  On  this  genetic  principle  we  may  divide 
all  rocks  into  three  primary  classes  or  groups. 

A.  Igneous  Rocks,  those  which  were  melted  and  have  solidified 
by  cooling.    Texture  glassy  or  crystalline. 

B.  Sedimentary  Rocks,  those  which  have  been  laid  down  (most 
commonly)  under  water,  by  mechanical,  chemical,  and  organic 
processes.     Rocks  composed  of  more  or  less  pounded  and  worn 
fragments,  seldom  crystalline. 

C.  Metamorphic   Rocks,   those   which   have   been   profoundly 
changed  from  their  original  sedimentary  or  igneous  character,  often 
with  the  formation  of  new  mineral  compounds  in  them.    Texture 
fragmental  or  crystalline. 

IGNEOUS   ROCKS 

The  igneous  rocks  have  a  deep-seated  origin  and  have  either 
forced  their  way  to  the  surface,  or  have  cooled  and  solidified  at 
varying  depths  beneath  it.  Though  rocks  of  this  class,  there  is 
every  reason  to  believe,  were  the  first  to  be  formed,  they  have  been 
made  all  through  the  recorded  history  of  the  earth,  and,  as  volcanoes 
show,  are  forming  now.  They  are  thus  the  primary  rocks  and  all 
the  others  have  been  derived,  either  directly  or  indirectly,  from 
them.  The  products  of  the  chemical  decomposition  or  mechanical 
abrasion  of  the  igneous  rocks  have  furnished  the  materials  out  of 


284  IGNEOUS   ROCKS 

which  the  sedimentary  rocks  were  formed,  at  least  in  the  first  in- 
stance. 

The  igneous  rocks  are  massive,  as  distinguished  from  stratified, 
and  though  sometimes  presenting  a  deceptive  appearance  of  strati- 
fication, may  always,  with  a  little  care,  be  readily  distinguished 
from  the  truly  stratified  rocks.  The  term  massive  is,  indeed, 
frequently  used  for  these  rocks  in  the  same  sense  as  igneous,  and 
eruptive  rocks  is  another  term  meaning  the  same  thing,  though 
eruptive  is  also  employed  in  a  more  restricted  sense.  Still  another 
term  which  should  be  defined  is  magma,  by  which  is  meant  a  con- 
tinuous molten  mass  before  solidification. 

Characteristic  differences  appear  between  those  igneous  masses 
which  have  solidified  deep  within  the  earth  and  have  been  brought 
to  light  only  by  the  denudation  and  removal  of  the  overlying 
rock-masses,  and  those  which  have  cooled  at  or  near  the  sur- 
face of  the  ground.  The  former  are  called  plutonic  (abyssal,  or 
intrusive)  and  the  latter  volcanic  (or  extrusive).  Between  the  two 
may  be  found  every  gradation,  and  the  term  hypabyssal  is  some- 
times employed  for  rocks  which  are  transitional  between  the 
typical  plutonic  and  the  typical  volcanic  kinds.  These  terms,  plu- 
tonic, hypabyssal,  and  volcanic,  are  used  to  describe  the  character 
of  rock-masses,  not  as  terms  of  classification. 

Texture.  —  The  texture  of  an  igneous  rock  means  the  size,  shape, 
and  mode  of  aggregation  of  its  constituent  mineral  particles.  Tex- 
ture is  a  very  important  means  of  determining  the  circumstances 
under  which  the  rock  was  formed,  and  hence  great  attention  is 
paid  to  it.  Since  texture  responds  so  accurately  to  the  circum- 
stances of  solidification,  rate  of  cooling,  pressure,  etc.,  all  the  varie- 
ties shade  into  one  another  by  imperceptible  gradations  and  form 
a  continuous  series.  Nevertheless,  it  is  necessary  to  distinguish 
and  name  the  more  important  kinds. 

Among  the  igneous  rocks  are  found  four  principal  types  of 
texture,  with  several  minor  varieties :  — 

i.  Glassy.  —  Here  the  rock  is  a  glass  or  slag,  without  distinct 
minerals  in  it,  though  the  incipient  stages  of  crystallization,  in  the 


TEXTURE  285 

form  of  globules  and  hair-like  rods,  are  often  observable  with  the 
microscope.  (See  Fig.  24,  p.  75.)  When  the  glass  or  slag  is  made 
frothy  by  the  bubbles  of  escaping  steam  and  gas,  the  texture  is 
said  to  be  vesicular,  scoriaceous,  or  pumiceous  (see  Figs.  27,  210), 
according  to  the  abundance  of  the  bubbles.  These  are  varie- 
ties of  the  glassy  texture,  though  other  kinds  may  also  be  vesic- 
ular. A  vesicular  rock  in  which  the  steam-holes  have  been  filled 
up  by  the  subsequent  deposition  of  some  mineral  is  called  amyg- 
daloidal,  a  term  derived  from  the  Greek  word  for  almond. 

2.  The  Compact  (or  Felsitic)   texture  is  characterized  by  the 
formation  of  exceedingly  minute  crystals,  too  small  to  be  seen  by 
the  unassisted  eye,  giving  the  rock  a  homogeneous    but  stony 
and  not  glassy  appearance.     If  the  crystals  are  too  minute  to  be 
identified  even  by  the  aid  of  the  microscope,  the  rock  is  said  to  be 
cryptocrystalline,  and  when    such    identification    can    be  made, 
it  is  called  micro  crystalline. 

3.  Porphyritic.  —  In  rocks  of    this  texture  are  large,  isolated 
crystals,  called  phenocrysts,  embedded  in  a  ground  mass,  which 
may  be  glassy  or  made  up  of  small  crystals.     The  phenocrysts  may 
have  sharp  edges  and  well-formed  faces,  or  they  may  have  irregu- 
lar and  corroded  surfaces.     The  porphyritic  texture  indicates  two 
distinct  phases  of  crystallization.     The  first  is  the  formation  of  the 
phenocrysts,  which  remain   suspended   in  the  molten  mass,  or 
magma,  and  are  often  corroded  and  partially  redissolved  (resorbed) 
by  it.    These  crystals  are  said  to  be  of  intratelluric  origin,  because 
formed  before  the  eruption  of  the  lava,  and  such  crystals  are 
showered  out  of  certain  active  volcanoes  at  the  present  time. 
Stromboli  (see  p.  75),  for  example,  ejects  quantities  of  large  and 
perfect  augite  crystals.    There  is  reason  to  believe,  however,  that 
not  all  phenocrysts  are  thus  intratelluric,  but  that  the  first  phase  of 
crystallization  sometimes  takes  place  after  the  ejection  of  the  molten 
mass.    The  second  phase  consists  in  the  formation  of  the  ground 
mass,  which  may  be  glassy,  finely  crystalline,  or  both.    Mineral 
particles  having  distinct  crystalline  form  are  called  idiomorphic. 

4.  Granitoid.  —  In  this  texture  the  rock  is  wholly  crystalline, 


286 


IGNEOUS   ROCKS 


without  ground  mass  or  interstitial  paste.  The  component  grains, 
which  may  be  fine  or  very  coarse,  are  of  quite  uniform  size,  and 
as  the  crystals  have  interfered  with  one  another  in  the  process  of 
formation,  they  have  rarely  acquired  their  proper  crystalline  shape. 
Such  grains  are  said  to  be  allotriomorphic. 

An  additional  texture  which  should  be  mentioned  is  the  frag- 
mental.  This  is  represented  by  the  accumulations  of  the  frag- 
mental  products  ejected  by  volcanoes  (see  p.  79),  agglomerates, 


FlG.  144. —  Slab  of  polished  porphyry,  natural  size.     Phenocrysts  of  felspar 

bombs,  lapilli,  ashes,  etc.  Many  such  materials  accumulate  in 
bodies  of  water  and  are  there  sorted  and  stratified  and,  it  may  be, 
mingled  with  more  or  less  sand  and  mud  and  other  sedimentary 
material.  Rocks  formed  in  this  manner  partake  of  the  nature 
of  both  the  igneous  and  sedimentary  classes,  and  may  be  regarded 
as  a  series  intermediate  between  the  other  two  and  in  a  measure 
connecting  them.  These  rocks  will  here  be  treated  as  a  special 
subdivision,  under  the  name  of  pyroclastic  rocks. 

In  our  studies  of  the  products  of  modern  volcanoes,  we  saw  that 


SOLIDIFICATION  287 

the  same  molten  mass  will  give  rise  to  rocks  of  very  different 
appearance  in  its  different  parts,  according  to  the  circumstances 
of  rapidity  of  cooling,  pressure,  etc.  We  may  now  express  this  in 
somewhat  more  general  form  and  say  that  the  texture  of  an  igne- 
ous rock  is  determined  by  the  several  factors  which  affect  the 
molten  mass  during  consolidation.  Of  such  factors  may  be  men- 
tioned the  chemical  composition,  temperature,  rate  of  cooling, 


FIG.  145.  —  Hand  specimen  of  granite,  natural  size 

degree  of  pressure,  and  the  quantity  present  of  dissolved  vapours 
and  gases,  which  are  called  mineralizers. 

Solidification.  —  Chemical  composition  determines  the  fusibility 
of  a  rock  at  a  given  temperature.  The  least  fusible  rocks  are,  on 
the  one  hand,  those  which  contain  large  quantities  of  silica,  60  to 
75%,  and,  on  the  other,  those  which  contain  less  than  40%  of 
silica.  The  most  fusible  rocks  are  those  with  an  intermediate 


288  IGNEOUS  ROCKS 

percentage  of  silica,  and  among  these  the  fusibility  increases,  as  the 
percentage  of  silica  diminishes,  until  the  lower  limit  is  reached. 
The  effect  of  chemical  composition  upon  texture  is  seen  in  the 
rapidity  with  which  the  less  fusible  rocks  chill  and  stiffen,  and 
therefore  the  greater  frequency  with  which  they  form  glasses. 

Chemical  composition  is,  however,  important  in  this  connection 
chiefly  through  its  effect  upon  the  rate  of  solidification.  We  have 
already  learned  (p.  9),  that  solidification  very  generally  takes 
place  by  a  process  of  crystallization,  and  this  requires  time.  Hence, 
very  rapid  cooling  results  in  a  glass,,  but  the  microscope  reveals 
the  incipient  stages  of  crystallization  in  many  of  even  the  glassy 
rocks.  A  somewhat  slower  rate  of  solidification  produces  a 
cryptocrystalline  rock,  and  successively  slower  rates  bring  about 
the  porphyritic,  microcrystalline,  and  granitoid  textures.  Large 
crystals  form  slowly,  and  other  things  being  equal,  the  larger 
the  component  crystals  of  a  rock,  the  more  slowly  has  it  consoli- 
dated. 

Pressure  is  of  importance  in  preventing  the  rapid  escape  of  the 
vapours  and  gases  contained  in  the  molten  mass,  and  hence  frothy, 
scoriaceous,  and  vesicular  textures  cannot  be  produced  under  high 
pressures.  Pressure  is  also  believed  to  be  necessary  for  the  for- 
mation of  many  phenocrysts  in  porphyritic  rocks. 

The  mineralizers,  such  as  steam,  hydrochloric  acid,  and  other 
vapours,  determine  the  crystallization  of  many  minerals,  which 
refuse  to  crystallize  in  the  absence  of  such  vapours.  Variations  in 
the  quantity  of  mineralizers  present  in  different  parts  of  the  same 
mass  occasion  corresponding  differences  in  the  local  textures. 
The  well-known  Obsidian  Cliff,  in  the  Yellowstone  National  Park, 
is.  formed  by  a  great  lava-sheet,  made  up  of  alternating  layers  of 
glassy  and  microcrystalline  rock,  a  difference  which  is  referred  to 
varying  proportions  of  mineralizers  present  in  different  parts  of 
the  molten  mass. 

It  must  not  be  supposed  that  a  molten  magma  consists  merely 
of  a  number  of  fused  minerals,  mechanically  mixed  together  and 
having  no  effect  upon  one  another.  If  such  were  the  case,  the 


SOLIDIFICATION  289 

minerals  in  cooling  should  all  crystallize  in  the  order  of  their  fusi- 
bility, the  least  fusible  forming  first,  and  the  most  fusible  last.  This 
is  not  what  we  find,  and  many  facts  which  cannot  be  discussed 
here  have  led  petrographers  to  the  belief  that  a  molten  magma 
is  a  solution  of  certain  compounds  in  others,  and  that  crystalliza- 
tion occurs  in  the  order  of  solubility,  as  the  point  of  saturation  for 
particular  compounds  is  successively  reached  by  the  cooling  mass. 

Similar  phenomena  may  be  observed  among  the  metals.  If 
strips  of  copper  be  thrown  into  a  vessel  of  melted  tin,  the  latter 
will  dissolve  the  copper  at  a  temperature  far  below  that  at  which 
the  copper  would  melt  alone. 

In  a  rock  magma  the  crystallization  of  the  more  and  more 
soluble  minerals  will  proceed  regularly,  provided  the  pressure  and 
rate  of  cooling  continue  constant.  As  these  conditions  are,  how- 
ever, subject  to  variation,  it  frequently  happens  that  the  more 
soluble  minerals  begin  to  crystallize  before  the  less  soluble  have 
all  been  formed,  and  thus  the  periods  of  formation  of  two  or  more 
kinds  of  minerals  partly  overlap. 

Usually,  the  order  of  formation  of  the  different  kinds  of  min- 
erals in  a  solidifying  magma  is  as  follows.  First  to  form  are  apatite, 
the  metallic  oxides  (magnetite,  ilmenite) ,  and  sulphides  (pyrite), 
zircon,  and  titanite.  "  Next  come  the  ferro-magnesian  silicates, 
olivine,  biotite,  the  pyroxenes,  and  hornblende.  Next  follow  the 
felspars  and  felspathoids,  nepheline  and  leucite,  but  their  period 
often  laps  well  back  into  that  of  the  ferro-magnesian  group.  Last 
of  all,  if  excess  of  silica  remains,  it  yields  quartz.  In  the  variations 
of  pressure  and  temperature,  it  may  and  often  does  happen  that 
crystals  are  again  redissolved,  or  resorbed,  as  it  is  called,  and  it 
may  also  happen  that  after  one  series  of  minerals,  usually  of 
large  size  and  intratelluric  origin,  have  formed,  the  series  is  again 
repeated  on  a  small  scale,  as  far  back  as  the  ferro-magnesian 
silicates.  Minerals  of  a  so-called  second  generation  thus  result, 
but  they  are  always  much  smaller  than  the  phenocrysts  and  are 
characteristic  of  the  ground  mass. 

"  It  results  from  what  has  been  said  that  the  residual  magma  is 
u 


2QO  IGNEOUS  ROCKS 

increasingly  siliceous  up  to  the  final  consolidation,  for  the  earliest 
crystallizations  are  largely  pure  oxides.  It  is  also  a  striking  fact 
that  the  least  fusible  minerals,  the  felspars  and  quartz,  are  the 
last  to  crystallize."  (Kemp.) 

A  very  considerable  number  of  minerals  are  found  in  the  igne- 
ous rocks,  but  comparatively  few  in  any  large  quantity.  It  thus 
becomes  necessary  to  distinguish  between  the  essential  minerals 
of  a  rock  and  the  accessory  ones.  The  essential  minerals  are 
those  which  characterize  a  given  kind  of  rock,  while  the  accessory 
minerals  are  those  which  occur  in  small  quantities  and  which  may 
be  present  or  absent,  without  materially  affecting  the  nature  of 
the  rock.  The  distinction  is  necessary  and  useful,  but  is  rather 
arbitrary. 

Another  necessary  distinction  is  that  between  original  and 
secondary  minerals.  Original  minerals  were  formed  with  or  be- 
fore the  rock  of  which  they  are  constituents,  and  secondary  min- 
erals are  produced  by  the  alteration  or  reconstruction  of  the 
original  ones. 

With  comparatively  few  exceptions,  the  igneous  rocks  are  made 
up  of  some  felspar  or  felspathoid,  together  with  one  or  more  of 
the  pyroxenes,  amphiboles,  micas,  olivine,  or  quartz.  Magnetite 
is  also  very  common. 

Differentiation.  —  Different  parts  of  the  same  continuous  rock- 
mass  frequently  display  chemical  and  mineralogical  variations, 
resulting  from  a  process  of  differentiation,  or  segregation,  of  the 
magma.  How  this  is  brought  about,  is  far  from  certain,  but  there 
can  be  little  or  no  doubt  as  to  the  fact.  "  When  large  areas  of 
eruptive  rocks  are  carefully  investigated,  it  is  found  that  there  is 
a  perfect  and  gradual  transition  of  one  kind  into  another  —  all  in- 
termediate varieties  existing  —  and  that  quantitatively  no  special 
part  of  the  series  is  universally  predominant,  although  there  are 
often  immense  masses  of  nearly  uniform  character,  and  there  may 
be  smaller  bodies  of  quite  variable  composition."  (Iddings.) 
It  is  further  found  that  in  a  given  volcanic  district,  or  petrographi- 
cal  province,  the  rocks  erupted  at  a  particular  geological  period 


ASSIMILATION  29 1 

have  certain  peculiarities  which  distinguish  them  from  those  of 
other  provinces.  It  thus  appears  probable  that  the  igneous  rocks 
of  such  a  province  were  derived  from  the  same  magma,  and  the 
relationship  between  the  various  kinds  of  rocks  of  the  province  is 
called  consanguinity. 

The  existence  of  these  petrographical  provinces  does  not  imply 
that  the  rocks  of  each  one  differ  from  those  of  all  the  others.  In 
fact,  similar  or  identical  groups  of  rocks  are  found  in  many  parts 
of  the  world,  but  each  province  differs  more  or  less  from  the  sur- 
rounding ones.  Thus,  rocks  which  from  the  genetic  point  of  view 
are  closely  related,  are,  by  any  scheme  of  chemical  or  mineralogical 
classification,  often  placed  in  widely  separated  groups. 

Assimilation.  —  When  we  examine  in  the  field  the  igneous  rocks 
in  their  relations  to  other  rock-masses,  we  frequently  find  cases 
where  it  is  exceedingly  difficult  to  account  for  the  presence  of  the 
igneous  mass,  except  upon  the  assumption  that  the  magma  made 
way  for  itself  by  fusing  and  incorporating  the  rocks  which  must 
formerly  have  occupied  its  present  position,  the  surrounding  rocks 
showing  no  evidence  of  being  merely  pushed  aside  by  the  ascending 
magma.  In  certain  instances,  such  a  melting  and  incorporating 
of  opposing  rocks  would  seem  to  be  clear,  as  when  a  sheet  of  magma 
has  made  its  way  into  a  series  of  strata,  parallel  with  the  bedding 
planes,  without  increasing  the  thickness  of  the  series.  This  in- 
corporating of  freshly  fused  material  with  the  intruding  magma 
is  called  assimilation,  but,  save  on  a  very  small  scale,  its  reality  is 
a  subject  of  much  dispute,  and  some  of  the  highest  authorities  al- 
together reject  it.  Nevertheless,  many  observed  facts  strongly 
favour  this  assimilation  theory,  which  has  a  most  important  bear- 
ing upon  some  of  the  fundamental  problems  of  geology.  T  Accord- 
ing  to  this  view,  the  ascending  magma  is  at  an  extremely  high 
temperature  and  very  fluid,  and  it  forces  its  way  upward  partly 
through  crevices  and  fissures,  partly  by  detaching  joint-blocks, 
which  sink  into  the  molten  mass  and  are  dissolved  by  it,  thus  greatly 
modifying  the  chemical  constitution  of  the  magma.  Subsequently, 
the  magma  becomes  differentiated,  so  that  the  different  varieties. 


2Q2  IGNEOUS   ROCKS 

of  rock  separate  from  one  another  in  the  manner  already  described. 
Perhaps,  as  Daly  has  suggested,  there  is  a  universal  subcrustal 
magma,  of  basic  composition,  which,  owing  to  the  pressure 
of  the  overlying  crust,  is  only  potentially  fluid,  liquefying  along  lines 
of  a  partial  release  of  pressure.  The  overlying  solid  rocks  are,  on 
the  average,  more  acid  than  the  subcrustal  magma  and  thus,  by 
assimilation  followed  by  differentiation,  the  many  varieties  of 
igneous  rock  are  formed. 

Classification.  —  What  was  said  above  with  regard  to  the  diffi- 
culty of  classifying  rocks,  applies  more  especially  to  the  igneous 
group,  because  of  the  way  in  which  the  various  kinds  shade  into 
one  another,  since  even  the  same  molten  mass  may  differentiate 
into  several  species,  showing  not  only  differences  of  texture,  but 
marked  changes  of  chemical  and  mineralogical  composition.  In 
an  elementary  work,  like  the  present,  only  a  meagre  outline  of  the 
subject  can  be  attempted,  for  the  microscopic  study  of  rocks,  or 
petrography,  has  now  become  an  independent  science  of  great  scope 
and  interest  and  cannot  be  compressed  into  a  few  pages. 

The  classification  of  the  igneous  rocks  now  most  generally 
adopted  is  made  upon  a  threefold  method,  according  to  texture, 
and  chemical  and  mineralogical  composition.  In  the  following 
table  (modified  from  Kemp's)  the  textures  are  given  in  vertical 
order,  while  transversely  the  arrangement  is  mineralogical,  chiefly 
in  accordance  with  the  principal  felspar.  In  this  manner  the  acidic 
rocks  come  at  the  left  side  of  the  table  and  the  basic  at  the  right 
side.  The  percentages  of  silica  are  given  on  a  lower  line  of  the 
table. 

The  acid  rocks  are  so  called  because  they  are  rich  in  silica,  but 
they  have  only  small  quantities  of  lime,  magnesia,  and  iron;  hence 
they  are  very  infusible,  of  low  specific  gravity,  and  generally  of  light 
colours.  The  basic  rocks,  thus  named  because  of  the  predomi- 
nance of  the  bases,  have  much  smaller  percentages  of  silica  and 
higher  ones  of  lime,  magnesia,  and  iron;  the  latter  substances 
act  as  fluxes,  making  the  basic  rocks  much  more  fusible,  as  well 
as  giving  them  a  higher  specific  gravity  and  darker  colour.  The 


CLASSIFICATION 


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294  IGNEOUS   ROCKS 

distinction  between  acid  and  basic  rocks  cannot  be  very  sharply 
drawn,  because  the  two  kinds  are  connected  by  every  variety  of 
intermediate  gradation.  The  same  is  true,  however,  of  all  the 
divisions  given  in  the  table,  which  is  apt  to  produce  a  false  impres- 
sion of  sharply  distinguished  groups  of  rocks,  such  as  do  not  occur 
in  nature. 

As  a  general  rule,  the  glassy  and  porphyritic  textures  character- 
ize those  rocks  which  have  solidified  at  the  surface  of  the  ground, 
or  not  very  far  below  it,  while  the  granitoid  types  have  cooled 
slowly  and  at  great  depths;  but  there  are  exceptions  to  both  state- 
ments. Between  the  glassy  and  porphyritic  textures  at  one  end 
of  the  series  and  the  granitoid  at  the  other  comes  the  felsitic 
which  represents  an  intermediate  rate  of  cooling  and  intermediate 
depths  within  the  earth  as  the  place  of  solidification  (hypabyssal 
rocks). 

The  division  of  the  igneous  rocks  into  families  is  made  prima- 
rily in  accordance  with  the  mineralogical  composition,  with  sub- 
divisions according  to  texture.  This  method  gives  us  five  principal 
groups. 

I.  THE  GRANITE  FAMILY 

The  molten  magma,  which  on  solidification  gives  rise  to  the  rocks 
of  this  group,  is  very  rich  in  silica  (65  to  80%)  and  has  from  10 
to  15%  of  alumina;  the  quantity  of  alkalies  (Na  and  K)  is  rela- 
tively large  (6  to  8  %),  and  there  are  small  amounts  of  Iron  oxides 
(2  to  4%),  magnesia  (i  to  2%),  and  lime  (i  to  4%).  In  the  process 
of  consolidation  the  principal  minerals  formed  are  orthoclase  and 
quartz,  with  smaller  amounts  of  oligoclase,  iron  oxide,  and  of  the 
ferro-magnesian  minerals,  biotite  or  hornblende.  Differences  of 
texture,  produced  in  the  manner  already  described,  give  rise  to 
rocks  of  totally  different  appearance,  which  it  is  difficult  to  imagine 
are  of  similar  or  identical  composition. 

Obsidian  is  a  volcanic  glass,  which  is  usually  black  or  dark 
brown  or  green  (but  sometimes  blue,  red,  or  yellow).  It  breaks 
with  a  shell-like  fracture,  and  in  very  thin  pieces  is  translucent. 


THE  GRANITE  FAMILY  295 

The  microscope  shows  "  crystallites,"  the  incipient  stages  of  crys- 
tals, which  are  present  in  great  numbers.  The  name  obsidian  is 
used  for  the  various  kinds  of  volcanic  glass  in  which  the  percentage 
of  water  is  small,  and  so  for  exact  description  a  prefix  is  necessary, 
such  as  rhyolite  obsidian,  andesite  obsidian.  Though  the  glasses 
are  of  varying  composition,  by  far  the  greater  number  of  them  be- 
long to  the  granite  family.  When  the  glass  is  divided  by  concentric 
cracks,  due  to  shrinkage  on  cooling,  so  as  to  form  onion-like  spher- 
ules, it  is  called  Perlite. 

Pitchstone  has  much  the  same  appearance  as  obsidian,  but 
contains  from  5  to  10  %  of  water. 

Pumice  is  a  glass  blown  up  by  the  bubbles  of  escaping  steam 
and  other  vapours  into  a  rock  froth,  so  light  that  it  will  float  upon 
water.  A  very  similar  substance  is  produced  when  a  jet  of  steam 
is  blown  through  the  melted  slag  from  an  iron  furnace. 

It  not  infrequently  happens  that,  in  course  of  time,  the  volcanic 
rocks  become  devitrified,  losing  their  glassy  texture  and  assuming 
a  stony  one.  The  homogeneous  rock  becomes  converted  into  a 
mass  of  extremely  minute  crystals  of  quartz  and  felspar,  and  the 
original  glassy  texture  is  then  shown  only  by  the  lines  of  flow,  or 
by  the  perlitic  character,  which  are  not  affected  by  the  change. 
Devitrification  has  also  been  observed  in  artificial  glasses,  espe- 
cially when  the  glass,  owing  to  insufficient  annealing,  has  been 
subject  to  internal  stress. 

Rhyolite  ordinarily  occurs  as  the  lava  outflow  of  a  granitic  magma, 
cooled  rapidly,  but  yet  more  slowly  than  obsidian.  The  texture 
is  porphyritic,  the  phenocrysts  being  chiefly  quartz  and  the 
glassy  form  of  orthoclase  known  as  sanidine,  while  the  ferro-mag- 
nesian  minerals  are  present  in  very  much  smaller  quantities,  and 
of  these  the  commonest  is  biotite.  The  phenocrysts  are  em- 
bedded in  a  ground  mass  of  minute  felspar  crystals  and  a  varying 
proportion  of  glass.  Other  names  used  for  rhyolite  are  liparite 
and  quartz  trachyte.  The  rhyolites  are  exceedingly  common  in  the 
western  part  of  the  United  States.  The  Felsites  are  very  dense, 
fine-grained,  and  light-coloured  rocks,  in  which  phenocrysts  are 


296  IGNEOUS   ROCKS 

absent  or  scanty;  they  are  rocks  which  have  been  formed  in  differ- 
ent ways,  by  the  devitrification  of  obsidians  and  rhyolites,  by  the 
recrystallization  of  tuffs,  and  by  original  cooling  from  fusion. 

Quartz  Porphyry  shades  imperceptibly  into  rhyolite  or  felsite 
on  the  one  hand,  and  into  granite  on  the  other;  it  is  made  up 
of  phenocrysts  of  quartz,  or  of  quartz  and  orthoclase,  in  a  crystal- 
line ground  mass  of  the  same  minerals.  If  the  phenocrysts 
are  very  abundant  and  the  ground  mass  rather  coarse  grained,  the 
rock  is  called  granite  porphyry.  Syenite  porphyry,  Diorite  por- 
phyry, etc.,  bear  similar  relations  to  the  other  members  of  their 
respective  families  and  need  no  further  description. 

Granite.  —  The  granites  are  thoroughly  crystalline  rocks,  of 
typically  granitoid  texture,  to  which  they  have  given  the  name, 
and  without  any  ground  mass.  The  grains  have  not  their  proper 
crystalline  shape,  the  separate  minerals  interfering  with  each  other 
in  the  process  of  crystallization.  The  characteristic  minerals  are 
quartz,  orthoclase,  some  acid  plagioclase,  muscovite,  biotite,  and 
hornblende;  magnetite  and  apatite  are  always  present,  though  in 
small  quantities.  The  variations  in  granite  are  principally  in  the 
ferro-magnesian  minerals.  Thus  we  have  muscovite  granite,  with 
white  mica  only;  granitite,  with  biotite  only;  hornblende  granite, 
the  hornblende  replacing  the  mica,  or  in  addition  to  biotite;  augite 
granite,  with  augite  and  biotite.  Those  in  which  the  percentage 
of  soda  is  high  are  called  soda-granites.  When  the  ferro-magnesian 
silicates  are  all  absent,  the  rock  is  called  a  binary  granite,  or  Aplite. 

The  colour  of  granite  is  dark  or  light  in  accordance  with  the 
proportion  of  dark  silicates  present,  while  the  shades  of  the  felspar 
determine  whether  the  rock  shall  be  red,  pink,  or  white.  The 
texture  of  granite  varies  from  fine  to  very  coarse,  and  in  some 
cases  becomes  nearly  porphyritic.  A  very  coarse-grained  granite 
is  called  Pegmatite,  or  giant  granite. 

II.   THE  SYENITE  FAMILY 

In  this  family  the  magma  much  resembles  that  of  the  granite 
group,  except  that  the  quantity  of  silica  is  less  (50  to  65  %);  hence 


THE  DIORITE  FAMILY  297 

it  is  nearly  or  quite  taken  up  in  the  formation  of  silicates,  leaving 
little  or  none  to  crystallize  out  separately  as  quartz,  and  orthoclase 
is  thus  the  chief  mineral.  The  two  families  are  connected  by 
many  transitional  rocks. 

Syenite  Obsidian  is  indistinguishable,  except  by  chemical  analy- 
sis, from  the  glasses  of  the  preceding  family,  but  it  is  much  less 
common. 

Trachyte  is  a  volcanic  rock,  consisting  of  phenocrysts  of  sani- 
dine  in  a  ground  mass  of  minute  felspar  crystals,  but  having  little 
or  no  glass,  together  with  more  or  less  biotite,  amphibole,  or 
pyroxene,  according  to  which  we  get  the  varieties  mica,  amphi- 
bole, or  pyroxene  trachyte.  In  America  the  trachytes  are  very 
much  less  abundant  than  the  rhyolites. 

Phonolite  differs  from  trachyte  in  the  higher  percentage  of 
soda  which  it  contains,  and  in  the  presence  of  the  felspathoid 
nepheline  or  leucite,  or  both.  The  name  is  derived  from  the 
ringing  sound  which  thin  plates  of  the  rock  give  out  when  struck 
with  a  hammer.  Phonolites  are  quite  rare  rocks,  and  in  this 
country  the  best-known  locality  for  them  is  the  Black  Hills  region 
of  South  Dakota. 

Syenite  is  a  thoroughly  crystalline  rock,  without  ground  mass, 
and  much  resembling  granite  in  appearance,  but  having  no  quartz. 
It  is  composed  typically  of  orthoclase  and  hornblende,  with  plagio- 
clase,  apatite,  and  magnetite  as  accessories.  When  the  hornblende 
is  replaced  by  biotite,  the  rock  is  called  mica  syenite,  and  when  by 
augite,  augite  syenite.  The  name  syenite  is  sometimes  given  to 
the  rock  we  have  called  "  hornblende  granite." 

Nepheline  Syenite  is  marked  by  the  presence  of  nepheline,  and 
bears  the  same  relation  to  phonolite  as  syenite  does  to  trachyte, 
being  the  granitoid  crystallization  of  the  same  magma. 

The  syenites  occur  just  as  do  the  granites,  but  are  not  nearly 
50  frequent. 

III.   THE  DIORITE  FAMILY 

The  magma  of  these  rocks  has  about  the  same  silica  percent- 
ages (50  to  65  %)  as  have  the  syenites,  but  the  quantity  of  alkalies 


298  IGNEOUS   ROCKS 

is  less,  while  that  of  the  lime  and  magnesia  is  greater.  Hence 
orthoclase  is  absent  or  much  less  important,  and  the  principal 
mineral  is  a  soda-lime  felspar.  The  textures  display  the  usual 
variety  from  glassy  to  granitoid. 

The  glasses  of  this  family  (andesite  obsidian)  can  be  distin- 
guished from  those  of  the  preceding  groups  only  by  chemical 
analysis,  but  they  are  rare. 

Andesites  are  dark-coloured  lavas  of  porphyritic  or  compact 
texture,  composed  of  a  glassy  plagioclase  felspar  and  some  ferro- 
magnesian  mineral,  embedded  in  a  ground  mass  of  felspar  needles 
and  glass.  In  accordance  with  the  nature  of  the  predominant 
ferro-magnesian  mineral,  we  have  hornblende  andesite,  biotite  ande- 
site,  and  several  varieties  of  pyroxene  andesite.  These  rocks  are 
very  common  in  the  western  United  States  and  along  the  Pacific 
coast  of  both  North  and  South  America;  they  are  named  from 
the  Andes. 

The  Dacites  differ  from  the  andesites  in  having  quartz,  and 
therefore  a  higher  percentage  of  silica. 

The  Diorites  are  the  plutonic  equivalents  of  the  andesites  and 
dacites,  having  granitoid  texture.  The  ferro-magnesian  mineral 
is  usually  green  hornblende,  but  augite  and  other  pyroxenes  and 
biotite  occur  in  the  different  varieties.  Most  diorites  have  a  little 
quartz;  but  when  this  mineral  becomes  abundant,  it  gives  a  quartz 
diorite,  which  is  related  to  the  dacites  as  the  typical  diorite  is  to  the 
andesites. 

IV.  THE  GABBRO  FAMILY 

In  the  magmas  of  this  series  the  percentage  of  silica  is  much 
less  than  in  the  preceding  groups  (40  to  55%),  and  the  quantity 
of  alkalies  is  small,  while  that  of  iron,  magnesia  and  lime  is  much 
greater.  They  are  heavy,  dark-coloured  rocks.  The  principal 
minerals  are  a  plagioclase  felspar,  rich  in  lime  (labradorite  or  anor- 
thite),  some  kind  of  pyroxene,  magnetite,  and  frequently  olivine. 
There  is  a  wide  range  of  mineralogical  composition  and  many  varie- 
ties of  rock  occur  in  this  family. 


THE  GABBRO  FAMILY  299 

Tachylyte  is  a  basaltic  glass,  which  is  not  at  all  common. 

Basalt  is  a  name  of  wide  application  covering  many  varieties. 
The  basalts  are  very  common  volcanic  rocks,  and  most  of  the  active 
volcanoes  of  the  present  day  extrude  basaltic  lavas.  In  texture  the 
basalts  are  ordinarily  porphyritic,  but  they  may  be  without  pheno- 
crysts,  and  consist  of  a  finely  crystalline  mass.  The  ground  mass  is 
made  up  of  tiny  crystals,  mingled  with  a  dark  glass. 

The  basalts  are  closely  related  to  the  andesites  and  connected 
with  them  by  a  number  of  transitional  forms,  but  in  the  andesites 
the  phenocrysts  are  principally  felspars,  which  is  not  the  case  in 
the  basalts.  Those  basalts  which  contain  olivine  in  notable  quan- 
tities are  called  olivine  basalt ;  while  those  in  which  the  felspar  is 
replaced  by  leucite  or  nepheline  are  called  leudte  and  nepheline 
basalt,  respectively. 

Trap  is  a  useful  field  name  for  various  sorts  of  dark,  granular 
rocks,  which  cannot  readily  be  distinguished  by  inspection.  The 
term  is  often  applied  to  diorite  and  especially  to  diabase. 

Dolerite  is  a  coarsely  crystalline  basaltic  rock,  which  is  either 
porphyritic  or  granitoid  in  texture. 

Diabase  is  a  rock  of  peculiar  texture;  the  felspar  crystals  are 
long,  narrow,  and  lath-shaped,  and  contain  the  dark  minerals  in 
their  interstices.  The  trap  rocks  of  the  Palisades  of  the  Hudson, 
and  many  localities  in  the  Connecticut  valley,  New  Jersey,  Mary- 
land, Virginia,  and  North  Carolina,  are  diabase. 

Gabbro  is  a  term  which  is  now  used  comprehensively  to  include 
the  coarse-grained,  plutonic  phases  of  the  various  basaltic  rocks, 
which  are  typically  composed  of  plagioclase  and  pyroxene.  Olivine 
gabbro  and  hornblende  gabbro  are  names  that  explain  themselves. 
Norite,  or  hypersthene  gabbro,  contains  orthorhombic  pyroxene. 
Anorthosite  is  nearly  pure  labradorite  in  large  crystals,  with  little 
or  no  pyroxene ;  great  masses  of  it  occur  in  Canada  and  the  Adi- 
rondack Mountains  of  New  York.  Gabbros  are  present  on  a 
great  scale  in  the  Adirondacks,  and  occur  in  the  White  Moun- 
tains, on  the  Hudson,  near  Baltimore,  around  Lake  Superior,  in 
California  and  various  parts  of  the  West. 


300  PYROCLASTIC   ROCKS 

V.    PERIDOTITE  FAMILY 

These  rocks  have  no  felspars,  and  in  most  of  them  the  quantity 
of  silica  is  below  45  %,  while  that  of  magnesia  is  from  35  to  48  %; 
they  are  composed  almost  entirely  of  ferro-magnesian  minerals. 

Limburgite  is  made  up  of  crystals  of  augite,  olivine,  and  mag- 
.netite,  embedded  in  a  glassy  ground  mass. 

Augitite  is  a  similar  rock,  but  without  olivine. 

Pyroxenite  is  a  holocrystalline,  plutonic  rock  composed  of  one 
or  more  varieties  of  pyroxene. 

Hornblendite  is  a  similar  rock  made  up  of  hornblende. 

The  Peridotites  are  likewise  plutonic  rocks  which  are  principally 
composed  of  olivine,  with  iron  ore  and  some  of  the  pyroxenes  or 
hornblende. 

The  Serpentines  are  products  of  alteration,  and  many  of  them 
have  been  formed  from  peridotites,  though  some  have  been  derived 
from  augitic  rocks,  such  as  gabbro,  and  others  from  hornblendic 
rocks.  In  rarer  instances  they  have  arisen  from  the  alteration 
of  acid  rocks. 


APPENDIX 

THE  PYROCLASTIC  ROCKS 

THESE  rocks  are  formed  out  of  the  fragmental  materials  ejected 
from  volcanoes.  The  materials  are  of  course  igneous,  but  the 
rocks  themselves  differ  from  the  typical  igneous  rocks  in  several 
important  respects.  They  have  not  been  formed  in  their  present 
state  of  aggregation  by  cooling  from  a  molten  mass,  and  in  many 
cases  they  are  more  or  less  distinctly  stratified.  It  seems  best, 
therefore,  to  group  them  separately,  under  the  name  pyroclastic. 

Volcanic  Agglomerate,  or  Breccia,  is  a  mass  of  angular  blocks  of 
lava,  with  which  may  be  mingled  fragments  of  sedimentary  rocks, 
which  the  volcano  has  torn  off  from  the  sides  of  its  chimney.  The 
blocks  may  be  loose  or  cemented  together  into  hard  rock  by  a 


PYROCLASTIC   ROCKS  30 1 

filling  of  finer  materials.  Ordinarily  the  breccia  is  formed  only 
near  the  vent,  but  sometimes  it  is  developed  on  a  great  scale,  as 
in  the  eastern  part  of  the  Yellowstone  Park. 

Tuffs  are  masses  of  volcanic  ashes  and  dust,  which  accumulate 
in  beds,  either  on  the  land  or  in  bodies  of  water.  Even  in  falling 
through  the  air,  the  particles  are  sorted,  in  some  degree,  in  ac- 
cordance with  their  size,  and  the  tuffs  are  thus  usually  stratified, 
and  frequently  have  fossils  in  them.  When  accumulated  under 
water,  the  ashes  are,  of  course,  stratified  and  may  be  mingled  with 
more  or  less  sedimentary  debris.  Such  subaqueous  tuffs  pass  into 
the  ordinary  sedimentary  rocks,  by  the  gradual  diminution  of  the 
volcanic  material.  When  examined  under  the  microscope,  even 
the  finest  tuffs  are  found  to  consist  of  crystals  and  particles  of  glass. 

The  volcanic  breccias  and  tuffs  may  best  be  classified  in  accord- 
ance with  the  nature  of  the  component  fragments.  Thus,  we  find 
rhyolite  tuffs  and  breccias,  andesite  tuffs  and  breccias,  basaltic 
tuffs  and  breccias,  and  the  like. 


CHAPTER   XI 
THE   SEDIMENTARY   ROCKS 

THE  materials  of  which  the  sedimentary  rocks  are  composed 
were,  in  the  first  instance  at  least,  derived  from  the  chemical 
decay  or  mechanical  abrasion  of  the  igneous  rocks,  and  hence 
they  are  often  called  derivative  or  secondary.  They  have  been 
laid  down  under  water,  or  on  land,  and  are  therefore  almost  always 
stratified  and,  for  the  most  part,  are  composed  of  rounded  fragments, 
seldom  crystalline. 

Almost  all  the  minerals  which  we  have  found  in  the  igneous 
rocks  also  occur,  in  a  more  or  less  worn  and  comminuted  condi- 
tion, in  the  sedimentary  class.  However,  with  the  exception  of 
quartz,  the  great  bulk  of  the  sedimentary  materials  consists  of 
simpler  and  more  stable  compounds  than  the  igneous  minerals, 
from  the  decomposition  of  which  they  have  been  derived.  The 
principal  minerals  which  compose  the  sedimentary  rocks  are  quartz 
(SiO2),  kaolinite  (A12O3,  2  SiO2,  2  H2O),  and  calcite  (CaCO3). 

Quartz  is  a  very  simple  and  stable  chemical  compound,  and 
hence,  in  the  ordinary  process  of  rock  decay,  it  remains  un- 
changed further  than  being  broken  up  into  smaller  pieces  and 
rounded  by  the  action  of  wind  or  running  water.  Kaolinite  is 
derived  principally  from  the  decay  of  the  felspars,  and  the  lime 
of  calcite  from  the  complex  silicates  containing  lime,  which  are  so 
frequent  in  the  igneous  rocks.  These  rocks  also  yield  the  iron 
oxides  which  are  so  widely  diffused  in  the  sedimentary  class,  though 
comparatively  seldom  in  any  very  great  quantity.  Very  many 
varieties  of  rocks  are  produced  by  the  mixture  of  the  siliceous 
(quartz),  argillaceous  (clay),  and  calcareous  (lime)  materials  in 
varying  proportions.  The  sorting  out  of  material  by  water,  accord- 


AQUEOUS   ROCKS  303 

ing  to  its  chemical  nature,  is  usually  imperfect  (although  siliceous 
and  calcareous  concentrations  are  often  remarkably  pure),  and 
changes  from  point  to  point,  so  that  the  sedimentary  rocks  have  an 
even  less  definite  chemical  composition  than  have  the  igneous. 

It  is,  unfortunately,  not  yet  practicable  to  apply  to  the  sedi- 
mentary rocks  the  arrangement  employed  for  modern  continental 
and  marine  deposits,  and  the  most  useful  classification  at  present 
of  the  sedimentary  rocks  is,  primarily,  according  to  the  mode  of 
their  formation,  and  secondarily,  according  to  their  composition. 
This  gives  two  principal  divisions:  I,  the  Aqueous  Rocks,  or  those 
laid  down  under  water  ;  II,  the  jEolian  Rocks,  those  which  were 
accumulated  on  land,  which  are  of  more  limited  extent  and  im- 
portance. 

The  aqueous  rocks  may  be  further  divided  into  three  classes: 
i,  Mechanical  Deposits;  2,  Chemical  Precipitates;  3,  Organic 
Accumulations. 

I.   AQUEOUS  ROCKS 

The  rocks  laid  down  under  water  form  the  larger  and  more  im- 
portant part  of  the  sedimentary  series. 

I.     MECHANICAL  DEPOSITS 

These  have  resulted  from  the  accumulation  of  debris  derived 
from  the  destruction  of  preexisting  rocks,  carried  in  mechanical 
suspension  by  moving  water,  whether  waves,  currents,  or  streams, 
and  dropped  when  the  velocity  of  the  moving  water  was  no  longer 
sufficient  to  carry  them.  The  study  of  the  dynamical  processes 
has  already  taught  us  that  such  accumulations  are  forming  to-day 
in  all  kinds  of  bodies  of  water,  and  an  examination  of  the  rocks 
will  show  that  similar  accumulations  have  been  made  since  the 
beginning  of  recorded  geological  time.  Mineralogically,  the 
mechanical  deposits  are  of  two  principal  kinds,  the  siliceous  and 
the  argillaceous.  The  sorting  power  of  water  has  been  sufficient 
to  separate  them  more  or  less  completely,  though  we  find  mixtures 
of  the  two  in  all  proportions. 


304  THE  SEDIMENTARY   ROCKS 

a.    Siliceous  Rocks 

In  these  rocks  the  principal  component  is  quartz  in  fragments 
of  greater  or  less  size,  either  angular,  or  more  or  less  rounded  by 
wear.  Of  the  common  rock-forming  minerals  quartz  is  the  hardest 
and  the  one  which  best  resists  chemical  change.  Small  quantities 
of  other  minerals,  such  as  magnetite,  mica,  felspar,  garnet,  etc.,  are 
generally  present. 

Sand  is  made  up  of  fine  grains  of  quartz,  not  compacted  to- 
gether, but  forming  a  loose,  incoherent  mass.  River  sands  and 
those  formed  by  the  atmospheric  disintegration  of  rocks  commonly 
have  angular  grains,  due  to  the  splitting  up  of  the  quartz  fragments 
along  preexisting  flaws,  though  desert  and  wind-blown  sands  are 
apt  to  be  fine-grained,  rounded  and  pitted  by  abrasion.  Beach 
sand  is  somewhat  rounded,  due  to  the  constant  wash  of  the  surf. 

Sandstone  is  a  rock  of  varying  degrees  of  hardness,  the  grains 
of  sand  being  held  together  by  a  cement.  The  most  important 
cementing  substances  are  carbonate  of  lime,  the  oxides  of  iron 
and  silica.  The  sandstones  with  calcareous  cement  usually  yield 
quickly  to  the  action  of  the  weather,  because  of  the  solubility  of 
the  cement.  Those  with  ferruginous  cement  are  much  more 
durable  and  more  highly  coloured,  being  of  various  shades  of  red, 
yellow,  and  brown.  Most  durable  of  all  are  the  siliceous  cements. 

Varieties  of  sandstone  are  produced  by  the  conspicuous  admix- 
ture of  other  minerals;  thus,  micaceous  sandstone  has  abundant 
flakes  and  spangles  of  mica  deposited  along  the  planes  of  strati- 
fication. Argillaceous  sandstone  is  composed  of  a  more  finely 
grained  sand  than  the  more  typical  sandstones,  contains  consider- 
able quantities  of  clay,  and  is,  in  general,  more  thinly  bedded. 
The  flagstones,  so  largely  used  for  pavements,  are  examples  of  such 
a  rock,  and  split  readily  into  slabs  of  almost  any  desired  size. 

Arkose  is  a  sandstone  containing  considerable  quantities  ol 
felspar  in  a  mechanically  subdivided  but  undecomposed  state. 

Gravel  is  composed  of  rounded,  Water- worn  pebbles,  varying 
in  size  from  a  pin-head  up  to  cobblestones  and  boulders.  The 


ARGILLACEOUS   ROCKS 


305 


coarser  kinds  are  often  called  shingle.  Gravel  may  be  composed 
of  almost  any  kind  of  rock  material,  but  the  commonest  pebbles 
are  of  quartz,  because  of  its  greater  resistance  to  wear.  Masses 
of  quartz  will  be  only  rounded  into  pebbles,  when  other  substances 
are  ground  into  fine  silt,  or  chemically  disintegrated,  and  so 
washed  into  deeper  water. 

Conglomerate  is  a  cemented  gravel.  Different  names  are  given 
to  conglomerate,  according  to  the  character  of  the  pebbles,  as 
quartz  conglomerate,  flint  conglomerate,  limestone  conglomerate, 
granite  conglomerate,  etc. 


FIG.  146.  —  Hand  specimen  of  conglomerate,  natural  size 

b.  Argillaceous  Rocks 

Clay  — Mud.  —  Clay  consists  of  kaolinite  nearly  always  with  large 
admixtures  of  other  substances,  such  as  exceedingly  fine  grains  of 
x 


306  THE  SEDIMENTARY   ROCKS 

quartz,  felspathic  mud,  and  the  like.  When  moist,  clay  is  plastic, 
differing  in  this  respect  from  mud.  The  particles  .of  clay  and  mud 
are  extremely  fine  and  are  carried  for  long  distances  before  settling 
to  the  bottom.  Hence  the  muds  and  clays  are  distributed  over  wider 
areas  than  the  gravels  and  sands,  and  deposits  of  them  indicate 
quieter  and,  usually,  but  not  always,  deeper  waters  than  the  con- 
glomerates and  sandstones. 

Clay  is  found  in  very  different  conditions  of  purity.  Kaolin, 
or  porcelain  clay,  is  nearly  pure,  while  Potter's  and  Brick  Clay 
contain  finely  divided  quartz,  and  the  latter  in  addition,  lime,  mag- 
nesia, iron,  and  alkalies.  Clays  with  considerable  percentage  of 
iron  burn  red  on  firing. 

Fire-clay  is  a  nearly  pure  mixture  of  sand  and  clay,  with  only 
traces  of  iron,  magnesia,  or  lime,  and  therefore  burns  to  white 
or  buff-coloured  bricks,  which  will  resist  very  high  temperatures. 
Fire-clays  occur  frequently  beneath  coal  seams,  representing  the 
ancient  soil  in  which  the  coal  plants  grew.  Such  ancient  fire-clays 
are  often  hard  rocks,  and  must  be  ground  up  before  using. 

Mudstone  is  a  rock  which  is  composed  of  solidified  clay  or  fels- 
pathic mud,  or  a  mixture  of  the  two,  and  which  crumbles  rapidly 
into  mud  when  exposed  to  the  action  of  the  weather. 

Shale  is  a  finely  stratified  or  laminated  clay  rock,  formed  from 
the  solidification  of  mud  and  silt.  In  some  of  the  paper  shales 
there  are  as  many  as  thirty  or  forty  laminae  to  the  inch,  each  repre- 
senting a  separate  stage  of  deposition.  Shales  ordinarily  contain 
more  or  less  sand,  and  as  this  increases  in  quantity,  they  shade 
gradually  into  arenaceous  shales  and  argillaceous  sandstones,  or 
by  the  increase  of  calcareous  matter  into  limestones.  Bituminous 
shale  is  coloured  very  dark  or  black  by  the  carbonaceous  matter 
with  which  it  is  saturated.  When  distilled,  the  bituminous  shales 
yield  hydrocarbons,  and  are  of  considerable  economic  importance; 
the  carbonaceous  matter  may  be  of  either  animal  or  vegetable 
origin.  Shales  of  this  class  grade  into  coals. 

Marl  is  clay  containing  carbonate  of  lime,  which  rapidly  crum- 
bles on  exposure  to  the  weather. 


CHEMICAL  PRECIPITATES  307 


2.    CHEMICAL  PRECIPITATES 

Rocks  which  have  been  principally  or  entirely  formed  by  chemi- 
cal processes  are,  for  the  most  part,  of  locally  restricted  extent,  and 
are  not  at  all  comparable  to  the  great  masses  of  mechanical  and 
organic  sediments.  This  arises  from  the  fact  that  the  chemical 
processes  occur  in  a  conspicuous  way  only  around  the  mouths  of 
certain  classes  of  springs  (p.  191),  and  in  closed  bodies  of  water 
without  outlet  and  subject  to  evaporation. 

The  chemical  precipitates  may  be  classed  under  the  following 
heads:  a,  Precipitates  of  the  alkalies  and  alkaline  earths;  b.  sili- 
ceous precipitates;  c,  ferruginous  precipitates. 

a.   Precipitates  of  the  Alkalies  and  Alkaline  Earths 

Calcareous  Tufa  or  Sinter,  Travertine,  Stalactite,  Onyx  Marbles, 

are  all  forms  of  carbonate  of  lime  deposited  from  solution,  either 
around  the  vents  of  springs,  or  by  percolating  waters  in  limestone 
caverns,  or  in  lakes  and  streams.  These  deposits  are  made  of  cal- 
cite  (or  aragonite),  are  often  very  pure,  and  usually  white,  and  more 
or  less  translucent,  though  they  may  be  stained  by  other  substances 
dissolved  with  the  lime.  In  structure  they  are  banded  and  show 
rings  of  growth,  which  distinguishes  them  from  the  organic  lime- 
stones. The  so-called  "  Mexican  onyx  "  or  "  onyx  marble  "  is  a 
beautifuly  banded  travertine  derived  from  ancient  spring  deposits. 
Oolite  is  a  limestone  composed  of  minute  spherules  of  carbonate 
of  lime,  cemented  into  a  more  or  less  compact  mass,  somewhat 
resembling  fish-roe,  whence  is  derived  the  name,  meaning  "  egg 
rock."  The  spherules  are  made  up  of  concentric  layers  of  car- 
bonate of  lime,  deposited  from  solution  around  some  nucleus,  it 
may  be  a  particle  of  sand  or  dust,  or  a  calcareous  fragment.  The 
beach  rock  of  a  coral  reef  (p.  264)  is  made  in  this  fashion,  and 
calcareous  sinter  often  has  a  similar  structure.  When  the  spheres 
are  larger,  resembling  peas  in  size  and  shape,  the  rock  is  called 
pisolite. 


308 


THE  SEDIMENTARY   ROCKS 


Gypsum  (CaSO4.2H2O)  is  deposited  from  solution  in  salt  lakes 
0,nd  lagoons,  in  which  evaporation  balances  the  influx  of  water 
(p.  224).  When  pure,  gypsum  is  white,  but  it  is  often  coloured 
gray,  brown,  or  red,  by  iron  stains,  and  it  may  even  be  black. 
It  forms  compact,  crystalline,  or  fibrous  beds,  looking  like  lime- 
stone, but  much  softer  and  not  effervescing  with  acid;  portions 
of  the  beds  may  consist  of  transparent  selenite  crystals.  The 
mineral  sometimes  occurs  in  the  form  of  anhydrite  (CaSO4),  but 
it  is  not  known  under  what  conditions  the  anhydrous  sulphate 
has  been  deposited  from  solution. 


FIG.  147. —  Piece  of  banded  travertine  polished,  natural  size 

Rock  Salt  (NaCl)  is  precipitated  by  evaporation  from  the  dense 
brine  of  salt  lakes  and  lagoons,  following  the  deposition  of  gypsum, 
which  explains  the  very  common  association  of  the  two  rocks  in 
successive  beds.  The  salt  may  be  present  only  as  an  ingredient  of 
shale  (saline  shale),  or  may  form  thin  layers,  indicating  brief  periods 
of  deposition,  followed  by  freshening  of  the  water.  Again,  it  may 
occur  in  enormously  thick  masses,  the  result  of  long-continued 


ORGANIC  ACCUMULATIONS  309 

precipitation.  One  such  mass,  near  Berlin,  exceeds  4000  feet  in 
thickness.  Rock  salt  is  often  very  pure,  and  then  it  is  transparent 
and  colourless;  but  it  is  frequently  stained  by  iron,  or  mingled 
with  dust  blown  into  the  lake  or  lagoon  which  deposited  the  salt, 
or  mixed  with  clay  and  other  mechanical  sediments. 

b.   Siliceous  Precipitates 

These  are  much  less  common  and  extensive  than  the  calcareous, 
and  are  formed  under  exceptional  conditions. 

Geyserite,  or  Siliceous  Sinter,  is  deposited  in  dense  and  hard 
masses  around  the  mouths  of  geysers,  partly  by  the  evaporation 
of  the  water  which  holds  the  silica  in  solution,  and  partly  by  the 
action  of  Algae  (see  p.  192).  Large  terraces  of  this  rock  have 
been  built  up  by  the  geysers  of  the  Yellowstone  Park.  Geyserite 
also  occurs  as  an  uncompacted  white  powder. 

Chert  (Flint  or  Hornstone)  forms  exceedingly  dense  and  fine- 
grained masses,  which  the  microscope  shows  to  be  made  up  of 
very  minute  grains  of  chalcedony  mixed  with  more  or  less  amor- 
phous silica  and  crystals  of  quartz.  The  mode  of  origin  of  these 
masses  is  not  at  all  well  understood,  but  is  believed  to  be  by 
precipitation  from  sea-water. 

c.   Ferruginous  Precipitates 

Bog  and  Lake  Iron  Ore  results  from  the  oxidation  and  conse- 
quent precipitation  of  iron  circulating  in  solution  in  the  soluble 
ferrous  condition.  The  deposits  often  have  a  concretionary  struc- 
ture, not  uncommonly  becoming  oolitic,  and  consist  of  impure 
limonite,  sometimes  mingled  with  siderite. 

3.     ORGANIC   ACCUMULATIONS 

The  organically  formed  rocks  are  those  whose  materials  were 
accumulated  by  living  beings,  on  the  death  of  which  more  or  less 
of  their  substance  was  preserved,  added  to  by  successive  genera- 


3IO  THE  SEDIMENTARY   ROCKS 

tions,  and  finally  compacted  into  rock.  In  preceding  chapters 
we  have  read  of  these  processes  as  going  on  at  the  present  time, 
in  peat  bogs,  in  the  coral  reefs,  shell-banks,  limestone  plateaus, 
and  organic  oozes  of  the  ocean.  Similar  processes  have  been 
at  work  in  all  the  recorded  ages  of  the  earth's  history  since  the 
first  appearance  of  living  things,  and  very  extensive  rocks  have 
thus  been  built  into  the  solid  crust  of  the  globe.  An  exact 
classification  would  require  us  to  place  certain  of  these  rocks 
among  the  mechanical  sediments,  because  the  actual  work  of 
accumulation  was  performed  by  mechanical  agencies,  such  as 
waves  and  currents.  But  it  will  be  more  convenient  to  examine 
together  all  those  rocks  which  are  principally  made  up  of  organic 
materials,  especially  as  it  is  not  always  easy  to  distinguish  the 
results  <"f  one  mode  of  formation  from  those  of  the  other. 

a.  Calcareous  Accumulations 

Limestone  is  a  very  abundant,  important,  and  widely  distributed 
rock,  the  commonest  of  the  organic  accumulations.  It  is  com- 
posed of  carbonate  of  lime  in  varying  degrees  of  purity,  hardness, 
fineness  of  grain,  and  crystalline  texture.  Sand  or  clay  is  fre- 
quently present  as  an  impurity,  and  by  an  increase  in  these  mate- 
rials, the  limestones  pass  gradually  into  sandstones  and  shales. 
In  some  varieties  of  limestone  the  organic  nature  of  the  rock  is 
most  obvious,  shells,  corals,  crinoid  stems,  and  the  like  being  con- 
spicuously shown,  especially  on  weathered  surfaces.  In  other 
kinds  the  microscope  is  required  to  make  this  organic  nature 
clear;  while  in  others,  again,  the  calcareous  materials  have  been  so 
ground  up  by  the  action  of  the  waves,  or  so  completely  modified 
by  crystallization,  that  all  traces  of  organic  structure  have  disap- 
peared. The  example  of  the  reef  rock  now  forming  in  many  coral 
reefs  (p.  264)  is  a  warning  that  the  absence  of  even  microscopic 
structure  in  a  limestone  cannot  be  relied  upon  as  a  proof  that  the 
rock  is  not  of  organic  origin. 

The  great  limestones  are  almost  entirely  of  marine  origin,  though 


CALCAREOUS  ACCUMULATIONS 


quite  extensive  fresh-water  limestones  are  known.  The  chemically 
formed  ones  are  never  very  widely  extended,  though  they  may 
form  quite  thick  masses.  As  a  rule,  the  limestones  are  deposited 
in  deeper  water  than  the  sandstones  and  shales,  but  not  necessarily 
so,  freedom  from  large  amounts  of  terrigenous  sediments  being 
more  important  than  depth  of  water.  This  is  shown  by  the  great 
calcareous  banks  of  the  Gulf  of  Mexico  and  •  the  Caribbean  Sea 
(p.  259),  and  coral  reefs  are  always  formed  in  water  of  less  than 
twenty  fathoms  in  depth. 

The  classification  of  the  limestones  is  very  difficult,  and  cannot 
be  readily  made  on  any  single  principle;  mode  of  formation, 
purity,  texture,  and  nature  of  organic  material,  all  being  employed 
for  the  purpose. 

Shell  Marl  is  an  incoherent  and  crumbling  rock,  formed  prin- 
cipally at  the  bottom  of  fresh-water  lakes  and  ponds,  by  the 
accumulation  of  shells;  it  frequently  occurs  beneath  peat  bogs, 
and  is  an  indication  that  the  bog  arose  from  the  choking  up  of  a 
lake  by  vegetable  growth.  When  the  shells  are  cemented  into 
a  hard  rock  they  form  &  fresh-water  limestone. 

Chalk  is  a  soft  limestone  of  friable,  earthy  texture,  and  fre- 
quently very  pure;  in  colour  it  may  bs  snowy  white,  pale  gray,  or 
buff.  The  microscope  reveals 
the  fact  that  chalk  is  principally 
composed  of  the  shells  of  Fora- 
minifera,  and  closely  resembles 
the  foraminiferal  oozes  forming 
to-day  at  the  bottom  of  the  sea 
(p.  270).  A  chalky  deposit  may, 
however,  be  formed  from  the 
debris  of  corals  ground  up  by 
the  waves. 

The  ordinary  massive  marine 
limestones  are  named  from  the 
character  of  the  organic  mate- 
rial which  predominates  in  them.  Thus,  we  have  coral  lime' 


FIG.  148.  —  Chalk  from  Kansas  X  45. 
(Drawn  from  a  photograph  by  the 
Geological  Survey  of  Iowa) 


312  THE  SEDIMENTARY   ROCKS 

stone,  foraminiferal  limestone,  made  up  of  the  shells  of  very  large 
extinct  forms  of  the  Foraminifera  (Fusulina,  Nummulites,  Orbito- 
lites,  etc.),  crinoidal  limestone,  shell  limestone,  and  the  like. 

Though  much  the  larger  part  of  the  limestones  is  of  animal  origin, 
yet  certain  seaweeds  contribute  extensively  to  the  formation  of  these 
rocks,  and  there  is  much  reason  to  believe  that  chemical  precipita- 
tion is  of  greater  or  less  importance  in  nearly  all  varieties  of  the 
rock.  Many  of  the  massive  limestones,  which  show  little  or  no 
sign  of  disturbance,  are  quite  completely  crystalline,  due  to  the 
action  of  water  upon  them.  Calcite  recrystallizes  with  the  greatest 
ease,  and  the  interior  of  coral-masses,  which  are  still  alive  on  the 
outside,  may  be  so  crystallized  as  to  obliterate  all  traces  of  their 
original  structure. 

Dolomite,  or  Magnesian  Limestone,  is  a  compact,  granular  rock 
of  white,  gray,  or  yellow  colour,  composed  of  the  carbonates  of 
lime  and  magnesia.  Nearly  all  limestones  contain  some  carbonate 
of  magnesia,  but  the  name  dolomite  is  given  only  to  those  with  a 
considerable  percentage  of  that  substance  (5  to  20%).  How  far 
this  rock  is  made  up  of  the  mineral  dolomite,  and  how  far  it  is 
merely  a  mixture  of  the  two  carbonates,  is  uncertain,  as  is  also  the 
way  in  which  the  rock  was  formed.  Dolomite  contains  a  much 
larger  proportion  of  magnesia  than  the  shells  or  tests  of  any  known 
animals,  and  this  ingredient  must  therefore  have  been  added  after 
the  accumulation  of  the  calcareous  organisms.  Opinions  differ  as 
to  just  how  this  has  been  accomplished,  but  probably  the  magnesia 
has  been  derived  from  the  strong  brine  of  lagoons  and  salt  lakes. 
The  frequent  association  of  dolomite  with  gypsum  gives  additional 
probability  to  this  view.  A  similar  process  has  been  observed  in 
the  lagoons  of  coral  reefs  at  the  present  time  (p.  266),  and  it  has 
been  shown  that  dolomitization  takes  place  much  more  readily 
when  the  CaCO3  is  in  the  form  of  aragonite,  as  is  the  case  in  the 
shells  and  tests  of  many  marine  animals. 

Green  Sand  is  not  strictly  a  calcareous  deposit,  but  has  a  natural 
connection  with  that  series  of  rocks.  Green  sand  is  seen  by  the 
microscope  to  be  largely  composed  of  internal  casts  of  foraminiferal 


SILICEOUS  ACCUMULATIONS  313 

shells  in  the  mineral  glauconite  (p.  19).  The  dead  foraminiferal 
shells  which  lie  upon  certain  areas  of  the  ocean  floor  are  gradually 
filled  up  with  glauconite,  and  then  the  shells  are  dissolved,  leaving 
the  grains  of  the  mineral,  which  retain  the  form  into  which  they 
were  moulded.  This  process  is  still  going  on,  and  has  been  ob- 
served at  several  points  (p.  269).  Glauconite  also  forms  on  the 
sea-floor  in  nodules,  quite  independently  of  foraminiferal  shells. 


b.   Siliceous  Accumulations 

The  siliceous  deposits  of  organic  origin  are  very  much  less  com- 
mon and  less  extensively  developed  than  the  calcareous,  because 
of  the  relatively  small  amount  of  silica  which  is  in  solution  in 
ordinary  waters,  and  of  the  comparatively  few  organisms  which 
secrete  shells  or  tests  of  it.  Nevertheless,  these  beds  are  of  suf- 
ficient importance  to  require  mention. 

Infusorial  Earth  is  a  fine  white  power  composed  of  the  micro- 
scopic tests,  or  frustules  of  the  minute  plants  called  diatoms.  The 
fineness  and  excessive  hardness  of  the  particles  make  this  an 
excellent  polishing  powder.  Beds  of  this  earth  occur  in  both 
marine  and  fresh-water  deposits.  At  Richmond,  Virginia,  is  a 
celebrated  deposit  of  this  kind. 

Siliceous  Oozes  are  exceedingly  rare  as  rocks  of  the  land;  they 
consist  of  the  tests  of  Radiolaria,  such  as  are  now  accumulating 
in  the  deeper  parts  of  the  ocean  (p.  172).  The  only  land  areas 
in  which  such  deposits  have  been  found  occur  in  certain  of  the 
West  Indian  Islands  (Barbadoes,  Cuba,  and  others). 

Flint  or  Chert  occurs  in  nodules  or  beds,  especially  in  marine 
limestones,  though  it  is  also  found  among  the  sands  and  clays  of 
certain  fresh-water  formations,  as  in  Wyoming.  Microscopic 
examination  sometimes  reveals  the  presence  of  sponge  spicules 
and  other  siliceous  organisms,  but  this  is  by  no  means  always  the 
case.  As  we  have  seen,  the  structureless  cherts  are  believed  to 
have  been  formed  by  chemical  precipitation. 


314  THE  SEDIMENTARY   ROCKS 


c.    Ferruginous  Accumulations 

The  iron  deposits  which  can  be  referred  to  the  activity  of  living 
creatures  are  of  small  extent  and  importance,  but  certain  of  the 
bog-iron  ores  are  believed  to  be  due  to  the  agency  of  diatoms, 
Bacteria  and  Algae,  which  extract  the  iron  from  its  dissolved  state. 


d.   Carbonaceous  Accumulations 

The  rocks  of  this  group  are  formed,  almost  entirely,  by  the 
accumulation  of  vegetable  matter  and  its  progressive,  though  in- 
complete, decay  under  water.  This  decay  is  cf  such  a  nature  that 
the  gaseous  constituents  diminish,  while  the  carbon  is  removed 
much  less  rapidly,  consequently  the  proportion  of  the  latter  sub- 
stance steadily  rises.  All  the  varieties  of  carbonaceous  rocks  pass 
into  one  another  so  gradually,  that  the  distinction  between  them 
seems  somewhat  arbitrary.  From  fresh  and  unchanged  vegetable 
matter  to  the  hardest  anthracite  there  is  an  unbroken  series  of 
transitions. 

Peat  is  a  partially  carbonized  mass  of  vegetable  matter,  brown 
or  black  in  colour  and  showing  its  vegetable  nature  on  the  most 
superficial  examination,  though  the  parts  which  have  been  longest 
macerated  are  often  as  homogeneous  and  as  fine  grained  as  clay, 
and  reveal  their  true  nature  only  under  the  microscope. 

Lignite  or  Brown  Coal  is  a  brown  or  black  mass  of  mineralized 
and  compressed  peat,  and  though  still  plainly  showing  its  vegetable 
nature,  it  does  so  less  obviously  than  peat,  being  more  carbonized. 
It  is  an  inferior  fuel,  though  often  very  valuable  in  regions  where 
other  fuel  is  scarce  or  entirely  wanting. 

Coal  is  a  compact,  dark  brown  or  black  rock,  in  which  vegetable 
structure  cannot  be  detected  by  the  unassisted  eye,  though  micro- 
scopic inspection  seldom  fails  to  reveal  it.  Coal  is  found  in  beds  or 
strata,  interstratified  with  shales,  sandstones,  and,  less  commonly, 
limestones.  The  different  kinds  of  coal  vary  much  in  hardness  and 


CARBONACEOUS  ACCUMULATIONS  31$ 

chemical  composition,  but  they  are  all  connected  by  intermediate 
gradations.  Bituminous  Coal  has  (neglecting  the  ash)  70  to  75  % 
of  carbon  and  25  to  30  %  of  volatile  matters,  chiefly  hydrocarbons, 
which  are  driven  off  on  destructive  distillation.  Under  the  term 
bituminous  are  included  many  varieties  of  coal,  which  differ 
much  in  their  behaviour  and  in  their  value  for  different  purposes. 
Anthracite  is  a  hard,  lustrous  coal,  that  is  nearly  pure  carbon 
(aside  from  the  ash)  and  has  little  or  no  volatile  matter;  it  burns 
without  smoke  or  flame  and  gives  an  intense  heat.  Semibitumi- 
nous  or  Steam  Coal  is  intermediate  in  character  and  composition 
between  the  bituminous  and  anthracite  varieties. 

Cannel  Coal  does  not  belong  in  the  series  of  coals  above  enu- 
merated, but  forms  a  very  distinct  variety.  It  occurs  in  lenticular 
patches,  not  in  beds,  and  is  very  compact,  though  not  very  hard  or 
heavy.  .This  coal  has  from  70  to  85  %  of  carbon  and  the  high  pro- 
portion of  6  to  7  %  of  hydrogen,  giving  off  large  quantities  of  gas 
when  heated,  and  burning  with  a  white,  candle-like  flame.  Even 
with  the  microscope,  it  is  difficult  to  detect  the  vegetable  structure  of 
cannel,  so  thoroughly  has  the  material  been  macerated.  Evidently, 
cannel  is  an  exceptional  coal  and  has  been  formed  in  a  somewhat 
peculiar  way.  While  the  ordinary  coals  evidently  represent  ancient 
peat  bogs,  which  by  subsidence  allowed  the  sea,  or  other  body  of 
water,  to  overflow  them  and  were  thus  sealed  up  and  buried  under 
sedimentary  deposits,  cannel  was  formed  in  pools  of  clear  water, 
in  which  vegetable  matter  was  accumulated  and  very  completely 
disintegrated.  This  is  shown  not  only  by  the  shape  of  the  coal 
patches,  but  also  by  the  fossil  fish  not  infrequently  found  in  cannel. 

The  following  table  (from  Kemp)  displays  the  composition  of 
the  typical  varieties  of  coal,  not  including  the  ash:  — 

C.  H.  O.  N. 

Wood 50  6  43  i 

Peat      .         ...         .         .         .         .59  6  33  2 

Lignite  .         .        .        .        .         .69  5.5  25  0.8 

Bituminous  Coal 82  5  13  0.8 

Anthracite  95  2.5  2.5  trace 


316  THE  SEDIMENTARY   ROCKS 

The  Hydrocarbons.  —  The  great  economic  importance  of  these 
substances  requires  that  brief  mention  of  them  be  made,  though 
they  can  hardly  be  considered  abundant  as  rocks.  The  natural 
hydrocarbons  of  the  earth's  crust  belong  principally  to  the  methane 
series,  with  the  general  formula  CwH2n+2-  The  most  abundant 
are  marsh  gas  (CH4),  petroleum,  a  mixture  of  several  hydrocarbons, 
which  are  liquid  at  ordinary  temperatures,  and  asphalt,  which  is 
solid  or  extremely  viscous,  and  results  from  the  oxidation  of 
hydrocarbons.  The  hydrocarbons  impregnate  porous  or  shattered 
rocks,  which  they  have  invaded  from  below,  and  are  frequently 
retained  under  great  pressure  by  overlying  impervious  beds.  Natu- 
ral gas  and  petroleum  tend  to  collect  in  the  upward  arches  (anti- 
clines) of  folded  beds,  and  when  these  reservoirs  are  tapped  by  the 
drill,  the  oil  and  gas  rise  in  spouting  wells  which  may  continue  to 
flow  for  many  years. 

While  certain  eminent  chemists  have  maintained  the  inorganic 
origin  of  the  hydrocarbons,  there  is  no  evidence  that  they  actually 
were  formed  in  this  way,  and  nearly  all  geologists  are  agreed  that 
they  have  been  derived  from  the  fatty  and  oily  parts  of  organic 
accumulations,  both  animal  and  vegetable,  at  high  temperatures 
and  pressures.  That  such  a  mode  of  generation  is  at  least  possible 
has  been  demonstrated  experimentally,  and  the  geological  mode 
of  occurrence  of  these  hydrocarbons  renders  the  hypothesis  of  their 
derivation  from  organic  substances  extremely  probable.  Petro- 
leum is  found  in  rocks  of  a  very  wide  range  in  geological  time,  and 
the  various  oil-fields  of  the  United  States  are  of  very  different  geo- 
logical dates. 

Asphalt  is  found  in  beds  interstratified  with  ordinary  sediments 
or  in  cavities  and  fissures  of  the  rocks,  or  impregnating  porous  lime- 
stones and  sandstones. 


II.    ^OLIAN   ROCKS 

The  rocks  formed  on  dry  land  form  less  of  the  earth's  crust 
than  do  the  aqueous  rocks,  but  they  have  a  special  importance 


AEOLIAN   ROCKS  317 

because  of  the  hints  which  they  often  give  as  to  the  physical  geog- 
raphy of  the  place  and  time  of  their  formation. 

Blown  Sand  is  heaped  up  by  the  wind  into  dunes,  and  displays 
an  irregular  kind  of  stratification.  The  sand-grains,  abraded  by 
their  contact  with  hard  substances,  are  smaller,  more  rounded, 
and  kss  angular  than  the  grains  of  river  or  even  beach  sands. 

Drift- sand  Rock  (also  called  seolian  rock)  is  the  consolidated 
sand  of  dunes.  If  the  sand  contains  any  considerable  quantity 
of  calcareous  matter,  the  solution  and  redeposition  of  this  by  per- 
colating waters  binds  the  sand  into  quite  a  firm  rock.  The  cal- 
careous sands  of  Bermuda  give  an  often  quoted  example  of  this. 

Talus  gathers  at  the  foot  of  cliffs  in  large  masses,  and  in  many 
deserts  it  forms  great  sheets. 

Breccia  is  a  rock  composed  of  angular  fragments  cemented  by 
deposition  of  material,  commonly  CaCO3,  in  the  interstices;  the 
fragments  may  be  any  kind  of  rock.  Breccia  is  also  found  in  zones 
of  fracturing  and  shattering  of  the  rocks  along  fault-planes, 
and  is  then  called  a  fault-breccia. 

Soil.  —  In  Chapter  IV  it  was  shown  that  soil  is  mainly  the 
residual  product  left  by  the  atmospheric  decay  of  rocks,  and  that 
its  surface  layers  contain  more  or  less  organic  matter  and  are  filled 
with  the  roots  of  plants.  Soils  may  be  buried  under  aqueous 
deposits  by  floods,  or  after  subsidence  marine  deposits  may  be 
built  up  upon  the  soils,  which  are  then  interstratified  with  marine 
rocks.  Ancient  soils  have  been  frequently  preserved  in  this  manner, 
filled  with  fossil  roots,  and  sometimes  with  the  stumps  of  trees  still 
standing  upon  them. 

Loess.  —  A  very  fine  grained  terrestrial  deposit,  usually  unstrati- 
fied  and  with  a  vertical  cleavage.  It  is  quite  firm  and  may  even 
become  hard  and  stony  (see  p.  190). 

In  logical  order,  the  Metamorphic  Rocks  would  next  come  up 
for  consideration;  but  since  we  have,  as  yet,  learned  nothing  of 
the  processes  by  which  these  rocks  are  formed,  it  will  be  best  to 
defer  the  study  of  this  class  to  a  future  chapter,  when  the  rocks 
and  their  mode  of  formation  will  be  examined  together. 


CHAPTER  XII 
THE  STRUCTURE  OF  ROCK  MASSES— STRATIFIED  ROCKS 

IN  the  preceding  chapter  we  have  studied  the  rocks  which  make 
up  the  crust  of  the  earth,  so  far  as  that  is  accessible  to  observation. 
It  remains  for  us  to  inquire  how  these  rocks  are  arranged  on  a 
large  scale,  and  to  what  displacements  and  dislocations  they  have 
been  subjected  since  the  time  of  their  formation.  Examined  with 
reference  to  the  simplest  and  broadest  facts  of  structure,  we  find 
that  rock  masses  fall  into  two  categories:  (i)  Stratified  Rocks,  and 
(2)  Unstratified  or  Massive  Rocks.  A  very  brief  examination  will 
show  us  that  these  two  categories  correspond  respectively  to  the 
sedimentary  and  igneous  divisions  of  the  classification  according  to 
mode  of  origin,  neglecting,  for  the  present,  the  metamorphic  class. 

We  shall  begin  our  study  of  rock  masses  with  the  stratified 
series,  because  their  structure  and  mode  of  occurrence  are,  on  the 
whole,  the  simplest  and  most  intelligible,  and  tell  their  own  story. 
The  unstratified  series,  on  the  other  hand,  can  be  understood  only 
by  determining  their  relation  to  the  former. 

The  stratified  rocks  form  more  than  nine-tenths  of  the  earth's 
surface,  and  if  the  entire  series  of  them  were  present  at  any  one 
place,  they  would  have  a  maximum  thickness  of  about  thirty  miles, 
but  no  such  place  is  known.  The  regions  of  greatest  sedimentary 
accumulation  are  the  shallower  parts  of  the  oceans,  while  those 
regions  which  have  remained  as  dry  land,  through  long  ages,  may 
not  only  have  had  no  important  additions  to  their  surfaces,  but 
have  lost  immense  thicknesses  of  rock  through  denudation.  The 
great  oceanic  abysses  are  also  areas  of  excessively  slow  sedimen- 
tation, and  thus  the  thickness  of  the  stratified  rocks  varies  much 
from  point  to  point,  a  variation  which  has  been  increased  by  the 


STRATIFICATION  3 1 9 

irregularities  of  upheaval  and  depression  and  of  different  rates  of 
denudation.  Even  with  this  irregularity  in  the  formation  and  re- 
moval of  the  stratified  rocks,  it  would  be  exceedingly  difficult,  if 
not  impossible,  to  investigate  the  entire  series  of  them,  if  they  had 
all  retained  the  original  horizontal  positions  in  which  they  were 
first  laid  down.  In  many  places,  however,  the  rocks  have  been 
steeply  tilted  and  then  truncated  by  erosion,  so  that  their  edges 
form  the  surface  of  the  ground,  and  thus  great  thicknesses  of  them 
may  be  examined  without  descending  below  the  surface. 

Stratification,  or  division  into  layers,  is  the  most  persistent  and 
conspicuous  characteristic  of  the  sedimentary  rocks.  In  studying 
the  sedimentary  deposits  of  the  present  day  (Chapter  VII)  we 
learned  that  by  the  sorting  power  of  water  and  wind,  heterogeneous 
material  is  arranged  into  more  or  less  homogeneous  beds,  separated 
from  one  another  by  distinct  planes  of  division,  and  the  same  thing 
is  true  of  the  sedimentary  rocks  of  all  ages.  This  division  into 
more  or  less  parallel  layers  is  called  stratification,  and  the  extent 
to  which  the  division  is  carried  varies  according  to  circumstances. 

A  single  member,  or  bed,  of  a  stratified  rock,  whether  thick  or 
thin,  is  called  a  layer,  though  for  purposes  of  distinction,  exces- 
sively thin  layers  are  called  lamince.  Each  layer  or  lamina  repre- 
sents an  uninterrupted  deposition  of  material,  while  the  divisions 
between  them,  or  bedding  planes,  are  due  to  longer  or  shorter 
pauses  in  the  process,  or  to  a  change,  if  only  in  a  film,  of  the  material 
deposited.  A  stratum  is'the  collection  of  layers  of  the  same  mineral 
substance,  which  occur  together  and  may  consist  of  one  or  many 
layers.  However,  the  term  is  not  always  employed  in  just  this 
sense  and  often  means  the  same  as  layer.  The  passage  from  one 
stratum  to  another  is  generally  abrupt  and  indicates  a  change  in 
the  circumstances  of  deposition,  either  in  the  depth  of  water,  or 
in  the  character  of  the  material  brought  to  a  given  spot,  or  m 
both.  So  long  as  conditions  remain  the  same,  the  same  kind  of 
material  will  accumulate  over  a  given  area,  and  thus  immense  thick- 
nesses of  similar  material  may  be  formed.  To  keep  up  such 
equality  of  conditions,  the  depth  of  water  must  remain  constant, 


320 


THE   STRUCTURE  OF   ROCK  MASSES 


and  hence  the  bottom  must  subside  as  rapidly  as  the  sediment 

accumulates. 

Usually,  a  section  of  thick  rock  masses  shows  continual  change 

of  material  at  different  levels.     Figure  149  is  a  section  of  the  rocks 

in  Beaver  County,  Pennsylvania,  in  which  several  different  kinds  of 

beds  register  the  changes  in  the  physical  geography  of  that  area. 

At  the  bottom  of  the  section  is  a  coal  seam  (No.  i),  the  con- 
solidated and  carbonized  vegetable  matter 
which  accumulated  in  an  ancient  fresh- 
water swamp.  Next  came  a  subsidence  of 
the  swamp,  allowing  water  to  flow  in,  in 
which  were  laid  down  mixed  sands  and 
gravels  (No.  2).  The  accumulations  even- 
tually shoaled  the  water  and  enabled  a 
second  peat  swamp  to  establish  itself;  this 
is  registered  in  the  second  coal  bed  (No. 
3),  the  thinness  of  which  indicates  that  the 
second  swamp  did  not  last  so  long  as  the 
first.  Renewed  subsidence  again  flooded 
the  bog,  as  is  shown  by  the  stratum  of 
shale  (No.  4)  which  overlies  the  second 
coal  bed.  Next,  the  water  was  shoaled  by 
an  upheaval,  and  argillaceous  sands  were 

FIG.  149.— Section  in  coal  laid  down,  which  now  form  the  flaggy  sand- 
measures  of  western  stones  (No.  5)  overlying  the  shale.  The 
Pennsylvania.  (White)  twenty_fiye  feet  of  sandstonej  aided  by 

continued  slow  rise,  silted  up  the  water  and  allowed  a  third 
peat  bog  to  grow,  the  result  of  which  is  the  third  coal  seam  (No. 
6),  while  a  repetition  of  the  subsidence  once  more  brought  in 
the  water,  in  which  were  laid  down  the  seventy  feet  of  gravel  at  the 
top  of  the  section.  In  this  fashion  the  succession  of  strata  records 
the  changes  which  were  in  progress  while  those  strata  were  forming. 
Whether  the  beds,  other  than  the  coal  seams,  were  laid  down  in 
fresh  water,  or  in  salt,  by  a  lake,  a  flooded  river,  or  the  sea,  may  be 
determined  from  the  fossils  contained  in  those  beds.  In  the 


STRATI  FICATION  J2 1 

absence  of  fossils  it  is  not  always  possible  to  make  the  dis- 
tinction. 

Somewhat  similar  changes  in  the  strata  may  be  occasioned  by 
the  steady  lowering  of  a  land  surface  through  denudation.  This 
diminishes  the  velocity  of  the  streams,  which,  in  its  turn,  changes 
the  character  of  the  materials  which  the  rivers  bring  to  the  sea. 

We  have  no  trustworthy  means  of  judging  how  long  a  time  was 
required  for  the  formation  of  any  given  stratum  or  series  of  strata, 
but  it  is  clear  that  different  kinds  of  beds  accumulate  at  very 
different  rates.  The  coarser  materials,  like  conglomerates  and 
sandstones,  were  piled  up  much  more  rapidly  than  the  shales  and 
limestones;  so  that  equal  thicknesses  of  different  kinds  of  strata 
imply  great  differences  in  the  time  required  to  form  them.  Com- 
paring like  strata  with  like,  we  may  say  that  the  thickness  of  a 
group  of  rocks  is  a  rough  measure  of  the  time  involved  in  their 
formation,  and  that  very  thick  masses  imply  a  very  long  lapse  of 
time,  but  we  cannot  infer  the  number  of  years  or  centuries  or 
millennia  required. 

Geological  chronology  can  be  relative  only.  Such  a  relative 
chronology  is  given  in  the  section  that  we  have  examined  by  the 
order  of  succession  of  the  beds.  Obviously  the  lowest  stratum  is 
the  oldest  and  the  one  at  the  top  the  newest.  This  may  be  put  as 
a  general  principle,  that,  unless  strata  have  lost  their  original  posi- 
tion through  disturbance  or  dislocation,  their  order  of  superposi- 
tion is  their  order  of  relative  age.  It  is  for  this  reason  that  in 
geological'  sections  the  strata  are  numbered  and  read  from  below 
upward. 

Change  in  the  character  of  the  strata  takes  place  not  only  verti- 
cally, but  also  horizontally,  since  no  stratum  is  universal,  even  for 
a  single  continent.  Our  study  of  the  processes  of  sedimentation 
which  are  now  at  work,  showed  us  that  the  character  of  the  bottom 
in  the  ocean  or  in  lakes  is  subject  to  frequent  changes,  varying 
with  the  depth  of  water  and  other  factors.  The  same  is  true  of 
the  ancient  sea  and  lake  bottoms,  now  represented  by  the  strati- 
fied rocks  of  the  land.  Strata  may  persist  with  great  evenness 


322 


THE  STRUCTURE  OF  ROCK  MASSES 


and  uniform  thickness  over  vast  areas,  and  in  such  cases  the  bed- 
ding planes  remain  sensibly  parallel.  But  sooner  or  later,  the  beds, 
whenever  they  can  be  traced  far  enough,  are  found  to  thin  out  to 
edges  and  to  dovetail  in  with  beds  of  a  different  character.  When 
the  strata  are  of  constant  thickness  for  considerable  distances,  and 
the  bedding  planes  remain  parallel,  the  stratification  is  said  to  be 
regular.  In  many  cases  these  changes  take  place  rapidly  from 
point  to  point,  and  then  the  strata  are  plainly  of  lenticular  shape, 
thickest  in  the  middle,  thinning  quickly  to  the  edges.  Here  the 

_  ^___     bedding  planes    are   distinctly   not 

parallel,    and   the   stratification    is 
irregular. 

An  example  of  rapid  horizontal 
changes  is  given  in  the  two  accom- 
panying parallel  sections  (Fig.  150), 
taken  through  the  same  beds,  only 
twenty  feet  apart.  In  these  sec- 
tions the  differences  of  thickness  of 
the  coal  seams  and  of  the  sands 
and  clays  which  separate  them  are 
very  striking. 

The  finer  details  of  structure  of 
the  stratified  rocks,  such  as  cross- 
bedding,  ripple  and  rill-marks,  rain- 
prints,  tracks  of  animals,  and  the 
like,  likewise  afford  valuable  testi- 
mony as  to  the  circumstances  under 
which  the  rocks  were  laid  down. 

Concretions,  or  Nodules,  are  de- 
veloped after  the  formation  of 


FlG.  150.  —  Parallel  sections  near 
Colorado  Springs,  Col.  (Hay- 
den) 


strata.  They  are  balls  or  irregular  lumps  of  a  material  differing 
from  that  of  the  stratum  in  which  they  o?cur.  They  are  not  peb- 
bles, which  are  older  than  the  stratum  which  contains  them  and 
which  were  embedded  just  as  we  find  them,  but  are  younger  than 
the  stratum  and  were  formed  subsequently.  This  is  shown  by  the 


CONCRETIONS 


323 


fact  that  the  planes  of  stratification  may  often  be  traced  through 
the  concretion,  and  that  fossils  are  sometimes  found  partly  within 
and  partly  without  the  nodule.  In  shape  the  concretions  vary 
greatly,  from  almost  true  spheres,  to  grotesque  aggregations,  but 
always  with  rounded  form,  and  almost  as  great  a  variety  of  mate- 
rial is  found  among  them.  Very  often  a  foreign  body,  like  a  fossil 
shell  or  leaf,  forms  the  centre  or  nucleus  of  the  nodule,  which  has 


FlG.  151.  —  Concretions  in  Laramie  sandstone,  exposed  by  weathering. 
(U.  S.  G.  S.) 

been  built  up,  often  in  concentric  layers,  around  the  nucleus. 
One  form  of  concretion,  known  as  a  sepiarium,  is  divided  inter- 
nally by  radial  cracks,  which  were  subsequently  filled  up  with  some 
mineral  deposited  from  solution  by  percolating  waters. 

The  agency  which  produces  concretions  cannot  as  yet  be  ex- 
plained.   The  material  of  which  they  are  made  must  have  been 


324 


THE  STRUCTURE  OF   ROCK   MASSES 


scattered  through  the  stratum  and  then  gathered  together  at  a 
later  period.  Such  nodules  have  been  observed  in  the  process  of 
formation  in  modern  sediments,  and  it  has  further  been  noticed 
that  when  finely  powdered  substances  are  mixed  together,  certain 
of  them  do  segregate  into  lumps.  These  observations,  however, 
merely  confirm  the  conclusion  that  concretions  are  due  to  segre- 
gation of  scattered  material  in  the  stratum;  they  give  us  no  ex- 
planation of  the  fact. 


FIG.  152. —  Ironstone  concretion,  split  open  to  show  the  fossil  leaf  which  forms  the 
nucleus ;  Mazon  Creek,  Illinois 


The  commonest  concretions  are  those  of  clay  in  various  kinds 
of  rock,  of  flint  and  chert  in  limestone,  and  of  ironstone  in  clay 
rocks. 

DISPLACEMENTS  OF  STRATIFIED  ROCKS 

It  is  evident  that  the  stratified  rocks  which  form  the  land  must 
have  been  changed,  at  least  relatively,  from  the  position  which 
they  originally  occupied,  since  the  great  bulk  of  them  were  laid 
down  under  the  sea.  Originally  they  must  have  been  nearly 
horizontal,  for  this  is  a  necessary  result  of  the  operation  of  gravity. 
Just  as  a  deep  fall  of  snow,  when  not  drifted  by  the  wind,  gradually 


DISPLACEMENTS   OF   STRATIFIED    ROCKS  325 

covers  up  the  minor  inequalities  of  the  ground  and  leaves  a  level 
surface,  so  on  the  sea-bottom  the  sediments  are  spread  out  in 
nearly  level  layers,  disregarding  ordinary  inequalities.  We  must 
remember,  however,  that  this  original  horizontality  is  not  exact, 
and  departures  from  it  are  not  infrequent.  On  a  large  scale,  these 
departures  from  the  horizontal  position  are  very  slight,  while  those 
that  are  conspicuous  are  local. 

Examples  of  such  original  deviations  from  horizontality  are  the 
following:  (i)  When  a  sediment-laden  stream  or  current  empties 
abruptly  into  a  deep  basin  with  steeply  sloping  sides,  the  sediment 
is  rapidly  deposited  in  oblique  layers,  which  follow  the  slope  of 
the  sides  (i.e.  foreset  beds).  (2)  Alluvial  cones,  or  fans  (p.  202), 
have  steeply  inclined  layers,  for  a  similar  reason.  Both  of  these 
cases  resemble  the  artificial  embankments  which  are  built  out  by 
dumping  earth  or  gravel  over  the  end,  until  each  successive  section 
is  raised  to  the  necessary  level.  In  such  embankments  the  obliquity 
of  the  layers  is  often  plainly  visible.  (3)  Sand  beaches  often  have  a 
considerable  inclination,  as  much  as  8  %,  and  newly  added  layers 
follow  this  slope.  (4)  On  a  large  scale,  the  great  sheets  of  sedi- 
ment that  cover  the  sea-bottom  generally  have  a  slight  inclination 
away  from  the  land,  with  a  somewhat  increased  slope  along  lines 
of  depression.  These  slight  original  inclinations  of  sedimentary 
masses,  either  as  a  whole,  or  along  certain  lines,  are  called  initial 
dips,  and  have  an  important  bearing  upon  the  results  of  subsequent 
movements  of  displacement. 

The  displacements  to  which  strata  have  been  subjected  after 
their  formation  are  of  two  principal  kinds:  (i)  In  the  first  kind, 
the  strata  have  been  lifted  vertically  upward,  often  to  great  heights, 
without  losing  their  horizontality.  Over  great  areas  of  our  Western 
States  and  those  of  the  Mississippi  valley,  the  beds  are  almost  as 
truly  horizontal  as  when  they  were  first  laid  down.  In  some  of 
the  lofty  plateaus  through  which  the  Grand  Canon  of  the  Colorado 
has  been  cut,  almost  horizontal  strata  are  found  10,000  feet  above 
the  sea-level.  (2)  More  frequent  and  typical  are  the  displacements 
of  the  second  class,  by  which  the  beds  are  tilted  and  inclined  at 


326  THE  STRUCTURE  OF   ROCK   MASSES 

various  angles,  sometimes  bringing  the  strata  into  a  vertical  posi- 
tion, and  occasionally  even  overturning  and  inverting  them.  In 
the  comparatively  sm  ill  exposures  of  strata  which  may  be  seen 
in  ordinary  sections  in  cliffs  and  ravines,  the  rocks  appear  to  be 
simply  inclined,  and  the  strata  themselves  to  be  nearly  straight. 
But  when  the  structure  is  determined  on  a  large  scale,  it  is  often 
found  that  this  appearance  is  due  to  the  limited  area  visible  in  one 
view,  and  that  the  apparently  straight  beds  are  really  portions  of 
great  curves.  Such  curves  are  called  folds. 

Dip.  —  The  angle  of  inclination  which  a  tilted  stratum  makes 
with  the  plane  of  the  horizon  is  called  the  dip,  and  is  measured  in 
degrees.  The  line  or  direction  of  the  dip  is  the  line  of  steepest  in- 
clination of  the  dipping  bed,  and  is  expressed  in  terms  of  compass 
bearing.  For  example,  a  stratum  is  said  to  have  a  dip  of  15°  to 
the  northwest.  The  angle  of  dip  is  measured  by  means  of  an  instru- 
ment called  a  clinometer,  of  which  many  kinds  are  in  use. 

Strike. — The  line  of  intersection  formed  by  the  dipping  bed 
with  the  plane  of  the  horizon  is  called  the  line  of  strike  and  is 
necessarily  at  right  angles  to  the  line  of  dip.  (See  Fig.  1 54.)  If  a 
piece  of  slate  be  held  in  an  inclined  position  and  lowered  into  a  ves- 
sel of  water,  the  wet  line  will  represent  the  strike.  As  long  as  the 
direction  of  the  dip  remains  constant,  the  line  of  strike  is  straight, 
but  as  the  direction  of  the  dip  changes,  the  strike  changes  also, 
always  keeping  at  right  angles  to  the  dip,  and  in  such  cases  as  the 
Appalachian  Mountains  the  lines  of  strike  are  sweeping  curves. 

Outcrop  is  the  line  along  which  a  dipping  bed  cuts  the  surface  of 
the  ground,  and  is,  of  course,  due  to  erosion,  which  has  truncated 
the  folds  of  strata.  Except  in  the  case  of  fractured  beds,  which 
will  be  considered  in  the  following  section,  if  there  were  no  erosion, 
there  could  be  no  outcrop.  When  the  surface  of  the  ground  is 
level,  outcrop  and  strike  become  coincident,  because  the  surface 
then  is  practically  a  horizontal  plane.  With  the  dip  remaining  con- 
stant, the  more  rugged  and  broken  the  surface  becomes,  the  more 
widely  do  strike  and  outcrop  diverge.  For  a  given  form  of  surface, 
outcrop  and  strike  differ  more  when  the  beds  dip  at  a  low  angle 


FOLDS 


327 


than  when  the  dip  is  steep,  for  when  the  strata  are  vertical,  outcrop 
and  strike. again  coincide,  and  the  more  nearly  the  strata  approach 
vertically,  the  more  closely  do  the  two  lines  come  together. 

Having  digressed  to  make  these  necessary  definitions,  we  may 
now  return  to  the  subject  of  folds. 

Folds  present  themselves  to  observation  under  many  different 
aspects,  all  of  which  may  be  regarded  as  modifications  of  three 
principal  types. 


FIG.  153.  —  Symmetrical  folds;  anticline  on  left,  and  syncline  on  right. 
(U.  S.  G.  S.) 

(i)  The  Anticline  is  an  upward  fold  or  arch  of  strata,  from 
the  summit  of  which  the  beds  dip  downward  on  both  sides.  The 
curve  of  the  arch  may  be  broad  and  gentle,  or  sharp  and  angular, 
or  anything  between  the  two.  The  line  along  which  the  fold  is 
prolonged  is  called  the  anticlinal  axis  and  may  be  scores  of  miles 
in  length,  or  only  a  few  feet.  This  may  be  illustrated  by  an  or- 
dinary roof,  which  represents  the  two  sides  or  limbs  of  the  anti- 
cline, while  the  ridge-pole  will  represent  the  anticlinal  axis. 


328 


THE  STRUCTURE  OF   ROCK   MASSES 


Whether  long  or  short,  the  fold  eventually  dies  away,  and  thus  the 
summit  of  the  arch  is  not  perfectly  level,  but  more  or  less  steeply 
inclined,  and  this  inclination  is  called  the  pitch  of  the  fold.  In 


FlG.  154.  — *Model  of  anticline.     P,  axis  pitching  to  the  left ;  6"  5,  line  of  strike ; 
D,  line  of  dip.     The  dotted  line  is  the  plane  of  the  axis.     (Willis) 

accordance  with  the  length  of  the  axis  and  the  steepness  of  the 
pitch  the  uneroded  anticline  is  either  short  and  dome-like,  or 
elongate  and  cigar-shaped. 

(2)  The  Syncline  is  the  complement  of  the  anticline,  and  in 
this  the  beds  are  bent  into  a  downward  fold  or  trough,  dipping 
from  both  sides  toward  the  bottom  of  the  trough,  which  forms  the 


FIG.  155.  —  Model  of  syncline.     (Willis) 


longitudinal  synclinal  axis.  As  in  the  anticline,  the  axis  may  be 
long  or  short,  with  gentle  or  steep  pitch,  forming  long,  narrow, 
"  canoe-shaped  "  valleys,  or  oval,  even  round,  basins.  In  section 
the  syncline  may  be  shallow  and  widely  open,  or  with  steep  sides 
and  angular  bottom. 


FOLDS 


329 


Domes  and  Basins  are  special  cases  of  anticlines  and  synclines. 
The  dome  is  an  anticlinal  fold  in  which  the  axis  is  reduced  to 
zero,  the  dip  of  the  beds  being  downward  in  all  directions  from 
the  summit  of  the  dome.  As  the  dip  changes,  the  strike  changes, 
describing  an  oval  or  circle.  Similarly,  the  basin  is  a  syncline 
with  axis  reduced  to  zero,  the  beds  dipping  downward  from  all 
sides  to  the  bottom  of  the  basin,  and  the  strike  forming  the  edge 


FIG.  156.  —  Anticline  near  Hancock,  Md.     (U.  S.  G.  S.) 

of  the  basin.  The  term  basin  is  used  in  different  senses,  and  it 
is  necessary  to  distinguish  carefully  between  a  basin  of  folding  and 
one  which  has  been  excavated  by  erosion. 

It  is  rare  to  find  a  single  anticline  or  syncline  occurring  by  itself; 
very  much  more  frequently  they  are  found  in  more  or  less  parallel 
series,  each  pair  of  anticlines  connected  by  a  syncline.  At  one 
end  of  the  system  we  may  find  several  axes  converging  and  unit- 


330  THE  STRUCTURE  OF   ROCK   MASSES 

ing  into  a  single  fold,  and  they  all  die  away  sooner  or  later,  the 
pitch  of  the  folds  coinciding  with  the  dip  of  the  beds. 

Antidinorium  and  Synclinorium. — The  system  of  roughly 
parallel  folds  which  are  grouped  together  may  be,  when  regarded  as 
a  whole,  either  anticlinal,  rising  up  into  a  great  compound  arch,  or 
synclinal,  depressed  into  a  great  compound  trough.  The  former 
is  called  an  anticlinorium,  and  the  latter  a  synclinorium.  The 
secondary  folds  which  compose  one  of  these  systems  may  them- 
selves be  compound  and  made  up  of  many  subordinate  folds,  the 
smallest  of  which  can  be  detected  only  with  the  microscope. 


FlG.  157. — Synclinorium,  Mt.  Greylock,  Mass.     (Dale) 

Geanticline  and  Geosyncline.  —  The  folds  and  flexures  which 
we  have  so  far  examined  are  those  which  affect  the  strata  at  the 
surface  or  at  comparatively  moderate  depths.  It  is  quite  impos- 
sible that  the  whole  crust  can  be  involved  in  folds  of  such  small 
amplitude.  The  crust  is,  however,  subject  to  flexures  of  its  own, 
which  are  characterized  by  their  great  width  and  gentle  slope. 
Such  flexures  have  been  named  by  Dana  geanticlines  and  geo- 
synclines,  to  express  their  importance  for  the  earth  as  a  whole. 
The  great  thickness  of  sediments  which  form  the  Appalachian 
Mountains  (exceeding  25,000  feet)  was  laid  down  in  an  immense 
geosynclinal  trough,  which  through  long  ages  slowly  sank  as  the 
sediments  accumulated.  The  rate  of  subsidence  so  nearly  equalled 
the  rate  of  deposition,  that  almost  the  entire  thickness  was  accumu- 
lated in  shallow  water,  as  is  indicated  by  the  character  of  the  rocks 
themselves.  Geanticlines  are  less  easy  to  detect,  but  there  is  evi- 
dence to  show  that  they  do  occur  on  an  equally  great  scale. 

Folds  may  be  classified  either  in  accordance  with  the  relation 


FOLDS 


331 


which  their  opposite  limbs  bear  to  each  other  >  or  with  reference 
to  the  degree  of  compression  to  which  they  have  been  subjected. 
Using  the  first  method,  we  may  distinguish  the  following  varieties. 


fr'iG.  158. — Diagrams  of  folds.  (Willis.)  i.  Upright  or  symmetrical  open  folds. 
2.  Asymmetrical  fold,  open.  3.  Asymmetrical  fold,  closed  and  overturned. 
4.  Symmetrical  fold,  closed.  5.  Closed  anticline,  overturned.  6.  Closed  anti- 
cline, recumbent 

Upright  or  Symmetrical.  —  In  this  case  the  two  limbs  of  the  fold 
dip  at  the  same  angle  in  opposite  directions,  the  plane  of  the  axis  of 
the  flexure  is  vertical  and  bisects  the  fold  into  equal  halves.  In 
asymmetrical,  or  inclined,  folds  the  opposite  limbs  have  different 


332  THE   STRUCTURE  OF   ROCK   MASSES 

angles  of  dip,  the  axial  plane  is  oblique  and  divides  the  flexure  into 
more  or  less  dissimilar  parts.  When  one  limb  has  been  pushed 
over  past  the  perpendicular,  the  fold  is  said  to  be  overturned  or  in- 
verted, and  when  this  has  gone  so  far  that  one  of  the  limbs  becomes 
nearly  or  quite  horizontal,  the  fold  is  recumbent. 


FlG.  159.  —  Asymmetrical  open  fold,  High  Falls,  N.Y.     (Photograph  by  van  Ingen) 

According  to  the  second  mode  of  classification,  we  have  a  some- 
what different  series  of  terms;  but  both  methods  have  their  uses 
and  must  be  employed.  Open  folds  are  those  in  which  the  limbs 
are  widely  separated;  strata  with  open,  gentle  flexures  are  said  to 
be  undulating.  Closed  folds  are  those  in  which  the  limbs  of  the 
flexures  are  in  contact  and  any  further  compression  must  be  re- 
lieved by  a  thinning  of  the  beds.  Contorted  strata  are  thrown 
into  closed  folds,  which  are  connected  by  sharp,  angular  turns. 


FOLDS 


333 


Plications  are  intense  crumplings  and  corrugations  of  the  strata. 
Isoclinal  folds  are  those  which  have  been  so  bent  back  on  them- 
selves that  the  limbs  of  the  flexures  are  all  parallel,  or  nearly  so. 
When  a  series  of  isoclines  has  been  planed  down  by  erosion  to  a 
level,  the  strata  show  a  continuous,  uniform  dip  and  present  a 
deceptive  appearance  of  being  a  simple  succession  of  tilted  beds. 
A  still  further  compression  of  isoclinal  folds  produces  fan  folds. 


FIG.  i6o.  —  Overturned  sharp  fold;  Big  Horn  Mountains,  Wyoming.  The  con- 
spicuous white  stratum  is  sharply  bent  on  itself  about  the  middle  of  the  moun- 
tain face,  in  reversed  Z-shape.  (U.  S.  G.  S.) 

In  this  structure  the  anticline  is  broader  at  the  summit  than  at 
the  base  and  the  syncline  broader  below  than  above,  a  reversal 
of  the  normal  arrangement. 

The  isoclinal  and  fan  folds  may  be  upright,  inclined,  inverted, 
or  recumbent.  In  the  closed  folds  there  has  been  such  enor- 
mous compression  that  the  same  strata  are  of  different  thickness 


334 


THE  STRUCTURE  OF   ROCK   MASSES 


FIG.  161.  —  Closed  recumbent  fold,  East  Tennessee.     (U.  S.  G.  S.) 


BBBBHH 

FlG.  162.  —  Plicated  gneiss,  Montgomery  Co.,  Pa.     (U.  S.  G.  Sj.) 


FOLDS 


335 


in  different  parts  of  the  flexure.  This  is  especially  marked  in  fan 
folding,  in  which  the  beds  are  much  thinner  on  the  limbs  than  at 
the  summit,  and  sometimes  the  central  beds  in  the  folds  have 
been  actually  forced  to  flow  upward  or  downward,  forming  iso- 
lated masses  cut  off  from  their  original  connections. 


FIG.  163.  —  Inclined  isoclinal  folds,  eroded.     (Willis) 

Besides  the  simple  folds  above  described,  there  are  frequently 
found  complex  systems  of  flexures,  in  which  the  compressing  force 
has  acted  simultaneously  or  successively  in  different  directions, 
producing  highly  complicated  cross  folds.  These  are,  however, 
often  extremely  difficult  to  work  out',  and  in  an  elementary  book, 
intended  for  the  beginner,  it  is  not  necessary  to  do  more  than 
mention  them. 


FlG.  164.  —  Diagram  of  monoclinal  fold 

(3)  The  monoclinal  fold  is  a  somewhat  exceptional  type,  which  can 
hardly  be  regarded  as  a  modified  form  of  the  anticline.     A  mono- 


336 


THE  STRUCTURE  OF   ROCK   MASSES 


FIG.  165.  — Monoclinal  fold,  Selmas  Valley.     (U.  S.  G.  5. 


FIG.  166.  —  Monoclinal  fold.  Mead  River,  Alaska.     (U.  S.  G.  S.) 


FOLDS  337 

clinal  flexure  is  a  single,  sharp  bend  connecting  strata  which  lie  at 
different  levels  and  are  often  horizontal  except  along  the  line  of 
flexure.  Folds  of  this  character  are  very  common  in  many  parts 
of  the  West,  especially  in  the  high  plateau  region  of  Utah  and 
Arizona. 


CHAPTER  XIII 
FRACTURES   AND   DISLOCATIONS   OF   ROCKS 

THE  rocks  are  often  unable  to  accommodate  themselves  by  bend- 
ing or  plastic  flow  to  the  stresses  to  which  they  are  subjected,  and 
therefore  break,  usually  with  more  or  less  dislocation.  A  simple 
fracture,  not  accompanied  by  dislocation,  is  called  a,  fissure.,  and 
the  strata  on  the  two  sides  of  the  fracture  are  the  same  at  correspond- 
ing levels,  so  that  the  crack  was  evidently  made  through  continuous 
beds. 

FAULTS 

When  the  strata  on  one  side  of  a  fissure  have  been  shifted  in  any. 
direction  relatively  to  the  beds  on  the  other  side,  so  that  the  strata, 
which  were  once  continuous  across  the  fracture,  are  now  separated 
by  a  vertical  interval  and  lie  at  different  levels,  the  structure  is 
called  a,  fault.  We  have  learned  that  faulting  is  an  accompaniment 
of  many  great  earthquakes  (see  Chapter  I),  and  these  modern 
faults  show  that  the  movement  may  be  in  any  direction,  vertical, 
horizontal,  oblique,  or  rotational.  "  Whenever  the  rocks  of  the 
earth's  crust  are  subjected  to  strain,  fractures  take  place  in  them  as 
in  any  other  body  under  similar  conditions,  and  the  different  parts 
of  the  rock  tend  to  move  past  one  another  along  the  fracture-planes, 
seeking  to  obtain  relief  from  the  strain  and  to  accommodate  them- 
selves to  new  conditions.  In  this  movement  one  part  of  the  frac- 
tured rock-mass  may  move  upon  the  other  in  any  direction,  up, 
down,  sidewise  or  obliquely,  according  to  the  conditions,  which  are 
different  in  each  instance."  (Spurr.) 

It  is  obvious  that  faulting  displays  highly  complex  phenomena, 
which  cannot  be  adequately  presented  by  diagrams,  since  three 

338 


FAULTS 


339 


dimensions  are  involved,  and  the  simple  cross-section  may  be 
altogether  misleading.  To  add  to  the  difficulty,  movements  fre- 
quently occur  at  intervals  along  the  same  fracture-planes,  but,  it 
may  be,  in  entirely  different  directions,  so  a  vertical  movement 
may  be  succeeded  by  a  horizontal  one,  or  vice  versa,  and  the 
final  outcome  may  be  the  resultant  of.  many  different  movements. 
It  is  very  unfortunate  that  several  of  the  terms  used  in  the  de- 


FlG.  167. —  Normal  fault,  fault-plane  hading  against  dip  of  beds;  b'  c,  throw;  b  c, 
heave;  b  b' ',  stratigraphic  throw,  which  in  this  case  is  measured  along  the  fault- 
plane,  because  the  latter  happens  to  be  at  right  angles  to  the  bedding-planes. 
The  angle  bb' c  is  the  angle  of  hade ;  b'bc,  the  angle  of  dip.  Foot-wall  to  the 
right  of  the  fault,  and  hanging  wall  to  the  left.  —  N.B.  The  line  b'  c  should, 
have  been  drawn  from  the  top  of  the  obliquely  lined  bed,  slightly  increasing 
both  throw  and  heave 


scription  of  faults,  adopted  from  the  miners  by  English  geologists, 
should  in  American  practice  have  acquired  meanings  quite  differ- 
ent from  those  originally  given  to  them,  so  that  the  student  finds  in 
different  books  the  same  terms  employed  in  different  senses.  The 
following  definitions  are  those  commonly  to  be  found  in  the  text 
books : — 

Faults    are    usually    inclinedj  and    the   angle  of    inclination, 


340 


FRACTURES  AND   DISLOCATIONS   OF   ROCKS 


measured  from  a  vertical  plane,  is  called  the  hade,  or  slope,  of  the 
fault,  while  the  dip  of  the  fault,  like  that  of  a  stratum,  is  measured 
from  a  horizontal  plane,  and  is  thus  the  complement  of  the  hade. 
For  example,  if  the  fault  is  vertical,  the  hade  =  o,  and  the  dip  =  90°; 
if  the  fault  is  horizontal,  the  hade  =  90°,  and  the  dip  =  o,  while  a 
hade  of  45°  gives  a  dip  of  the  same  amount.  The  side  on  which  the 
beds  lie  at  a  higher  level  than  their  continuations  on  the  other  side 
of  the  fault-plane  is  called  the  upthrow  side  and  the  other  is  the 
downthrow  side,  without  reference  to  the  actual  direction  of  the 
movement.  Owing  to  the  inclination  of  the  fault,  the  rocks  on  one 
side  project  over  those  on  the  other,  and  are  hence  called  the  hang- 


FiG.  168.  —  Normal  fault  hading  with  dip  of  beds, 
throw ;   CB,  heave 


DB,  stratigraphic  throw;  AC, 


ing  wall,  and  the.  side  which  projects  undeneath  the  other  is  called 
the  foot-wall.  Either  the  hanging  or  the  foot- wall  may  be  on  the 
upthrow  or  the  downthrow  side,  according  to  the  nature  of  the  fault. 
The  vertical  displacement  between  the  fractured  ends  of  a  given 
stratum  is  called  the  throw  (b'c,  Fig.  167)  and  the  heave,  or  hori- 
zontal throw,  is  the  horizontal  distance  through  which  one  end  of  a 
faulted  bed  has  been  carried  past  the  corresponding  end  on  the 
other  side  of  the  fault-plane  (be,  Fig.  167).  When  the  movement 
has  been  vertical,  the  heave  is  due  to  the  obliquity  of  the  fault  and 
therefore  increases,  in  proportion  to  the  throw,  as  the  hade  in- 
creases. A  fault  with  plane  perpendicular  to  the  surface  has  no 


FAULTS 


341 


heave,  for  it  has  no  hade.  Offset  is  the  distance  between  the  two 
corresponding  ends  of  a  faulted  bed,  measured  on  a  horizontal 
plane  and  usually  applied  to  the  outcrop  (see  Fig.  177,  III). 
The  stratigraphic  throw  is  the  thickness  of  beds  which  is  included 
between  the  two  fractured  ends  of  a  faulted  stratum  and  is  taken  at 
right  angles  to  the  bedding-planes.  (DB,  Fig.  168.) 

The  throw  of  faults  varies  greatly  in  different  cases,  from  a  frac- 
tion of  an  inch  up  to  thousands  of  feet.     In  those  of  small  throw 


FIG.  169.  —  Fault-breccia  of  limestone 

the  plane  of  fracture  is  frequently  a  clean,  sharp  break;  but  in  the 
greater  faults  the  rocks  in  the  neighbourhood  of  the  fault  are  often 
bent,  crushed,  and  broken,  forming  a  confused  mass  of  fragments, 
large  and  small,  which  may  be  cemented  into  a  breccia,  which  is 
then  called  fault-breccia  or  fault-rock.  In  soft  rocks  the  fault  is 
always  closed  by  the  immense  weights  and  pressures  involved,  but 
in  rigid  rocks  it  may  remain  partly  open,  especially  if  the  break  be 
not  a  plane,  but  of  curved,  warped,  and  irregular  course,  as  is 
usually  the  case.  The  term  fault-plane  is  thus  rarely  accurate, 


342 


FRACTURES   AND   DISLOCATIONS   OF   ROCKS 


though  it  is  constantly  employed  as  a  matter  of  convenience.     In 
faults  of  considerable  throw  the  ends  of  adjoining  strata  are  apt  to 


FIG.  170.  — Vertical  slickensides  ;  Rondout,  N.Y.     (Photograph  by  van  Ingen) 

be  bent  more  or  less  sharply  upward  or  downward,  in  accordance 
with  the  direction  of  movement.     This  is  drag  (Fig.  178). 


FAULTS 


343 


In  the  more  rigid  rocks  the  friction  of  the  masses  grinding  against 
one  another  on  the  fault-plane  grooves  and  polishes  them,  which 
produces  the  characteristic  appearance  known  as  slickensides. 
The  grooves  or  striae  indicate  the  direction  of  the  last  movement 
along  the  fault-plane,  for  ordinarily  this  last  movement  obliterates 
the  earlier  striae,  but  does  not  always  do  so,  for  we  sometimes  find 
cases  in  which  two,  or  even  three,  sets  of  striae  are  preserved,  each 
demonstrating  motion  in  a  different  direction. 


FlG.  171.  —  Limestone  faulted  on  bedding-planes,  with  vertical  slickensides;  Ron- 
dout,  N.Y.     (Photograph  by  van  Ingen) 

In  stratified  rocks  faults  usually  break  across  the  strata,  separat- 
ing each  bed  into  two  or  more  parts,  according  to  the  number 
of  dislocations,  yet  sometimes  the  fault-planes  coincide  with  the 
bedding  planes,  which  are  slickensided,  pushing  each  stratum 
upon  those  above  and  below  it,  but  without  fracture. 

The  preceding  discussion  of  faults  deals  only  with  those  of  strati- 


344 


FRACTURES  AND  DISLOCATIONS  OF   ROCKS 


fied  rocks,  but  this  is  merely  because  such  displacements  are  the 
easiest  to  observe.  As  a  matter  of  fact,  dislocations  may  and  do 
traverse  rocks  of  all  kinds,  but  it  may  be  quite  impossible  to  detect 
a  fault,  even  one  of  great  throw,  in  a  thick,  homogeneous,  crystalline 
mass,  for  lack  of  any  definite  points  of  reference  on  the  two  sides 
of  the  fault-plane.  On  the  other  hand,  in  thinly  laminated  rocks 


FIG.  172.  —  Minute  vertical  fault,  of  recent  date,  interrupting  glacial  striae. 
(G.  F.  Matthew) 

with  well-defined  colour  lines  the  most  minute  displacements  are 
strikingly  apparent. 

It  is  customary  in  geological  literature  to  apply  the  term  fault 
to  any  dislocation  of  the  rocks,  in  which  the  broken  ends  of  the  beds 
are  carried  past  one  another,  yet,  used  in  this  manner,  it  includes 
structures  of  very  different  significance,  produced  in  dissimilar 
ways.  It  therefore  seems  advisable  to  distinguish  between  the 
two  main  classes  of  structure  by  removing  thrusts  altogether  from 


RADIAL   FAULTS 


345 


the  category  of  faults.  Used  in  this  restricted  sense,  faults  are 
those  dislocations  which  generally  tend  toward  the  vertical  and  occur 
in  horizontal,  inclined,  or  but  slightly  folded  strata,  which,  to  all 
appearance,  have  been  subjected  to  tension  rather  than  compression, 
though  the  latter  frequently  occurs  locally,  while  thrusts  tend  to  be 
horizontal  and  are  found  in  regions  of  violent  compression.  The 
classification  of  faults  is  even  yet  the  subject  of  vigorous  discussion, 
and  no  general  agreement  has  been  reached,  so  that  the  following 
scheme  is  to  be  taken  as  merely  tentative,  though  it  departs  but 
little  from  the  customary  plan,  except  in  the  complete  separation 
between  thrusts  and  reversed  faults,  which  has  been  advocated  by 
many  writers.  In  the  present  state  of  knowledge,  however,  any 
scheme  of  classification  has  an  undue  appearance  of  exactness. 

DISLOCATIONS 


Faults  < 


Thrusts 


I.  Radial  Faults  are  those  in  which  the  principal  component  of 
the  movement  has  been  upward,  downward,  or  both,  though  sub- 
ordinate movements  of  tilting  and  rotation  frequently  occur. 
However,  it  is  not  always  possible  to  tell  from  the  observed  data, 
whether  the  chief  movement  was  vertical  or  horizontal,  for  in  cer- 
tain circumstances  the  results  are  so  deceptively  similar.  Accord- 
ing to  prevalent  belief,  the  radial  faults  comprise  the  great  majority 
of  dislocations,  and  horizontal  movements  are  comparatively  rare, 
but -recent  exact  studies  indicate  that  this  opinion  is  at  least 
an  exaggeration  and  that  horizontal  movements  are  far  from 
uncommon. 


I.    Radial    J1' 

I2' 
II.    Horizontal 

III.    Pivotal 

[I.   Scission 
II.    Fold 
III.    Surface 

(a.   Strike 
b.    Dip 
c.    Oblique 
Reversed 

346 


FRACTURES  AND  DISLOCATIONS  OF   ROCKS 


i.  NORMAL  FAULTS  (also  called  gravity  faults)  are  those  in  which 
the  fault-plane  inclines  or  hades  toward  the  downthrow  side,  which 
forms  the  hanging  wall.  "  It  seems  best  to  use  the  term  normal 
to  cover  those  faults  in  which,  using  the  horizontal  plane  as  datum, 
the  hanging  wall  has  dropped  relative  to  the  foot."  (J.  A.  Reid.) 
Locally,  at  least,  a  normal  fault  implies  an  extension  of  an  arc  of  the 
earth's  surface,  for  the  beds  occupy  a  greater  space,  measured  across 
the  fault,  than  they  did  before  faulting  occurred.  The  normal 
faults  may  be  divided  into  three  groups,  as  shown  in  the  table. 


FIG.  173.  —  Trough-fault  of  very  small  throw.     (U.  S.  G.  S.) 

a.  Strike-faults  are  those  which  run  parallel,  or  nearly  so,  to  the 
strike  of  the  beds.  To  this  group  belong  the  great  normal  faults, 
great  both  as  to  length  and  throw,  though  they  may  be  extremely 
minute,  or  of  any  order  of  magnitude  between  these  extremes. 
They  may  die  out  in  a  few  yards,  or  run  for  hundreds  of  miles,  and 
may  be  simple  or  compound,  single  or  branching.  A  compound 
fault  is  made  up  of  a  number  of  parallel  dislocations,  placed  close 


NORMAL    FAULTS 


347 


together,  which  may  hade  in  the  same,  or  in  opposite  directions, 
but  in  the  latter  case  one  hade  prevails  over  the  other.  A  series 
of  parallel  faults,  wider  apart  than  those  of  compound  faults,  and 
all  hading  in  the  same  direction,  are  called  step-faults.  If  two  par- 
allel dislocations  are  inclined  toward  each  other,  they  form  a 
trough-fault  and  include  a  wedge-shaped  mass  of  rock,  which  is  on 
the  downthrow  side  of  both  displacements,  while  if  they  incline 


FIG.  174.  —  Small  faults  in  the  roof  of  a  mine,  Idaho.     Near  the  right  end,  a  tiny 
Horst.     (U.  S.  G.  S.) 

away  from  each  other  the  inclined  mass  is  on  the  upthrow  side  of 
both.  For  the  latter  structure,  which  is  the  converse  of  the 
trough-fault,  there  is  no  English  term  —  many  geologists  have 
therefore  adopted  the  German  word  Horst. 

However  long  it  may  be,  a  simple  fault  sooner  or  later  dies  away 
by  diminution  of  the  throw,  until  it  vanishes.  This  implies  that  the 
rocks  are  bent  along  the  dislocation,  upward  on  the  upthrow  side 
and  downward  on  the  downthrow  side.  It  is  comparatively  sel- 


348 


FRACTURES  AND   DISLOCATIONS  OF   ROCKS 


dom  that  the  upthrow  side  of  a  fault  is  left  standing  as  a  line  of 
cliff,  or  fault- scarp,  which  depends  upon  the  length  of  time  during 
which  the  scarp  has  been  exposed  to  denudation.  In  the  majority 

of  instances  the  two  sides  of  the  fault 
are  worn  to  the  same  level  or  to  one 
continuous  slope,  so  that  there  is  no 
feature  of  surface  topography  to  indi- 
cate the  existence  of  the  fault,  which 
must  be  inferred  from  the  effects  of  the 
dislocation  upon  the  outcrops  of  the 
strata  involved  in  it.  These  effects 
are  determined  by  the  direction  and 
throw  of  the  fault,  and  by  the  attitude 
and  dip  of  the  beds.  Strike-faults  of 
moderate  throw  which  traverse  hori- 
zontal strata,  or  strata  inclined  so  that 
the  dip  of  the  beds  and  the  hade  of  the 
FIG.  175.  — Effect  of  strike-fault  fault  are  in  opposite  directions,  repeat 

on  outcrop ,      A   before  fault-      fa  f  the,bed      bringing  them 

mg ;  B,  with  fault  scarp  stand- 
ing;   c,  with  upthrow  and  again  to  the  surface,  as  shown  in  Fig. 
downthrow  sides  worn  to  a   jy^.     When  dislocated  by  a  series  of 
(Model  by  step_fauitSj   a    given    stratum    has    a 
number    of    outcrops  greater   by  one 

than  the  number  of  faults.     When  the  surface  has  been  worn 

down  to  one  continuous  slope, 

such  a  repetition  of   the  out- 
crops may  be  very  deceptive. 

In  Fig.   176,  for  example,  the 

observer  might  easily  be  misled 

into  believing  that  seven  seams 

of  coal  were  cropping  out  on  the 

hillside,  whereas  in  reality  there 

are  only  two  such  seams  with     FlG*  ^.-Effect  of J*P;f»ul<s  in  re; 

J  peatmg  outcrops.   ( Model  by  Sopwith) 

outcrops  repeated  by  faulting. 

When  the  hade  of  a  strike-fault  is  in  the  same  direction  as  the 


DIP-FAULTS 


349 


dip  of  the  beds,  a  certain  number  of  the  latter  abut  against  the 
fault-plane  and  fail  to  reach  the  surface,  their  outcrops  being 
cut  out  (Fig.  1 68).  In  great  faults,  with  displacements  of  many 
thousands  of  feet,  the  beds  cropping  out  on  the  two  sides  of  the 
fault  are  entirely  different.  The  deep-seated  strata  which  are 
exposed  by  denudation  on  the  upthrow  side,  are  carried  so  far 
down  on  the  downthrow  side  that  they  do  not  reach  the  surface  at 
all,  or,  at  least,  do  not  crop  out  in  the  neighbourhood  of  the  fault. 

b.  Dip-faults.  These  are,  in  general,  parallel  to  the  dip  of  the 
beds  and  therefore  cross  or  branch  out  from  the  strike-faults  of  the 
same  region,  more  or  less  at  right  angles; 
they  are  less  important  than  strike-faults, 
having  generally  a  smaller  throw  and  less 
length.  Dip-faults  cut  across  the  strike 
of  the  beds  and  interrupt  the  continuity 
by  producing  an  offset  in  the  outcrop. 
The  outcrop  of  a  given  stratum  ceases 
abruptly  at  the  fault-line  and  when  found 
on  the  other  side  is  seen  to  be  shifted  for 
some  distance  along  that  line.  How  such 
a  horizontal  shifting  is  brought  about  by  a 
vertical  movement,  is  shown  by  the  model 
(Fig.  177).  In  I  is  seen  the  model  before 
faulting,  the  black  band  representing  a 
dipping  bed;  in  II  the  block  has  been 
faulted,  the  upthrow  side  remaining  as  a 
fault-scarp,  while  III  shows  the  scarp  re-  FlG-  177-  — Model  showing 
moved  by  denudation.  On  the  down-  ^^um  offset  by  dip-fauit, 

I,  before  faulting ;  II,  with 

throw  side  the  outcrop  is  shifted  away  fault  scarp  standing;  ill, 
from  the  dip  of  the  beds  and  on  the  up-  with  scarP  removed  by 
throw  side  toward  the  dip. 

When  a  dip-fault  cuts  across  eroded  folds,  the  distance  between 
the  outcrops  of  the  same  stratum  in  the  two  limbs  of  an  anticline 
is  increased  on  the  upthrow  side,  diminishing  on  the  downthrow; 
in  the  synclines  this  arrangement  is  reversed.  This  is  due  to  the 


350 


FRACTURES  AND   DISLOCATIONS  OF   ROCKS 


fact  that,  when  both  sides  are  planed  down  to  the  same  level, 
the  surface  of  the  ground  cuts  the  beds  at  a  lower  stratigraphic 
level  on  the  upthrow  than  on  the  downthrow  side,  and  as  the  limbs 
of  an  anticline  diverge  downward,  the  outcrops  will  be  the  more 
widely  separated,  the  lower  the  level  at  which  they  reach  the  sur- 
face. The  limbs  of  a  syncline,  on  the  other  hand,  converge  down- 
ward and  the  effect  of  the  fault  is  therefore  just  the  reverse  of  what 
occurs  in  the  anticline. 


FIG.  178.  —  Drag  of  strata  near  fault-plane.     (U.  S.  G.  S.)     The  hole  is  an  artificial 
opening  along  the  fault 

c.  Oblique  Faults.  —  Dip-faults  do  not  always  follow  the  dip, 
and  strike-faults  often  deviate  considerably  from  the  strike  of  the 
beds,  and  sometimes  the  fault  is  neither  one  nor  the  other,  but  mid- 
way between  the  two,  and  then  is  called  an  oblique  fault.  The  out- 
crop of  a  given  bed,  obliquely  faulted,  has  an  offset,  as  in  the  case 
of  a  dip-fault,  but  if  the  fault  inclines  with  the  dip  of  the  strata, 
there  is  a  gap  between  the  two  adjacent  ends  of  the  outcrop,  the 


HORIZONTAL  FAULTS  351 

gap  widening  as  the  line  of  fault  approximates  that  of  strike.  If  the 
fault  hades  in  the  opposite  direction  from  the  dip,  the  two  ends 
of  the  outcrop  overlap. 

2.  REVERSED  FAULTS.  This  group,  as  usually  defined,  is  made  to 
include  thrusts  (q-v.),  but  the  latter  are  here  excluded  and  the  term 
reversed  fault  comprises  only  those  true,  radial  faults  in  which  the 
hanging  wall  has  been  pushed  up  over  the  foot-wall  and  there- 
fore forms  the  upthrow  side.  A  reversed  fault,  which  almost 
always  coincides  with  the  strike  of  the  beds,  implies  a  local  com- 
pression, for  the  beds  occupy  less  space  than  before  dislocation. 
In  a  large  faulted  area,  normal  and  reversed  faults  frequently 
occur  together,  compression  in  one  place  compensating  for  tension 
in  another,  and  the  two  kinds  of  displacement  appear  to  have  been 
formed  at  the  same  time,  or  in  close  succession. 

II.  Horizontal  Faults  (or  Heave-faults).  —  In  displacements 
of  this  class  the  principal  direction  of  movement  is  horizontal, 
and  in  horizontal  strata  may  readily  escape  detection.  When  the 
strata  are  inclined,  a  horizontal  displacement  produces  effects 
which  in  cross-section  cannot  be  distinguished  from  those  of  ordi- 
nary normal  and  reversed  faults,  except  when  the  striae  of  slicken- 
sides  remain  to  indicate  the  actual  direction  of  movement.  The 
deceptive  appearance  is  exactly  the  counterpart  of  that  which  re- 
sults from  the  vertical  movement  of  a  dip-fault,  by  which  the  offset 
of  the  outcrop  is  brought  about.  This  is  illustrated  by  the  model, 
Figs.  179,  180,  which  shows  that  if  the  hanging  wall  is  moved  in  a 
direction  opposite  to  that  of  the  dip  of  the  beds,  an  apparently 
normal  fault  results,  while  if  it  is  moved  in  the  same  direction  as 
the  dip,  an  apparently  reversed  fault  is  produced.  Horizontal 
faults  do  not  form  scarps,  for  there  is  no  vertical  movement,  but 
in  certain  cases,  as  shown  by  the  striae,  the  movement  is  obliquely 
upward.  It  is  thus  evident  that  what  would  ordinarily  be  re- 
garded as  normal  and  reversed  faults  of  the  typical  kind  may 
readily  be  formed  by  the  same  movement.  For  a  long  time  these 
heave-faults  were  supposed  to  be  very  rare,  but  they  are  now 
known  to  be  quite  common,  and  doubtless  very  many  faults, 


352 


FRACTURES   AND   DISLOCATIONS   OF  ROCKS 


which  have  been  regarded  as  normal  or  reversed,  will  on  further 
study  turn  out  to  be  heaves. 

III.     Pivotal  Faults.  —  In  faults  of  the  preceding  groups  there 


FlG.  179.  —  Model  illustrating  horizon- 
tal faulting,  with  hanging  wall  moved 
against  the  dip  and  producing  an 
apparently  normal  fault.  Upper  fig- 
ure (modified  from  Ransome)  block 
after  dislocation.  Lower  figure,  cross- 
section  on  plane  SSSS  :  BB,  B'B't 
stratum  of  reference.  FFFF,  fault- 
plane 


FlG.  180.  —  Model  illustrating  horizon- 
tal faulting,  with  hanging  wall  moved 
Sn  direction  of  dip  and  producing  an 
apparently  reversed  fault.  Upper  fig- 
ure (modified  from  Ransome)  block 
after  dislocation.  Lower  figure,  cross- 
section  on  plane  SSSS.  Lettering  as 
in  Fig.  179 


is  apt  to  be  more  or  less  rotation,  because  of  unequal  friction  and 
resistance  of  the  walls,  but  in  certain  cases  this  movement  of  rota- 
tion is  the  principal  one,  exceeding  any  movement  of  translation. 


PIVOTAL  FAULTS 


353 


and  these  are  the  pivotal  faults.  The  result  of  the  movement  is 
that  the  hanging  wall  drops  on  one  side  of  the  axis  of  rotation,  pro- 
ducing a  fault  of  the  normal  type,  and  rises  on  the  other  side,  form- 
ing a  reversed  fault.  Thus,  one  and  the  same  fault  is  "  normal  '- 
in  one  part  of  its  course  and  "  reversed  "  in  the  other. 

Systems  of  faults  of  different  dates  frequently  traverse  the  same 
region,  intersecting  and  crossing  one  another  at  all  angles.     An 


FIG.  181.  —  Horizontal  slickensides,  Oklahoma.     (U.  S.  G.  S.) 

older  fault  crossed  by  a  newer  one  is  itself  faulted  and  offset.  The 
intersecting  faults  divide  the  rocks  into  large  and  small  fault-blocks, 
which  are  generally  tilted  in  different  directions,  but,  as  a  rule, 
their  beds  are  not  strongly  folded.  As  was  pointed  out  in  the  dis- 
cussion of  earthquakes  (Chapter  I),  this  mosaic  of  fault-blocks 
is  an  important  element  in  the  production  of  seismic  disturbances. 
Though  faults  often  occur  in  regions  of  strata  that  are  not  folded, 

2  A 


354  FRACTURES  AND   DISLOCATIONS  OF   ROCKS 

there  is,  nevertheless,  frequently  a  close  connection  between  faults 
and  folds,  especially  monoclinal  flexures,  which  so  often  pass  into 
faults,  the  strata  bending  along  part  of  their  course,  fracturing  and 
dislocating  in  another  part. 


FlG.  182.  —  Model  illustrating  pivotal  faulting.     Upper  figure,  before  dislocation. 
Lower  figure,  after  dislocation.     (J.  A.  Reid) 

THRUSTS 

A  thrust  is  like  a  reversed  fault  in  that  it  is  the  result  of  compres- 
sion and  that  the  inclination  or  hade  of  the  fault  is  toward  the  up- 
throw side,  which  is  the  hanging  wall,  but  differs  in  the  tendency  to 


THRUSTS  355 

a  horizontal  position  of  the  plane  of  fracture  and  in  the  association 
with  violent  folding  and  plications.  In  his  latest  work  on  the  sub- 
ject Mr.  Willis  divides  thrusts  into  the  following  three  groups :  — • 
I.  Scission-thrusts  are  those  in  which  the  fault-plane  is  inde- 
pendent of  any  older  structures,  and  occur  chiefly  in  the  crystalline 


FIG.  183.  —  Fold  thrust,  near  Highgate  Springs,  Vt.     (U.  S.  G.  S.) 

schists  (metamorphic  rocks)  and  granite,  and,  as  a  rule,  de- 
part but  little  from  horizontality.  Thrusts  of  this  kind  are  de- 
veloped on  a  great  scale  in  the  southern  Appalachians,  especially 
in  eastern  Tennessee,  where  thrusts  of  20  miles  or  more  have  been 


356 


FRACTURES  AND   DISLOCATIONS  OF  ROCKS 


observed.  On  an  even  more  gigantic  scale  they  occur  in  the  High- 
lands of  Scotland  and  Norway,  where  the  movement  of  translation 
amounts  to  75  miles. 

II.  Fold-thrusts  are  intimately  connected  with  folds  and  occur 
only  among  folded  sedimentary  rocks;  they  may  arise  by  plication 
and  inversion,  usually  between  an  overturned  anticline  and  the  ad- 


FlG.  184.  —  Steep  fold-thrust,  Big  Horn  mountains,  Wyoming.    (U.  S.  G.  S.)    Strata 
of  hanging  wall,  on  left  of  thrust-plane,  show  drag 

joining  synclines.  Thrusts  of  this  character  are  very  widespread  and 
common  in  regions  of  strongly  folded  and  plicated  strata  and  repre- 
sent the  breaking  and  dislocation  of  rocks  in  the  process  of  folding. 
The  central  and  southern  Appalachians,  Arkansas,  and  Oklahoma, 
and  the  northern  Rocky  Mountains  are  the  regions  of  the  United 
States  where  great  thrusts  of  this  kind  are  most  frequent.  In  the 
latter  mountains,  on  both  sides  of  the  international  boundary 


SURFACE-THRUSTS 


357 


line,  great  fold-thrusts  have  carried  masses  of  strata  at  least  eight 
miles  to  the  eastward.  In  fold-thrusts  the  plane  of  dislocation  is 
somewhat  steeper  than  in  scission-thrusts  and  sometimes  approxi- 
mates the  steepness  of  typical  reversed  faults.  (See  Fig.  49^ 
p.  129.) 

III.   Surface-thrusts,  as  their  name  implies,  are  formed  at  the 
earth's  surface,  where  a  rigid,  gently  inclined  stratum  that  crops 


FIG.  185.  —  Surface-thrust  of  small  displacement.     (U.  S.  G.  S.) 

out  of  the  ground  is  subjected  to  lateral  compression  and  thrust 
forward  over  the  underlying  beds.  Such  a  condition  arises,  for 
example,  when  an  anticlinal  fold  has  been  planed  down  by  erosion, 
so  that  some  of  the  beds  lying  on  the  flanks  of  the  fold  are  trun- 
cated and  crop  out  freely;  when  renewed  compression  is  applied 
to  the  fold,  the  more  rigid  bed  will  be  pushed  forward  over  the  beds 
beneath,  or  it  may  be  fractured  and  overthrust  not  far  below  the 


358 


FRACTURES  AND  DISLOCATIONS   OF  ROCKS 


surface,  as  shown  in  the  figure  (Fig.  186).  Instances  of  surface- 
thrusts  have  not  been  identified  in  great  numbers,  though  there  is 
no  reason  to  doubt  that  they  are  common,  for  they  cannot  always 
be  distinguished  from  fold-thrusts  without  careful  study.  They 
have  been  found  in  the  southern  Appalachians  and,  on  a  great  scale, 
in  Montana. 


—  2 


FIG.  186.  —  Surface-thrust,  Holly  Creek,  Georgia.     (Hayes) 


THE  CAUSES  OF  FOLDING  AND  DISLOCATION 

Like  all  processes  which  take  place  deep  within  the  interior  of 
the  earth,  the  causes  of  crustal  deformations  are  very  obscure  and 
there  is  much  difference  of  opinion  concerning  them.  The  view 
which  is  held  as  to  the  physical  state  of  the  earth's  interior  will 
necessarily  condition  the  explanation  of  folding  and  faulting,  which 
is  but  one  special  aspect  of  the  general  problem.  Any  complete 
theory  must  of  course  contain  a  satisfactory  solution  of  all  the 
problems  involved,  but  such  a  theory  has  still  to  be  propounded  and 
for  the  present  we  must  be  content  with  tentative  hypotheses. 

The  first  step  in  the  inquiry  is  to  determine  the  direction  in 
which  the  folding  force  acted.  At  first  sight,  it  might  seem  natural 
to  suppose  that  the  direction  of  the  force  was  vertically  upward, 
acting  with  maximum  intensity  beneath  the  anticlines  and  with 
minimum  intensity  beneath  the  synclines.  But  such  an  explana- 
tion could  apply  only  to  open,  symmetrical,  and  simple  folds, 
and  even  in  these  cases  is  not  satisfactory.  Folded  strata  must 


THE  CAUSES   OF  FOLDING  AND   DISLOCATION         359 

either  occupy  less  space  transversely  than  they  did  before  folding, 
or  else  they  must  have  been  stretched  and  made  much  thinner,  but 
a  comparison  of  continuous  beds,  in  the  flexed  and  horizontal 
parts  of  their  course,  shows  no  such  thinning.  Again,  such  an  ex- 
planation is  obviously  insufficient  to  account  for  closed,  inclined, 
and  inverted  folds,  for  contortions  and  plications,  and  for  flexures 
of  different  orders,  one  within  another. 

If  the  folding  force  did  not  act  vertically,  it  must  have  acted 
horizontally,  and  this  is  the  explanation  now  almost  universally 
accepted.  A  horizontally  acting  force  would  compress  and  crumple 
up  the  beds,  producing  different  types  of  flexure  in  accordance 
with  varying  circumstances.  Furthermore,  the  microscopic  study 
of  intensely  folded  rocks  shows  that  they  have  actually  been  com- 
pressed and  mashed,  and  the  minutest  plications  are  visible  only 
under  the  microscope. 

Assuming,  then,  that  the  folding  force  was  one  of  compression 
and  acted  horizontally,  we  have  next  to  consider  the  circumstances 
which  modify  the  result,  producing  now  one  form  of  flexure  or 
fractare,  now  another.  Such  modifying  circumstances  are  the 
depth  to  which  a  given  stratum  is  buried,  its  thickness  and  rigidity, 
the  character  of  the  beds  which  are  above  and  below  it,  and  the 
intensity  and  rapidity  with  which  the  flexing  force  is  applied. 
When  in  a  mountain  region  one  sees  the  manner  in  which  vast 
masses  of  rigid  strata  are  folded  and  crumpled  like  so  many  sheets 
of  paper,  one  perceives  the  enormous  power  which  is  involved  in 
these  operations  and  the  gradual,  steady  way  in  which  that  power 
must  have  been  exerted.  When  strata  are  buried  under  a  suffi- 
cient depth  of  overlying  rock  to  crush  them,  they  become  virtually 
plastic  and  yield  to  the  compressing  force  by  bending.  The 
movement  would  seem  not  to  be  a  true  molecular  flow,  but  rather 
a  gliding  of  the  mineral  particles  one  upon  another.  At  such 
relatively  great  depths  cavities  cannot  exist,  and  if  the  compressed 
rock  should  be  broken  by  the  compression,  the  particles  are  again 
welded  together  into  a  firm  mass.  We  may  accordingly  distinguish 
a  shell  of  flowage,  in  which  the  rocks  all  yield  plastically,  a  more 


360 


FRACTURES  AND  DISLOCATIONS  OF  ROCKS 


superficial  shell  of  fracture,  in  which  all  but  the  softest  rocks  break 
on  compression,  and  between  the  two  a  shell  of  fracture  and  flow* 
age,  in  which  some  rocks  break  and  others  bend,  according  to 
their  rigidity.  The  depth  of  the  zone  of  flowage  is  estimated  at 
20,000  to  30,000  feet  below  the  surface. 


FlG.  187.  —  Folded  and  fractured  iron  ore  and  jaspilite,  Lake  Superior  region. 
About  %  natural  size 

Strata  which  have  not  been  buried  to  a  sufficient  depth  to 
make  them  plastic,  will  yield  to  compression  by  breaking,  though 
whether  a  given  bed  is  faulted  or  flexed,  will  often  depend  upon 
whether  the  folding  force  is  applied  slowly  or  with  comparative 
rapidity.  A  force  long  acting  in  a  slow  and  steady  fashion  will 


THE  CAUSES  OF   FOLDING  AND   DISLOCATION          361 


FIG.  188.  —  Plicated  beds  on  unfolded  ones ;  Mineral  Ridge,  Nevada.     (U.  S.  G.  S.) 


362 


FRACTURES  AND   DISLOCATIONS  OF  ROCKS 


produce  folds,  when  the  same  force  applied  more  suddenly  would 
shatter  the  beds.  Near  the  surface,  under  light  loads,  rigid  rocks 
will  always  break  rather  than  bend,  when  compressed.  Different 
stratified  rocks  differ  much  in  their  rigidity,  and  hence  a  load 
which  is  sufficient  to  cause  one  bed  to  bend  and  flow,  when  later- 
ally compressed,  will 
leave  another  unaf- 
fected, or  cause  it  to 
break,  if  the  compress- 
ing force  overcomes  its 
strength.  In  Bald 
Mountain,  New  York, 
the  stiff  limestones  are 
left  unchanged  by  a 
pressure  which  has 
crumpled  and  contorted 
the  soft  shales. 

A  certain  amount  of 
gentle  folding  may  take 
place  immediately  at 
the  surface  and  has 
actually  been  observed 
in  process  of  formation, 
even  in  rigid  rocks.  In 
Wisconsin  the  limestone 
bed  of  the  Fox  River 
suddenly  arched  upward 
into  a  low  anticline, 
crushing  and  bending 
the  steel  columns  of  a 
mill  which  had  been 
built  at  that  spot.  The 
bed  of  the  Chicago  drainage  canal,  also  in  limestone,  curved  up- 
ward in  similar  fashion,  when  the  excavation  had  removed  the 
overlying  load.  Very  many  other  surface  folds  are  demonstrably 


FIG.  189.  —  Plicated  limestone,  with  sheet  of  igne- 
ous rock,  near  Rockland,  Maine.  (U.  S.  G.  S.) 
The  limestone  has  flowed  under  compression, 
and  the  igneous  rock  has  fractured 


THE  CAUSES  OF  FOLDING  AND  DISLOCATION 


363 


of  very  recent  origin.  Folds  of  this  character  are  due  to  a  gentle 
and  gradual  compression,  to  which  the  strata  yield  by  a  read- 
justment of  the  joint-blocks  of  which  they  are  made  up. 


FIG.  190.  --  Model  showing  the  slip  of  folded  beds  upon  one  another.     (Willis) 

A  factor  of  much  importance  in  determining  the  character 
and  position  of  folds  is  the  mode  in  which  the  strata  were  originally 
laid  down.  As  we  have  already  learned,  the  sheets  of  sediment 
which  cover  the  sea-  •*• 

bottom  are,  on  a 
large  scale,  nearly 
level,  but  they  often 
show  slight  depar- 
tures from  such 
horizontality  along 
certain  lines.  These 
initial  dips  often 
determine  the  place 
of  flexures,  because 
they  divert  the  com- 
pression from  its 
horizontal  direc- 
tion. 

The  effects  of  lat- 
eral      compression 


are  shown  in  Figs. 
190  and  191, taken 


FIG.  191.  —  Model  showing  effects  of  lateral  compres- 
sion. A,  before  folding,  with  slight  initial  dip  at  x\  B< 
C,  D,  in  various  stages  of  compression.  (Willis) 


364  FRACTURES  AND   DISLOCATIONS  OF  ROCKS 

from  the  models  experimented  on  by  Mr.  Willis,  which,  when 
strongly  compressed,  imitate  with  remarkable  accuracy  the  struc- 
tures which  may  be  observed  in  folded  rocks.  Fig.  190  shows  that 
in  folding,  the  beds  must  slip  upon  each  other,  as  is  proved  by 
the  lines  perpendicular  to  the  bedding-planes,  which  were  contin- 
uous before  folding,  but  in  the  anticline  are  broken  by  the  differ- 
ential motion  of  the  layers,  each  bed  rising  farther  up  the  slope 
than  the  one  beneath  it.  The  same  thing  must  occur  in  folded 
rocks,  which  sometimes  show  polished  bedding-planes,  due  to  the 
slipping  of  the  beds  upon  one  another.  The  series  A  to  D  (in 
Fig.  191)  represents  a  model  before  and  in  various  stages  of  lateral 
compression,  and  exhibits  the  effect  of  the  slight  initial  dip  at  x 
in  determining  the  position  of  the  anticlinal  fold,  which  is  developed 
by  compression.  The  formation  of  one  fold  assists  in  the  develop- 
ment of  another,  for  it  both  changes  the  direction  of  compression 
and  redistributes  the  load  of  overlying  strata.  The  arch  of  the  anti- 
cline lifts  the  load  and  diminishes  the  weight  upon  the  beds  that 
lie  beneath  the  flexure,  but  increases  the  weight  upon  the  lines 
from  which  the  arch  springs. 

There  is  much  independent  evidence  to  show  that  folding  is  a 
gradual  process.  The  force  exerted  is  enormous,  but  so  is  also 
the  resistance  to  be  overcome,  and  a  steady  or  oft-renewed  com- 
pression, acting  upon  strata  under  a  great  load  of  overlying 
masses,  will  produce  regular  flexures,  where  a  sudden  compression, 
however  intense,  could  only  shatter  them. 

Thrusts  are  likewise  due  to  lateral  compression,  by  which  the 
rocks  have  been  sheared  and  broken,  and  the  beds  on  one  side  of 
the  plane  of  fracture  have  been  thrust  up  over  those  on  the  other. 
A  plication  or  overturned  fold  may  often  be  traced  into  a  thrust, 
in  a  way  that  shows  the  direction  of  movement  to  have  been  the 
same  in  both  fold  and  fracture.  Numerous  experiments  also  show 
that  lateral  compression  will  produce  just  such  structures.  A  re- 
duction of  the  overlying  load,  by  diminishing  the  plasticity  of  the 
rocks,  will  occasion  shearing  and  overthrusts,  when,  under  a 
greater  load,  the  same  strata,  exposed  to  an  equal  force  of  compres- 


THE  CAUSES  OF  FOLDING  AND  DISLOCATION 


365 


sion,  will  simply  flex  and  bend.  As  we  have  seen  (see  Fig.  186), 
an  anticlinal  fold  whose  load  has  been  reduced  by  erosion,  will,  on 
renewed  compression,  fracture  and  develop  a  thrust. 

While  thrusts  are  associated  with  violent  folding,  overturning 
and  plication  of  strata,  faults  occur,  as  a  rule,  in  regions  where 
folding  is  absent,  or  very  subordinate,  or,  if  in  areas  of  folded  rocks, 
the  faults  were,  generally  at  least,  formed  at  a  period  more  or  less 
subsequent  to-  the  period  of  folding.  The  association  of  the  differ- 


FIG.  192.  —  Model  illustrating  the  development  of  a  fold-thrust 

ent  classes  of  faults  shows  that  locally  tension  and  compression 
may  be  generated  in  the  same  area  and  probably  simultaneously. 
Reversed  and  horizontal  faults  are  due  to  compression,  the  force 
in  the  latter  case  acting  parallel  to  the  fault-plane  and  in  the  former 
case  across  it,  while  normal  faults  are  the  result  of  a  local  tension. 
It  is  still  an  open  question  how  these  local  compressions  and  ten- 
sions are  generated. 

One  explanation  is  that  such  phenomena  are  developed  in  re- 
gions that  have  been  raised  by  upwarping  above  a  position  of  ade- 


366  FRACTURES  AND   DISLOCATIONS   OF   ROCKS 

quate  support,  whence  results  a  system  of  fractures  and  the  settling 
and  readjusting  of  the  fault-blocks.  "  If  we  endeavour  to  restore 
a  system  of  normal  fault-blocks  to  the  relations  which  they  may  have 
had  before  faulting,  we  must  commonly  construct  a  dome-shaped 
figure  of  some  sort,  whose  surface  occupies  more  space  than  the 
displaced  blocks  occupy.  That  is  to  say,  in  any  cross-section  an 
elongated  arc  has  in  consequence  of  faulting  been  brought  into  a 
shorter  chord,  commonly  by  bringing  the  narrower  parts  of  wedges 
into  juxtaposition.  .  .  .  The  doming  may  produce  elongation  or 
stretching  in  superficial  sections  at  least,  and  thus  tend  to  provide 
the  opportunity  for  the  development  of  planes  whose  attitude  is 
that  of  the  normal  fault-plane.  In  so  far  as  the  inadequacy  of 
support  gives  rise  to  vertical  displacements  pari  passu  with  the 
stretching,  the  blocks  will  adjust  themselves  with  reference  to  each 
other  by  relative  displacement  in  the  direction  of  maximum  stress 
and  least  resistance.  ...  In  this  process  elongation  is  the  pri- 
mary condition  and  a  settling  down  of  the  blocks  is  a  result. 
Through  that  settling  a  secondary  effect  of  compression  is  set  up. 
The  large  masses  become  wedged  against  one  another,  and  as  their 
magnitude  is  such  that  their  own  weight  is  sufficient  to  deform 
them,  they  suffer  more  or  less  folding  and  even  reversed  faulting  as 
an  after  effect.  .  .  .  We  may  reasonably  expect  to  find  some  re- 
versed faulting  in  connection  with  normal  faulting  wherever  the 
latter  is  developed  on  a  truly  large  scale*  The  absence  of  folding 
or  reversed  faulting  could  only  follow  in  case  the  blocks  were 
free  to  move  outward  to  the  extent  demanded  by  the  elongation 
due  to  the  attitudes  of  the  normal  fault-planes."  (Willis.) 

In  some  cases,  normal  faults  are  due  to  pressure  acting  along 
and  parallel  to  the  fault-plane  and  causing  the  strata  to  arch 
gently  upward  on  the  upthrow  side,  downward  on  the  downthrow 
side.  Faults  of  this  class  have  been  observed  in  central  Pennsyl- 
vania, Tennessee,  and  Alabama.  Though  due  thus  to  pressure, 
a  tension  is  developed  across  the  fault-plane. 

Quite  a  different  type  of  explanation  seeks  to  account  for  the 
phenomena  of  faulting  by  the  transfers  of  molten  magmas  deep 


THE  CAUSES  OF  FOLDING  AND   DISLOCATIO  36; 

within  the  earth.  In  certain  regions,  as  in  the  Tonopah  district 
of  Nevada,  it  has  been  made  exceedingly  probable  that  such  trans- 
fers are  the  actual  cause  of  the  fracturing  and  dislocation  of  strata, 
and  some  observers  would  give  this  principle  a  widespread,  if  not 
a  general,  application.  "  Not  only  are  the  violent  migrations  of 
igneous  material  the  cause  of  complex  faulting,  but  also  it  is  most 
reasonable  to  conceive  that  the  deeper  and  more  gradual  move- 
ments of  the  subcrust  are  the  cause  of  the  larger  fault  systems. 
.  .  .  Given  this  cause  of  faulting,  the  heretofore  puzzling  facts  are 
satisfactorily  and  easily  explained.  Compression  and  tension  still 
remain  true  causes  of  faulting,  but  mainly  as  local  and  proximate 
ones.  The  common  expression,  tilting  of  fault-blocks,  attains  a 
deeper  significance,  for  this  tilting  may  be  more  largely  the  result 
of  subcrust  migrations  than  of  the  mere  force  of  gravity.  Cases 
of  horizontal  motion  and  pivotal  motion  become  simple,  for  there 
is  no  necessary  unchangeable  relation  between  the  direction  of  the 
force  and  the  position  of  the  fracture-plane."  (J.  A.  Reid.) 

Even  if  it  be  granted  that  the  effective  forces  which  cause  the 
folding  and  dislocation  of  rocks  are,  in  the  last  analysis,  a  horizon- 
tal or  tangential  compression,  it  still  remains  to  inquire  how  this 
great  force  was  generated.  There  is  no  general  agreement  concern- 
ing the  solution  of  this  problem.  For  a  long  time  it  was  supposed 
that  a  satisfactory  solution  was  given  by  the  contraction  of  the  earth 
from  cooling,  and  perhaps  the  majority  of  geologists  still  adhere 
to  this  view,  which  may  be  briefly  expressed  as  follows:  The 
earth's  crust  long  ago  reached  a  state  of  fairly  constant  tempera- 
ture, but  the  highly  heated  interior  is  steadily  cooling  by  radiation, 
and  consequently  contracting.  As  the  crust  cannot  support  itself, 
it  must  follow  the  shrinking  interior,  and  is  thereby  crowded  into 
a  smaller  space,  thus  setting  up  irresistible  lateral  stresses.  If  the 
earth  were  homogeneous,  its  surface  would  be  wrinkled  all  over, 
as  is  the  skin  of  a  withered  apple,  of  which  the  pulp  contracts  from 
loss  of  water,  crowding  the  skin  into  a  smaller  space;  but  as  the 
crust  is  heterogeneous,  with  special  lines  of  weakness,  the  compres- 
sion results  in  the  formation  of  long,  narrow  belts  of  folded 


368  FRACTURES  AND   DISLOCATIONS  OF  ROCKS 

rocks,  separated  by  broad  areas  of  relatively  little  disturbed 
strata. 

The  contractional  hypothesis  has  been  attacked  from  many  points 
of  view,  and  very  serious  doubt  has  been  thrown  upon  its  adequacy 
to  explain  the  facts,  and  even  upon  its  reality.  A  modification 
of  this  hypothesis  has  been  proposed  by  Professor  Chamberlin, 
who  regards  the  downward  movement  of  segments  of  the  earth's 
crust  as  primary  and  the  horizontal  movements  as  incidental  to  the 
former.  The  lithosphere  is  regarded  as  made  up  of  a  number  of 
heavier  and  stronger  segments,  the  surface  of  which  forms  the  ocean 
basins,  and  of  lighter  and  weaker  segments  which,  on  the  surface, 
are  the  continental  platforms.  The  general  shrinkage  of  the  earth 
causes  the  oceanic  segments  to  descend,  compressing  the  lighter 
continental  segments  and  producing  belts  of  folded  rocks  upon  their 
borders. 

The  study  of  the  radio-active  substances  and  their  distribution 
in  the  rocks  of  the  earth's  crust  has  led  some  observers  to  the  con- 
clusion that  the  earth's  loss  of  heat  is  fully  compensated  by  radio- 
activity and  that  since  a  very  early  period  in  the  history  of  the  globe, 
there  has  been  no  shrinkage  at  all,  a  standpoint  which  others  have 
reached  from  entirely  different  lines  of  evidence  and  reasoning. 

An  elementary  text-book  is  not  the  proper  place  for  the  discus- 
sion, or  even  the  full  statement,  of  all  the  different  hypotheses 
which  have  been  proposed  in  explanation  of  these  most  difficult 
problems.  Suffice  it  to  say  that  all  the  questions  concerning  the 
mechanics  of  the  earth's  interior  are  bound  up  together  in  an  in- 
divisible unity  and  that  the  full  and  satisfactory  answer  to  any  one 
question  will  involve  the  solution  of  all  the  cognate  problems. 


CHAPTER   XIV 
JOINTS.  —  STRUCTURES   DUE   TO    EROSION 

WITH  the  exception  of  loose,  incoherent  masses,  such  as  soil, 
gravel,  sand,  etc.,  all  rocks  which  are  accessible  to  observation,  are 
divided  into  blocks  of  greater  or  less  size  by  systems  of  cracks 
and  crevices,  which  are  known  as  joints.  These  may  be  easily 
observed  in  any  stone-quarry,  where  they  are  taken  advantage  of 
in  getting  out  the  stone. 

In  the  igneous  rocks  all  the  division  planes  which  separate  the 
blocks  are  true  joints,  which  vary  greatly  in  their  number  and 
manner  of  intersection  and  in  the  consequent  shape  of  the  joint- 
blocks.  Fine-grained  basalts  display  a  very  general  tendency  to 
columnar  jointing,  forming  more  or  less  regularly  prismatic  columns, 
which  are  commonly  hexagonal.  Several  modern  lavas  (see  p.  76) 
display  these  hexagonal  columns,  as  do  the  ancient  basalts  of  very 
many  regions.  In  certain  cases,  as  in  the  famous  Giant's  Cause- 
way of  Ireland,  the  columns  are  divided  transversely  by  concave 
joints,  giving  a  ball-and-socket  arrangement  which  has  a  curiously 
artificial  appearance.  Although  the  regular  hexagonal  columns 
are  most  frequent  among  the  fine-grained  basalts,  they  also  occur 
in  the  coarser  rocks  of  the  gabbro  family  and  in  other  families 
also.  The  acid  glass  of  Obsidian  Cliff  (see  Fig.  26)  shows  colum- 
nar jointing,  and  the  phonolite  of  Mato  Tepee  in  South  Dakota  is 
jointed  in  magnificent  columns,  and  many  other  examples  might  be 
cited. 

In  many  of  the  granites  and  other  coarse-grained  igneous  rocks, 
the  joints  divide  the  mass  into  cubical  blocks,  or  into  long,  rectangu- 
lar prisms,  or  into  broad,  slab-like  plates.     In  others,  again,  the 
blocks  are  of  exceedingly  irregular  form  and  size. 
2B  369 


370 


JOINTS 


FlG.  193.  —  Platy  jointing  in  diabase ;  above,  spheroidal  weathering  and  transition  to 
soil.     Rocky  Hill,  N.  J.     (Photograph  by  Sinclair) 


JOINTS 


371 


In  sedimentary  rocks  the  joints  are  ordinarily  in  only  two  planes, 
the  third  being  given  by  the  bedding-planes.     In  homogeneous, 


FIG.  194,  —  Regular  jointing  in  gneiss,  near  Washington.     (U.  S.  G.  S.) 

heavily  bedded  sediments,  such  as  limestones  and  massive  sand- 
stones, the  joints  are  apt  to  form  cubical  or  rectangular-prismatic 


372  JOINTS 

blocks,  making  a  weathered  cliff  look  like  a  gigantic  wall  of  regular 
masonry.  Other  sedimentary  rocks  are,  as  a  rule,  more  irregularly 
jointed. 


FIG.  195.  —  Irregular  jointing  in  gneiss,  Little  Falls,  N.Y.    (Photograph  by  van  Ingen) 


JOINTS 


373 


FIG.  196.  — Jointing  in  shale,  Cayuga  Lake,  N.Y.     (U.  S.  G.  S.) 

Joints  are  of  very  different  orders  of  importance:  some,  the 
master  joints,  traverse  many  strata  and  remain  constant  for  long 
distances  and  considerable  depths,  while  each  layer  usually  has 
minor  joints  which  are  confined  to  that  bed.  One  set  of  joints, 


374  JOINTS 

the  strike  joints,  run  more  or  less  parallel  to  the  strike  of  the 
beds,  while  the  second  set,  the  dip  joints,  follow  the  dip;  the 
former  are  usually  the  longer  and  more  conspicuous.  Oblique 
or  diagonal  joints  intersect  the  other  two  systems,  and  many  irregu- 
lar cracks  may  occur.  In  general,  the  more  disturbed  the  rocks 
have  been,  the  more  broken  they  are. 

Cause  of  Joints.  —  With  regard  to  the  manner  of  their  produc- 
tion, joints  may  be  classified  into  two  series:  (i)  those  which  are 
due  to  tension,  the  rock  usually  parting  in  planes  normal  to  the 
directions  of  tension;  (2)  those  which  are  due  to  compression, 
the  cracks  forming  in  the  shearing-planes. 

(i)  Tension  Joints.  —  In  igneous  rocks  joints  are  caused  by 
the  cooling  and  consequent  contraction  of  the  highly  heated  mass. 
This  shrinkage  sets  up  tensile  stresses  in  the  mass  to  which  the  rock 
yields  by  cracking  and  parting,  the  shape  of  the  blocks  being  largely 
controlled  by  the  coarseness  or  fineness  of  the  mass.  Igneous  rocks 
are  subject  to  all  the  vicissitudes  which  affect  other  kinds  of  rocks; 
they  are  faulted,  compressed,  exposed  to  tension,  etc.  Hence, 
systems  of  joints  may  occur  in  them,  which  were  formed  subse- 
quently to  the  shrinkage-joints  due  to  the  contraction  of  cooling. 
In  some  cases  the  jointing  of  sedimentary  rocks  may  perhaps  be 
caused  by  a  shrinkage  of  the  mass  on  drying,  but  this  cannot  be 
an  important  method  of  producing  systems  of  joints. 

The  convex  sides  of  anticlinal  and  synclinal  folds  are  stretched, 
and  (provided  they  are  not  too  deeply  buried)  the  stretching  may 
result  in  a  system  of  cracks  radial  to  the  curves  which  follow  the 
strike  of  the  beds.  Folds  are  not  horizontal,  but  pitch  in  the 
direction  of  their  axes.  This  complex  folding  may  produce  two 
sets  of  tensil  2  stresses  perpendicular  to  each  other,  and  thus  cause 
two  series  of  joints,  one  following  the  strike  and  the  other  the  dip 
of  the  beds.  Complex  folding  must  produce  a  twisting  and  warping 
of  the  strata,  and  it  has  been  experimentally  shown  that  a  brittle 
substance,  when  twisted,  cracks  in  two  sets  of  fractures  which 
intersect  nearly  at  right  angles.  How  slight  is  the  twisting  and 
warping  needful  to  produce  joints  is  shown  by  the  fact  that  strata 


CAUSE  OF  JOINTS 


375 


which  are  perfectly  horizontal,  so  far  as  can  be  detected,  are 
jointed.  The  modern  limestones  which  are  formed  in  coral-reefs 
are  jointed,  even  in  cases  where  the  movements  resulting  in  fracture 
must  have  been  minimaL 

Tension  joints  produce  either  rough,  or  smooth  and  sharply 
cut  surfaces,  which  is  determined  by  the  character  of  the  rock. 
In  sandstones  which  are  weakly  cemented  the  cracks  pass  be- 


FlG.  197.  —  Jointing  in  limestone,  Black  Hills,  South  Dakota.     (U.  S.  G.  S.) 

tween  the  grains,  while  in  hard  and  firm  rocks  the  fractures  are 
clean. 

(2)  Compression  Joints  are  caused  when  the  rocks  yield  along 
the  shearing-planes.  In  simply  folded  strata  are  produced  two  sets 
of  strike  joints  which  are  inclined  toward  each  other,  but  whether 
dip  joints  will  be  made  by  complex  folding  is  not  certain.  In  some 
conglomerates  the  joint  planes  pass  through  the  hard  quartz  pebbles 
and  leave  a  smooth,  even,  shining  face.  Tension  would  pull  such 


376 


JOINTS 


a  pebble  out  of  its  socket  and  only  by  shearing  could  it  be  cleanly 
cut. 

The  whole  subject  of  joints  in  sedimentary  rocks  is  a  difficult 
one  and  the  explanations  given  of  them  are  not  altogether  satisfac- 
tory, for  several  other  agencies  may  be  involved  in  their  produc- 
tion. It  is,  however,  highly  probable  that  the  master  joints  which 
roughly  follow  the  strike  and  dip  of  the  strata,  have  been  caused 
by  the  forces  which  produce  folding. 


FIG.  198. — Joints  dying  away  downward,  shown  by  pinching  out  of  white  calcite 
veins.     (Photograph  by  van  Ingen) 

Joints  cannot  occur  in  the  shell  of  flowage,  and  are  best  devel- 
oped in  the  shell  of  fracture,  being  of  less  importance  in  the  transi- 
tion belt  between  the  two. 


STRUCTURES  DUE  TO  EROSION 

Unconformity.  —  We   have   hitherto   considered   the  stratified 
rocks  as  made  up  of  beds  which  follow  upon  one  another,  in  orderly 


UNCONFORMITY 


377 


sequence,  and  as  being  affected  alike  by  the  elevation  or  depression, 
folding  or  dislocation,  to  which  they  may  have  been  subjected. 
Strata  which  have  thus  been  laid  down  in  uninterrupted  succession, 
with  sensibly  parallel  bedding-planes,  and  which  have  been  simi- 
larly affected  by  movements,  are  said  to  be  conformable,  and  the 
structure  is  called  conformity.  In  many  places,  however,  the  strata 
exposed  in  a  section  are  very  obviously  divisible  into  two  groups, 
each  made  up  of  a  series  of  conformable  beds,  but  the  upper 


FIG.  199.  —  Unconformity  with  change  of  dip,  or  angular  unconformity 


group,  as  a  whole,  is  not  conformable  with  the  lower,  but  rests  upon 
its  upturned  edges,  or  its  eroded  surface.  The  two  groups  are  said 
to  be  unconformable  and  the  structure  is  named  unconformity. 
The  definition  of  unconformity  here  given  includes  certain  not  un- 
common structures,  which  must  be  distinguished  as  having  quite  a 
different  significance. 

Unconformity  is  of  two  kinds :  (i)  There  is  a  distinct  difference 
in  the  dip  of  the  two  sets  of  strata,  the  upper  beds  lying  across  the 
upturned  and  truncated  edges  of  the  lower.  This  is  the  more 


378 


STRUCTURES   DUE  TO   EROSION 


usual  kind  and  is  shown  in  Figs.  199-202.  The  structure  im- 
plies that  the  lower  series  of  beds  was  first  laid  down  under  water, 
and  that  these  beds  were  then  upturned,  tilted,  or  folded  to  form  a 
land  surface.  Erosion  next  truncated  the  folds,  planing  the  edges 
of  the  disturbed  beds  down  to  a  more  or  less  level  surface.  The 
land  surface  was  again  depressed  beneath  the  water,  and  the 
second  set  of  strata  was  deposited  upon  it.  Finally,  a  renewed 


FIG.  200.  —  Angular  unconformity,  Grand  Canon  of  the  Colorado. 
(Photograph  by  Sinclair) 

elevation,   accompanied   perhaps   with   folding  or   faulting,  has 
brought  both  series  of  strata  above  the  sea-level. 

While  the  older  beds  formed  a  land  surface,  they  were  eroded 
and  no  deposition  took  place  upon  them.  Consequently,  between 
the  two  sets  of  strata  is  a  gap,  unrecorded  by  sedimentation  (at 
that  point),  the  length  of  which  represents  the  time  that  the  older 
beds  were  above  water.  The  processes1  involved  in  an  unconform- 


UNCONFORMITY 


379 


ity  are  of  slow  operation,  so  that  the  gap  usually  implies  a  very 
long  lapse  of  time.  In  many  cases  whole  geological  ages,  of  in- 
calculable duration,  have  intervened  between  the  deposition  of  the 
two  groups  of  strata. 


FlG.  201.  —  Angular  unconformity,  old  gravels  on  hard  shale;  Kingston,  N.J. 
(Photograph  by  Sinclair.)  Note  the  smooth  joint-faces  of  the  shale,  in  con- 
trast to  the  rugged  fracture-surfaces 


38o 


STRUCTURES   DUE  TO   EROSION 


(2)  In  the  second  kind  of  unconformity,  the  two  groups  of 
strata  have  the  same  dip,  the  upper  series  resting  upon  the  eroded 
surfaces  of  the  lower.  The  processes  involved  in  this  kind  of  uncon- 
formity are  nearly  the  same  as  in  the  first,  so  far,  at  least,  as  the 
alternation  of  land  surface  and  sea-bottom,  elevation  and  depres- 
sion, are  concerned.  In  this  case,  however,  the  first  upheaval  was 
not  accompanied  by  any  folding  or  fracturing  of  the  beds.  An 


FlG.  202. — Angular  unconformity,  west  of  Altoona,  Pa.     (U.  S.  G.  S.) 

unconformity  of  the  second  class  is  sometimes  exceedingly  diffi- 
cult to  detect  and  then  is  called  a  deceptive  conformity.  Such  a 
case  arises  when  the  surface  of  the  ground  is  made  by  cutting 
down  strata  to  the  upper  surface  of  a  hard  bed,  which  is  then  de- 
pressed beneath  the  water,  as  a  flat  pavement,  upon  which  new 
material  of  a  similar  kind  is  laid  down  with  hardly  a  perceptible 
break.  In  the  Rocky  Mountain  region  remarkable  instances  of 


OVERLAP 


381 


this  deceptive  conformity  occur,  where,  in  the  middle  of  a  mass  of 
limestone  apparently  formed  without  any  interruption,  there  is,  in 
reality,  an  enormous  time-gap.  Long  and  careful  search  has 
made  clear  the  nature  of  the  contact  and  exposed  the  deception. 
The  existence  of  an  unconformity,  when  none  is  apparent,  may 
sometimes  be  detected  by  observing  certain  structural  features 
which  affect  the  lower  and  older  beds,  but  not  the  upper.  For 
example,  the  lower  strata  may  be  faulted,  or  intersected  by  a  dyke 
of  igneous  rocks,  the  fault  or  dyke  ending  abruptly  at  a  certain 
level  and  not  continuing  into  the  upper  series. 

The  lowest  member  of  the  upper  series  of  strata  in  an  uncon- 
formity is  very  frequently  a  conglomerate  or  coarse  sandstone,  and 


•""T^—  ,—^T~~^~i~  ~i~"i~~ 


FlG.  203.  —  Unconformity  without  change  of  dip,  and  overlap 

represents  the  beach  formation  of  the  sea  advancing  over  the  old 
land.  These  are  called  basal  conglomerates.  Such  coarse  beds 
are,  however,  not  always  present,  and  they  may  be  only  locally 
developed  along  a  particular  line. 

Unconformities  may  be  confined  to  relatively  restricted  regions, 
or  they  may  extend  over  whole  continents;  they  are  very  useful 
means  of  dividing  the  strata  into  natural  chronological  groups. 

Overlap.  —  When  a  .series  of  strata  is  deposited  in  a  basin  with 
sloping  sides,  or  one  sloping  side,  each  bed  will  extend  farther 
than  the  one  upon  which  it  lies,  and  thus  in  a  thick  mass  of  strata, 
if  the  shelving  bottom  be  gently  inclined,  the  upper  beds  will  ex- 
tend far  beyond  the  lower  ones,  or  overlap  them  (see  Fig.  203). 
Overlap  also  occurs  where  the  sea  is  advancing  or  transgressing 


382  STRUCTURES  DUE  TO  EROSION 

slowly  across  a  subsiding  land  surface,  the  rate  of  depression  not 
much  exceeding  the  rate  of  deposition.  Here  also  each  stratum 
extends  farther  across  the  old  land  surface  than  the  one  beneath 
it,  and  conceals  -the  edges  of  the  latter.  The  relation  of  overlap 
is  between  the  successive  layers  of  a  conformable  series. 

Overlap  may  be  a  structure  of  much  economic  importance, 
if  one  of  the  lower  strata,  say  a  coal-bed,  is  mined.  It  is  not 
safe  to  assume  that  wherever  the  upper  beds  of  such  a  series  are 
found,  the  lower  will  be  found  directly  beneath  them,  an  assump- 
tion which  may  result  in  costly  failure. 

Contemporaneous  Erosion.  —  It  was  stated  above  that  the  defi- 
nition of  unconformity,  as  given,  would  include  certain  structures, 
which,  nevertheless,  must  be  distinguished  from  it:  one  of  these 
is  contemporaneous  erosion.  This  structure  is  produced  when  a 
current  of  water  excavates  channels  for  itself  in  the  still  soft  and 
submerged  mass  of  sediment.  After  the  current  has  ceased  to 
flow,  renewed  deposition  fills  up  the  hollow  with  the  same  or  a 
different  kind  of  material  as  was  thrown  down  before.  This 
structure  requires  only  a  short  pause  in  deposition,  not  a  long, 
unrecorded  break,  and  does  not  necessarily  involve  movements 
of  elevation  and  depression.  Furthermore,  contemporaneous 
erosion  is  a  local  phenomenon,  and  though  in  a  limited  section 
it  may  not  always  be  easy  to  distinguish  it  from  an  unconformity, 
the  difference  becomes  apparent  when  a  wider  area  is  examined. 
If  the  structure  be  one  of  contemporaneous  erosion,  the  two 
series  of  strata  will  be  conformable  except  along  the  line  of  the  chan- 
nel or  channels.  Fig.  204  is  an  example  of  this  structure  and 
shows  where  a  channel  in  an  ancient  sea-bottom  was  filled  up  by 
a  later  deposition  of  material. 

The  clay  "  horses  "  (as  miners  call  them),  which  frequently 
interrupt  coal  beds,  are  the  channels  of  streams  which  meandered 
through  the  ancient  peat  bog,  and  which  were  filled  up  with  sedi- 
ment when  the  swamp  became  submerged.  The  "  horses  "  are 
usually  of.  the  same  rock  as  that  which  forms  the  cap  or  rooi 
of  the  coal  seam. 


OUTLIERS 


383 


Horizontal  and  Oblique  Bedding.  —  Another  kind  of  deceptive 
resemblance  to  unconformity  is  occasionally  caused  by  the  alterna- 
tion of  horizontal  and  oblique  bedding,  a  horizontal  bed  resting 
upon  a  series  of  inclined  layers.  A  conspicuous  example  of  this 
is  given  by  the  Le  Clair  limestone  of  Iowa,  which  was  at  one  time 
altogether  misunderstood,  but  the  deception  is  seldom  one  that  a 
little  care  will  not  expose. 


FlG.  204.  —  Contemporaneous  erosion,  cnannel  in  wall  of  Niagara  Gorge. 
(U.  S.  G.  S.) 

Outliers.  —  An  outlier  is  an  isolated  mass  of  strata,  which  is 
surrounded  on  all  sides  by  beds  older  than  itself.  This  definition 
does  not  imply  that  the  older  beds  must  actually  rise  to  the  level 
of  the  outlier  and  enclose  it,  but  as  mewed  on  a  map,  which  brings 
all  irregularities  of  surface  down  to  one  plane,  the  older  beds  appear 
to  surround  the  outlier.  An  outlier  has  been  cut  off  by  denudation 
from  its  former  connections,  from  which  it  is  separated,  in  some 
cases  by  a  few  feet,  in  others  by  scores  or  even  hundreds  of  miles. 
Outliers  thus  stand  as  monuments  which  show,  partially  at  least, 


384  .   STRUCTURES    DUE  TO   EROSION 

the  former  extension  of  strata  long  subject  to  denudation,  though 
we  never  can  be  sure  that  the  farthest  outlier  was  at  the  actual 
original  margin  of  the  beds,  and  generally  may  be  confident  that 
it  was  not.  Outliers  are  almost  always  composed  of  horizontal 
strata,  or  of  isolated  synclines. 

If  the  outlier  be  brought  to  the  surface  by  faulting,  it  is  called  a 
faulted  outlier,  in  distinction  from  one  which  is  entirely  due  to  ero- 
sion. A  faulted  outlier  may  be  found  on  the  downthrow  side  of  a 
fault-block,  especially  in  a  trough-fault,  which  is  downcast  with 
reference  to  the  blocks  on  each  side  of  it.  In  such  a  case  the  older 
beds  actually  surround  and  enclose  the  isolated  mass  of  newer  beds. 

Inliers  differ  from  outliers  in  not  necessarily  being  isolated 
masses  of  rock,  but  merely  isolated  outcrops  of  older  beds  which 
are  surrounded  by  newer  strata,  though  underground  they  may  be 
continuous  with  very  extensive  areas  of  beds.  An  inlier  is  thus  a 
larger  or  smaller  mass  of  rock  surrounded  by  beds  which  are  geo- 
logically younger  than  itself.  The  summit  of  an  anticline  or  dome 
which  has  been  truncated  by  denudation  exposes  older  strata  in 
the  middle,  newer  ones  on  the  sides.  Inliers  may  also  be  due  to 
faulting  and  occur  on  the  upthrow  side,  as  in  a  fault-block  which 
is  on  the  upthrow  side  with  reference  to  the  blocks  on  each  side 
of  it,  or  Horst. 

Outliers  may  be  converted  into  inliers  by  the  deposition  of  newer 
beds  around  them.  The  isolated  "  stacks  "  and  pillars  on  the  sea- 
coast,  as  shown  in  Figs.  76  and  77,  are  outliers,  but  a  movement 
of  depression  submerging  them  in  the  sea  would  eventually  result 
in  their  being  buried  in  newer  deposits,  thus  changing  them  into 
inliers.  There  is  abundant  evidence  that  such  changes  have  actu- 
ally occurred  in  past  times. 


CHAPTER   XV 
UNSTRATIFIED   OR  MASSIVE  ROCKS 

THE  unstratified  or  massive  rocks  have  risen  in  a  molten  state 
from  below  toward  the  surface,  though  by  no  means  always  reach- 
ing it,  and  have  forced,  or  perhaps  have  sometimes  melted,  their 
way  through  or  between  the  stratified  rocks.  One  of  the  most 
important  points  to  determine  with  regard  to  a  massive  rock  is  its 
relation  to  the  strata  in  which  it  occurs;  for  the  earth's  chronology 
is  given  by  the  stratified  rocks.  Considered  only  with  reference  to 
itself,  an  igneous  mass  gives  no  trustworthy  evidence  as  to  the  time 
when  it  was  formed.  The  term  eruptive  is  frequently  employed 
in  the  same  sense  as  unstratified,  because  of  the  belief  that  most 
igneous  masses  have  been  connected  with  volcanoes;  but  as  such 
a  belief  may  not  be  well  founded,  it  is  better  to  use  a  non-committal 
term. 

As  in  most  departments  of  geology,  there  are  unfortunately  con- 
siderable differences  in  the  meaning  attached  by  various  writers 
to  the  terms  used  in  the  description  of  the  igneous,  or  massive, 
rocks.  Since  it  is  highly  desirable  that  greater  uniformity  and 
exactness  of  nomenclature  should  be  attained,  the  usage  proposed 
by  Professor  Daly  will  be  followed  here,  though  his  classification  is 
more  elaborate  than  is  required  in  an  elementary  work. 

We  shall  first  take  up  the  volcanic  rocks,  because  modern  vol- 
canoes give  us  the  key  by  which  we  may  readily  interpret  them. 

I.    ANCIENT  VOLCANOES  AND  THEIR  ROCKS 

Volcanic  Necks.  —  Volcanoes,  like  all  other  mountains,  are  sub- 
ject to  the  destructive  effects  of  the  atmosphere,  rivers,  and  the 
2c  385 


386 


UNSTRATIFIED   OR  MASSIVE   ROCKS 


sea.  In  an  active  volcano  the  upbuilding  by  lava  flows  and  frag- 
mental  ejections  more  than  compensates  for  the  loss  by  weather- 
ing, and  the  cone  continues  to  grow  in  height  and  diameter. 
When  the  volcano  has  become  extinct,  the  destructive  agencies 
work  unopposed.  We  find  extinct  volcanoes  in  all  stages  of 
degradation,  from  those  which  look  as  though  their  activity 
might  be  renewed  at  any  moment,  to  those  which  require  the 


••••i^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
FIG.  205.  —  Volcanic  neck,  Colorado.     (U.  S.  G.  S.) 

careful  examination  of  a  skilled  geologist  to  recognize  them  for 
what  they  are. 

In  the  Pacific  States  may  be  found  admirable  examples  of 
volcanic  cones  in  various  stages  of  erosion.  In  northern  Arizona 
the  picturesque  San  Francisco  mountains,  themselves  volcanic,  are 
surrounded  by  numerous  small  and  very  perfect  cones,  hardly 
affected  by  weathering  (Fig.  21).  In  northern  California  stands 


VOLCANIC  NECKS 


387 


the  noble  peak  of  Mt.  Shasta  (Fig.  29),  which  was  active  till  a 
late  geological  date  and  still  shows  traces  of  activity  in  its  hot 
vapours,  but  has  begun  to  suffer  notably  from  weathering.  Still 
farther  north,  in  the  State  of  Washington,  is  Mt.  Rainier,  another 
volcanic  cone,  which  has  been  longer  exposed  to  the  destructive 
agencies  and  has  been  worn  into  an  exceedingly  rugged  peak. 


FIG.  206.  —  Diamond  mine,  showing  circular  form  of  volcanic  pipe  in  sandstone; 
Kimberley,  South  Africa.     (Photograph  by  Hancox) 

These  mountains,  however,  merely  exemplify  the  earliest  stages  of 
degradation;  as  time  goes  on,  the  loftiest  cones  will  be  worn  away, 
and  at  last  only  the  worn-down  and  hardly  recognizable  stump  of 
the  volcano  remains,  which  is  known  as  a  volcanic  neck.  The  neck 
consists  of  the  funnel  or  vent  filled  up  with  the  hardened  lava  of 
the  last  eruption,  or,  less  commonly,  with  a  mass  of  volcanic 
blocks.  Associated  with  this  plug  of  lava  may  be  preserved  the 
lowest  lava  flows  or  tuffs  of  which  the  cone  was  originally  built 
up.  If  the  land  upon  which  the  volcanic  neck  stands  be  covered 


388 


UNSTRATIFIED   OR   MASSIVE   ROCKS 


by  the  sea  or  other  body  of  water,  the  remnant  of  the  cone  will 
be  buried  beneath  sediments,  and  a  volcanic  island  may  be  simi- 
larly cut  down  and  covered  with  sediments.  Subsequent  up- 
heaval and  denudation  may  at  a  long  subsequent  time  once  more 
expose  the  buried  cone  to  view.  Several  examples  of  this  have  been 
found  in  Great  Britain. 


FlG.  207.  —  Irregularly  and  columnar-jointed  lava  flow  on  sandstone;  Island  of 
Staffa,  Scotland 


The  diamond  mines  of  South  Africa  are  in  almost  cylindrical 
pipes,  which  are  cut  through  stratified  rocks  and  are  filled  with  an 
irregular  agglomerate.  On  the  surface  the  pipes  show  no  topo- 
graphical indication  of  their  presence,  but  are  quite  level  with  the 
ground.  The  exact  nature  of  these  pipes  is  not  well  understood; 
if  they  are  truly  volcanic,  all  traces  of  the  cones  and  associated 
ejected  masses  have  been  removed  by  denudation. 


LAVA  FLOWS  AND  SHEETS 


389 


Lava  Flows  and  Sheets  which  were  poured  out  on  the  surface 
of  the  ground  may  be  recognized  by  the  aid  of  several  criteria. 
In  flows  of  only  moderate  antiquity,  which  have  suffered  little 
denudation,  the  nature  of  the  mass  may  be  determined  at  a  glance, 
and  traced  to  the  vent  whence  it  issued.  Successive  sheets,  piled 
one  over  the  other  in  a  rude  bedding,  are  also  evidence  that  the 
rocks  are  surface  lavas.  Surface  sheets  may  be  overlaid  by  sedi- 


FlG.  208.  —  Lava  flow  on  sandstone,  Upper  Montclair,  N.J.      (Photograph  by  van 
Ingen.)     The  white  line  shows  the  irregular  contact 

ments,  which  were  deposited  upon  a  submarine  flow,  or  after  de- 
pression of  the  land.  Such  a  flow  is  then  called  a  contemporaneous 
or  interbedded  sheet,  and  evidently  its  geological  age  follows  the 
rule  for  strata;  it  is  newer  than  the  bed  upon  which  it  lies  and 
older  than  the  one  which  rests  upon  it. 

Fragmental  Products  (Pyroclastic)  are  positive  proof  of  vol- 
canic action,  for  they  cannot  be  formed  underground.  Coarse 
masses  of  agglomerate,  blocks,  and  bombs  show  that  the  vent 


390 


UNSTRATIFIED   OR  MASSIVE   ROCKS 


from  which  they  issued  was  not  far  away,  while  beds  of  fine  ashes 
and  tuffs  may  be  made  at  great  distances  from  their  source.  All 
these  varieties  may  be  enclosed  in  true  sediments,  and  may,  in  part, 
escape  destruction  long  after  the  volcano  which  ejected  them  has 
been  cut  away.  The  fragmental  products  are  always  contempo- 
raneous', and  when  interstratified  with  sediments  are  newer  than 
the  underlying,  older  than  the  overlying,  stratum. 


FIG.  209.  —  Pumice,  natural  size 


II.      ROCKS   SOLIDIFIED   BELOW  THE   SURFACE    (PLUTONIC) 

We  now  come  to  a  series  of  rocks  which  no  one  has  ever  ob- 
served in  the  course  of  formation,  because  they  were  solidified  at 
greater  or  less  depths  underground.  When  such  masses  are 
exposed  to  view,  it  is  not  because  they  have  been  brought  to 
the  surface,  but  because  the  surface  has  been  eroded  down  to 
them.  Though  these  unstratified  masses  cannot  be  observed  ^n 


INJECTED   BODIES  391 

the  process  of  formation,  as  may  the  lavas  and  pyroclastic  rocks, 
yet  the  nature  of  the  rocks  themselves,  and  their  relations  to  the 
volcanic  and  stratified  rocks,  enable  us  to  explain  them  satisfac- 
torily. In  whatever  shape  they  occur,  these  masses  are  intrusive, 
and  have  forced  or  melted  their  way  upward,  filling  fissures  and 
cavities,  or  have  thrust  themselves  between  strata,  following  the 
path  of  least  resistance.  Intrusions  are  younger,  it  may  be  vastly 
so,  than  the  strata  which  they  penetrate  and  lie  over  or  beneath; 
their  geological  date  may  be  determined  by  a  process  of  elimination, 
finding  the  newest  strata  which  they  have  traversed  and  the  oldest 
which  they  have  not  reached. 

A  primary  division  of  the  plutonic  masses  is  into  (i)  injected 
and  (2)  subjacent  bodies.  "'An  injected  body  is  one  which  is  en- 
tirely enclosed  within  the  invaded  formations,  except  along  the 
relatively  narrow  openings  to  the  chamber  where  the  latter  has  been 
in  communication  with  the  feeding  reservoir."  (Daly.)  Sub- 
jacent bodies,  on  the  other  hand,  have  no  floor  upon  which  the 
intrusive  mass  rests,  the  communication  with  the  earth's  interior 
being  by  great  openings  which  enlarge  downward  indefinitely 
within  the  limits  of  observation. 

Both  injected  and  subjacent  bodies  may  be  either  simple,  i.e. 
composed  of  material  intruded  at  one  period,  multiple,  i.e.  com- 
posed of  material  of  the  same  kind  intruded  at  more  than  one  pe- 
riod, or  composite,  i.e.  made  up  of  material  derived  from  different 
kinds  of  magma  intruded  at  more  than  one  period  of  time. 

i.   Injected  Bodies 

These  are  of  manifold  variety  of  shapes  and  sizes  and  differ  in 
their  relations  to  the  enclosing,  or  country  rock,  and  different  terms 
are  accordingly  used  to  describe  them. 

Dykes.  —  A  dyke  is  a  vertical  or  steeply  inclined  wall  of  igneous 
rock  which  was  forced  up  into  a  fissure  when  molten  and  there 
consolidated.  Dykes  of  a  certain  kind  may  actually  be  seen  in 
the  making,  as  when  the  lava  column  of  a  volcano  bursts  its  way 


392 


UNSTRATIFIED   OR   MASSIVE   ROCKS 


through  fissures  in  the  cone.  The  ordinary  dyke  is  formed  in  fis- 
sures which  traverse  stratified  rocks,  breaking  across  the  bedding- 
planes  and  usually  approximating  a  vertical  position,  though  some- 
times it  cuts  through  older  and  already  consolidated  igneous  rocks. 
In  thickness  dykes  vary  from  less  than  a  foot  to  a  hundred  feet  or 
more,  and  pursue  nearly  straight  courses,  it  may  be  for  many 
miles.  The  rock  of  a  dyke  has  usually  a  compact  texture,  having 


FlG.  210.  —  Parallel  dykes,  Cinnabar  Mountain,  Montana.      (U.  S.  G.  S.)      The 
left-hand  dyke  forms  the  distant  peak 

cooled  more  slowly  than  the  volcanic  masses,  though  the  edges, 
chilled  by  contact  with  the  walls  of  the  fissure,  may  be  glassy.  If 
the  rock  displays  columnar  jointing,  the  prisms  are  horizontal, 
normal  to  the  cooling  surfaces. 

Dykes  may  be  so  numerous  as  to  form  a  regular  network  of 
intersecting  walls,  just  as  we  have  found  to  be  the  case  in  faults  and 
fissures. 


DYKES 


393 


The  commonest  rocks  in  dykes  are  basalt,  quartz  porphyry, 
andesite,  and  diabase. 

When  denudation  has  so  far  cut  away  the  surface  of  the  ground 
as  to  expose  the  dyke,  the  form  which  the  latter  takes  will  depend 


FlG.  211.  —  Dyke  trenched  by  weathering  faster  than  country  rock.     (U.  S.  G.  S.} 

upon  the  relative  destructibility  of  the  igneous  rock  and  the  enclos- 
ing strata.  If  the  latter  wear  away  more  rapidly,  the  dyke  will  be 
left  standing  above  the  surface  like  a  wall  (Fig.  210);  but  if  the 
igneous  mass  be  disintegrated  more  rapidly  than  the  strata,  a 
trench  will  mark  the  line  of  the  dyke. 


394 


UNSTRATIFIED   OR   MASSIVE   ROCKS 


Dykes  are  common  and  conspicuous  objects  in  the  Connecticut 
valley  and  in  the  sandstone  belt  which  runs,  with  interruptions, 
from  the  Hudson  River  to  North  Carolina. 

Intrusive  Veins  are  smaller  and  more  irregular,  frequently  branch- 
ing fissures  which  have  been  filled  with  an  igneous  magma;  they 
may  be  only  a  few  inches  in  thickness,  and  may  often  be  traced  to 
the  mass  which  gave  them  off.  The  nature  of  the  rock  in  a  vein 
may  be  much  modified  by  material  derived  from  the  walls. "  This 


FIG.  212.  —  Veins  of  granite  in  cliff,  near  Gunnison,  Col.     (U.  S.  G.  S.) 

vein  rock  is  often  so  coarsely  crystalline,  as  in  pegmatite  veins, 
that  it  has  been  suggested  that  it  could  not  have  solidified  from  fu- 
sion, but  was  deposited  from  solution  in  superheated  waters. 

Sills  or  Intrusive  Sheets.  —  These  are  horizontal  or  moderately 
inclined  masses  of  igneous  rock,  which  have  small  thickness  as 
compared  with  their  lateral  extent.  Sheets  conform  to  the  bed- 
ding-planes of  the  strata,  often  running  long  distances  between  the 
same  two  beds;  but  if  they  can  be  traced  far  enough,  they  may 


SILLS  OR  INTRUSIVE   SHEETS 


395 


generally  be  found  cutting  across  the  strata  at  one  point  or  an- 
other. In  thickness  they  vary  from  a  few  feet  to  several  hundreds 
of  feet.  The  Palisades  of  the  Hudson  are  formed  by  a  sheet 
of  unusual  thickness;  its  outcrop  is  70  miles  long  from  north 
to  south,  and  its  thickness  varies  from  300  to  850  feet. 

Sills  are  most  commonly  found  in  horizontal  strata,  which 
offer  less  resistance  to  horizontal  expansion  than  do  the  folded 
beds;  they  are  also  very  generally  of  the  most  fusible  kind,  the 


FIG.  213.  —  Granite  veins  intrusive  in  diorite  and  both  cut  by  a  small  dyke  of  aplite: 
coast  of  Maine.     (U.  S.  G.  S.) 


gabbro  family,  because  such  magmas  retain  their  fluidity  and 
flow  for  longer  distances  than  do  the  highly  siliceous  rocks.  It  is 
probable  that  intrusive  sheets  can  be  formed  at  only  moderate 
depths,  because  the  overlying  strata  must  be  lifted  to  an  amount 
equal  to  the  thickness  of  the  sheet,  although  certain  cases  are  known 
where  the  sill  appears  to  have  made  its  way  by  melting  and  incor- 
porating some  of  the  strata.  At  great  depths  the  weight  to  be 
lifted  is  so  enormous,  that  the  easiest  path  of  escape  must  be 
by  breaking  through  and  across  the  strata.  If  the  beds  are  sub- 


396 


UNSTRATIFIED   OR  MASSIVE   ROCKS 


jected  to  compression  after  the  intrusion  of  the  igneous  masses, 
the  latter  will  be  flexed  or  faulted  like  the  stratified  rocks. 

In  a  limited  exposure  it  is  often  difficult  to  distinguish  at  once 
between  a  sill  and  a  contemporaneous  sheet,  but  there  are  cer- 
tain characteristic  marks  which  enable  the  observer  to  decide. 
The  presence  of  scoriae  shows  that  the  sheet  is  contemporaneous. 
If,  on  the  other  hand,  the  overlying  stratum  be  baked  and  altered 


FlG.  214.  —  The  Palisades,  seen  from  Hastings,  N.Y.     (Photograph  by  van  Ingen) 

by  the  heat,  or  if  the  sheet  cuts  across  the  bedding-planes  at  any 
point,  or  if  it  can  be  traced  to  a  dyke  which  rises  above  it,  or  if  it 
gives  off  tongues  or  veins,  or  if  pieces  of  the  overlying  stratum 
be  torn  off  and  included  in  the  sheet,  it  must  be  intrusive.  The 
nature  of  the  contact  between  the  sheet  and  the  stratum  above  it^ 
is  also  significant;  if  the  former  be  contemporaneous,  the  cracks 
and  fissures  of  its  upper  surface  will  be  filled  with  the  sedimen- 


LACCOLITHS 


397 


tary  material.  Finally,  the  texture  of  the  igneous  mass  gives  valu- 
able evidence;  in  the  intrusive  sheet  the  texture  is  compact 
(without  glassy  ground  mass)  or  even  quite  coarsely  crystalline, 
while  the  contemporaneous  sheet  will  display  the  glassy  or  por- 
phyritic  texture  of  surface  flows. 

Laccoliths.  —  A  laccolith   (or  laccolite)   is  a  large,  lenticular 
mass  of  igneous  rock,  filling  a  chamber  which  it  has  made  for 


FlG.  215.  —  Contact  of  diabase  sill  with  shales  below.    Base  of  Palisades,  Wee- 
hawken,  N.J.     (U.  S.  G.  S.) 

itself  by  lifting  the  overlying  strata  into  a  dome-like  shape;  the 
magma  was  supplied  from  below  through  a  relatively  small  pipe  or 
fissure.  The  rock  of  which  laccoliths  are  made  is  nearly  always  of 
the  highly  siliceous  and  less  fusible  kinds,  so  that  it  can  more  easily 
lift  the  strata  than  force  its  way  between  them.  Intrusive  sheets 
are,  it  is  true,  often  given  off  from  a  laccolith,  but  these  are  of  quite 
subordinate  importance,  while  dykes  and  irregular  protrusions, 


398 


UNSTRATIFIED   OR  MASSIVE   ROCKS 


called  apophyses,  extend  into  the  fissures  of  the  surrounding  and 
overlying  strata.  Subsequent  erosion  may  remove  the  dome  of 

strata  and  cut  deeply  into  the 
igneous  mass  beneath,  leav- 
ing rugged  mountains,  the 
height  of  which  depends 
upon  the  amount  of  original 
uplift  and  the  subsequent 
denudation.  Laccoliths  in 

FIG.   216.  — Diagrammatic   vertical   section     various  Stages  of  denudation 
of  a  laccolith   (Gilbert).     The  full  black     occur    jn    different    parts    of 

the  West.     Fig.   219    shows 

Little  Sun-Dance  Hill  in  South  Dakota,  a  small  dome  from 
which  the  overarching  strata  have  not  been  removed  and  the 
igneous  core  has  nowhere  been  exposed,  yet  there  can  be  little  doubt 


FIG.  217.  —  Eroded  laccolith,  with  many  sills  and  apophyses ;  Colorado.     (Holmes) 

of  its  presence.  Bear  Butte  (Fig.  220)  represents  a  second  stage  of 
denudation;  the  strata  have  been  removed,  except  those  upturned 
around  the  foot  of  the  butte,  and  the  igneous  c,ore,  exposed,,  yet  but 


CHONOLITIIS  399 

little  eroded.  In  the  same  region  is  Mato  Tepee  (also  called  the 
Devil's  Tower),  a  magnificent  shaft  of  columnar  phonolite,  which 
rises  700  feet  above  a  platform  of  horizontal  strata.  This  tower 
is  the  remnant  of  a  laccolith  from  which  the  covering  strata,  and 
probably  much  of  the  igneous  core,  have  been  eroded  away.  In 
southern  Utah  the  Henry  Mountains  are  a  group  of  laccoliths 
from  which  several  thousand  feet  of  overlying  strata  have  been 
removed  and  the  cores  deeply  dissected.  In  the  Elk  Mountains 
of  Colorado  are  some  enormous  laccolithic  masses. 


FlG.  218.  —  Vertical  section  through  laccolith  shown  in  Fig.  217  before  denudation. 
aa,  present  surface:  full  black,  remaining  parts  of  intrusive  body;  vertical 
lines,  portion  of  laccolith  removed  by  denudation.  (Holmes) 


Chonoliths.  —  Sometimes  the  shape  of  an  intruded  igneous 
body  is  so  irregular  and  its  relations  to  the  country  rock  are  so  com- 
plex that  it  cannot  be  referred  to  any  of  the  preceding  categories. 
For  such  irregular  masses  Daly  has  proposed  the  term  chonolith, 
which  he  defines  as  follows:  "  an  igneous  body  (a)  injected  into 
dislocated  rock  of  any  kind,  stratified  or  not;  (b)  of  shape  and  rela- 
tions irregular  in  the  sense  that  they  are  not  those  of  a  true  dyke, 
vein,  sheet,  laccolith,  bysmalith  or  neck;  and  (c)  composed  of 
magma  either  passively  squeezed  into  a  subterranean  orogenic 
chamber,  or  actively  forcing  apart  the  country  rocks."  Chonoliths 
are  probably  much  more  numerous  than  true  laccoliths 


400 


UNSTRATIFIED   OR   MASSIVE  ROCKS 


2.  Subjacent  Bodies 

The  mode  in  which  the  plutonic  masses  of  this  group  have  reached 
their  present  position  is  highly  problematical  and  still  forms  the 
subject  of  a  lively  discussion,  to  which  attention  has  already  been 
called  in  another  connection  (see  p.  291).  From  the  purely  de- 
scriptive point  of  view,  the  special  characteristic  of  these  bodies 
is  that  their  diameter  increases  downward  to  unknown  depths  and, 
consequently,  that  they  do  not  rest  upon  a  floor  of  country  rock. 


FIG.  219.  —  Little  Sun-Dance  Hill,  South  Dakota.     (U.  S.  G.  S.) 

Stocks  or  Bosses  are  rounded  or  irregular  masses  of  intrusive 
rock,  which  vary  in  diameter  from  a  few  feet  to  several  miles;  they 
cut  across  the  country  rock,  which  they  have  sometimes  pushed 
aside  and  sometimes  cleanly  perforated,  and  with  which  the  contact 
is  steeply  inclined  or  vertical.  The  structure  of  the  country  rocks, 
such  as  bedding-planes,  has  no  effect  upon  the  shape  of  the  stock. 
From  many  stocks  are  given  off  tongues  or  apophyses,  which  pene- 
trate the  country  rock  as  veins,  dykes,  sills,  and  various  irregular 
protrusions.  Granite,  diorite,  and  gabbro  are  especially  common 
in  stocks  and  the  texture  frequently  becomes  coarser  from  the 


BATHOLITHS 


4OI 


circumference  to  the  centre  of  the  mass.  In  many  instances,  per- 
haps generally,  stocks  themselves  are  but  protrusions  from  larger 
masses. 

Batholiths  are  great  masses  of  plutonic  rock  hundreds  or  even 
thousands  of  miles  in  extent;  in  general  characteristics  they  agree 
with  stocks,  except  for  their  very  much  greater  size,  yet  small 
batholiths  and  large  stocks  grade  into  one  another,  so  that  any  line 
of  demarcation  between  them  must  be  arbitrarily  drawn;  probably 


FIG.  220.  —  Bear  Butte,  South  Dakota.     (U.  S.  G.  S.) 

all  true  stocks,  could  they  be  followed  down,  would  prove  to  be  pro- 
trusions from  batholiths.  Granite  is  the  commonest  batholithic 
rock,  and  in  such  masses  forms  the  core  of  many  great  mountain 
ranges,  like  the  Sierra  Nevada  and  the  Rocky  Mountains. 


THE  MECHANICS  OF  INTRUSION 

As  in  all  questions  which  deal  with  the  subterranean  agencies, 
the  exact  manner  in  which  molten  magmas  make  their  way  up 


9.  P 


402 


UNSTRATIFIED   OR   MASSIVE   ROCKS 


THE  MECHANICS   OF  INTRUSION  403 

through  the  overlying  rocks  is  veiled  in  obscurity;  in  fact,  it  is  the 
unsolved  problem  of  the  ascensive  force  of  lava  in  another  shape. 
The  great  variety  of  forms  assumed  by  the  intrusive  bodies  is  due  to 
the  complex  interaction  of  two  main  groups  of  factors  —  the  ascen- 
sive force  of  the  molten  magma,  however  that  may  be  generated, 
and  the  resistance  to  be  overcome.  With  these  are  frequently 
associated  factors  of  a  third  series,  the  orogenic  compression  of  the 
rocks,  which  may  squeeze  a  purely  passive  magma  into  the  cavities 
and  fissures  made  by  compression.  The  description  given  in  the 
preceding  section  of  the  various  plutonic  bodies  left  out  of  account 
the  fact  that  the  different  kinds  are  connected  by  all  sorts  of  transi- 
tions. Laccoliths  grade  into  sills,  on  the  one  hand,  and  into 
chonoliths,  on  the  other,  and  the  same  continuous  body  may  be  a 
dyke  in  part  of  its  course,  a  sill  in  another,  and  so  on.  The  char- 
acter of  the  magma  itself  is  also  of  importance  in  determining  the 
result,  whether  the  molten  mass  is  thoroughly  fluid  pr  merely 
pasty,  and  how  great  the  quantity  of  the  imprisoned  gases  and 
vapours.  In  the  complicated  play  of  these  different  factors  it  is 
often  extremely  difficult  to  distinguish  effect  from  cause,  and  it  is 
this  which  gives  rise  to  such  radical  divergences  of  opinion  in  inter- 
preting the  phenomena. 

Igneous  intrusions  are  most  abundant  in  regions  of  disturbed 
rocks,  and  we  find  great  areas  of  nearly  horizontal  strata,  such  as 
the  Great  Plains,  in  which  intrusions  are  not  known  to  occur. 
On  the  other  hand,  folded,  even  intensely  compressed,  strata  may 
have  no  igneous  rocks  associated  with  them.  The  Appalachian 
Mountains,  for  example,  are  singularly  free  from  intrusions. 
These  associations  have  been  differently  interpreted.  According 
to  one  view,  the  very  general  coincidence  of  extensive  intrusions 
and  orogenic  compression  implies  that  the  magma  is  for  the  most 
part  passive,  and  is  squeezed  by  the  compressing  force  into  the 
actual  or  potential  cavities  which  are  generated  by  the  compres- 
sion. On  the  other  hand,  there  is  a  growing  tendency  among 
many  geologists  to  regard  the  deep-seated  magmas  as  active  and 
energetic  agents  of  dislocation  and  to  find  in  them  the  origin  of  the 


404  UNSTRATIFIED  OR  MASSIVE   ROCKS 

compressive  force  itself.  We  have  met  with  this  tendency  already 
in  the  discussion  of  earthquakes  (p.  52),  dislocations  (p.  367),  etc., 
and  it  must  be  reckoned  with  in  all  attempts  to  solve  the  great  prob- 
lem of  subterranean  activities.  As  so  often  happens,  it  will  prob- 
ably be  found  that  the  truth  lies  between  the  extreme  views. 

In  the  chapter  on  the  igneous  rocks  (p.  291)  we  learned  that  very 
different  opinions  were  held  concerning  the  proper  answer  to  the 
question  whether  the  magmas  make  their  way  entirely  by  mechani- 


,  .FiG.  222.—  Inclusions  (xenoliths)  of  schist  in  granite.     (U.  S.  G.  S.) 

cal  means,  taking  advantage  of  fissures,  cavities,  and  lines  of 
weakness,  and  forcing  the  country  rock  aside,  or  whether  they  may 
make  room  for  themselves  by  dissolving,  fusing,  and  incorporating 
more  or  less  of  the  rocks  which  formerly  occupied  the  position 
now  held  by  the  plutonic  bodies.  So  far  as  the  injected  masses 
are  concerned,  it  is  seldom  necessary  to  assume  that  they  have 
done  more  than  lift  or  push  aside  the  enclosing  rock,  but  the  case 
is  very  different  with  the  subjacent  masses.  Frequently  the  contact 


THE  MECHANICS  OF  INTRUSION  405 

between  the  country  rock  and  a  stock  or  batholith  shows  no  evi- 
dence that  the  former  has  been  compressed  or  crowded  to  make 
room  for  the  intrusion,  and  it  seems  impossible  to  account  for  the 
presence  of  the  plutonic  mass  except  on  the  assumption  that  space 
has  been  gained  by  fusing  more  or  less  of  the  enclosing  country 
rock.  It  is  not  necessary  to  suppose  that  such  fusion  takes  place 
only  on  the  periphery  of  the  intruding  magma;  on  the  contrary 
it  seems  more  likely  that  the  magma  dislodges  the  joint-blocks 
which  then  sink  in  the  highly  heated  mass  and  are  gradually  dis- 
solved. It  must  be  admitted  that  this  hypothesis  has  not  been 
established.  Some  of  the  highest  authorities  maintain  that  it  is 
definitely  disproved  by  the  microscopic  and  chemical  examination 
of  the  batholithic  rocks,  which  are  not  affected  by  the  character 
of  the  country  rock  through  which  they  break.  We  have  here  a 
conflict  of  evidence  which  it  remains  for  future  studies  to  harmonize. 
The  energy  of  intrusion  is  eloquently  displayed  along  the  margins 
of  many  batholiths,  where  the  country  rock  is  shattered  and  great 
blocks  are  torn  off  and  embedded  in  the  plutonic  mass.  Such 
blocks  are  called  inclusions  or  xenoliths,  and,  on  a  small  scale,  they 
occur  in  other  plutonic  bodies,  such  as  sills  and  laccoliths.  The 
existence  of  these  blocks  in  their  undissolved  state  has  been  made 
an  argument  against  the  assimilation  hypothesis,  but  it  must  be 
remembered  that  the  intact  xenoliths  are  products  of  the  dying 
energy  of  intrusion,  when  the  magma  was  already  so  stiff  that 
the  blocks  were  no  longer  able  to  sink  in  it. 


CHAPTER    XVI 
METAMORPHISM    AND   METAMORPHIC    ROCKS 

BY  the  term  metamorphism  is  meant  the  profound  transforma- 
tion of  a  rock  from  its  original  condition  by  means  other  than 
those  of  disintegration.  The  incipient  changes  of  the  latter  class 
may  very  greatly  modify  a  rock  and  its  constituent  minerals,  but 
such  changes  are  distinguished  from  metamorphism  under  the  term 
alteration.  Metamorphism  usually  implies  an  increase  in  hardness 
and  in  the  degree  of  crystallization,  and  very  frequently  also  the 
generation  of  an  entirely  new  set  of  minerals,  which  take  on  a 
characteristic  arrangement.  The  degree  of  metamorphism  varies 
according  to  circumstances,  and  from  the  mere  consolidation  of 
loose  sediments  to  the  most  radical  reconstruction  of  the  rock 
there  is  every  possible  transition.  Fossils  may  be  found  in  those 
metamorphic  rocks  of  sedimentary  origin  which  have  not  been 
completely  changed.  The  more  thorough  the  reconstruction  of 
the  rock,  the  more  obscure  do  the  fossils  become,  and  in  advanced 
stages  nearly  or  quite  all  trace  of  them  is  obliterated. 

It  was  long  supposed  that  the  metamorphic  rocks  were  one  and 
all  transformed  sediments,  but  later  investigations  have  shown 
that  many  of  them  were  originally  igneous.  Indeed,  it  is  often 
quite  impossible  to  decide  whether  a  given  metamorphic  rock  has 
been  derived  from  a  sedimentary  or  an  igneous  original.  This  is 
not  surprising,  for  the  ultimate  chemical  (not  the  mineralogical) 
composition  of  a  basalt,  a  volcanic  tuff,  or  a  clay  shale,  may  be 
the  same,  and  the  metamorphic  processes  may  produce  an  iden- 
tical rock  from  any  one  of  these  three  as  a  starting-point.  Much 
yet  remains  to  be  learned  regarding  the  modes,  causes,  and  results 

406 


CONTACT   METAMORPH1SM  407 

of  metamorphism,  and  some  of  the  most  far-reaching  problems  of 
geology  are  bound  up  with  these  questions. 

Metamorphism  is  of  two  quite  distinct  kinds:  (i)  contact  or 
local,  and  (2)  regional  metamorphism. 

I.  CONTACT  METAMORPHISM 

This  is  the  change  effected  in  surrounding  rocks  by  igneous 
magmas.  There  is  a  difference  between  the  effects  produced  by 
\  surface  lava  flow  and  those  caused  by  a  plutonic  intrusive.  In 
the  former  case  the  results  are  usually  not  very  striking,  because  of 
the  way  in  which  a  lava  stream  surrounds  itself  with  non-conduct- 
ing scoriae,  and  are  such  as  may  be  referred  to  the  action  of  dry  heat. 
Bituminous  coal  is  changed  into  a  natural  coke  by  the  removal 
of  its  volatile  constituents;  clay  may  be  baked  into  a  hard  red  rock, 
looking  like  earthenware,  and  limestone  changed  to  quick  lime, 
by  driving  off  the  CO2.  Plutonic  intrusions,  on  the  other  hand, 
are  more  efficient  agents  of  change,  because  they  are  presumably 
of  a  higher  temperature  and  retain  their  heat  longer,  and  because 
the  vapours  and  gases  which  they  contain  cannot  escape  into  the 
atmosphere,  but  strongly  affect  the  invaded  rocks.  The  rock 
invaded  and  metamorphosed  may  be  either  sedimentary,  igneous, 
or  already  metamorphic,  and  the  effects  may  be  very  marked,  or 
surprisingly  small;  indeed,  it  is  often  quite  impossible  to  say  why 
the  changes  should  be  so  insignificant.  Magmas  which  contain  an 
abundance  of  the  mineralizing  vapours  (see  p.  287)  produce  much 
more  effect  than  those  with  only  a  small  quantity  of  such  vapours. 
For  this  reason  acid  magmas  are  more  effective  than  basic.  Much, 
too,  depends  upon  the  nature  of  the  invaded  rock;  sediments 
which  contain  large  percentages  of  alumina  and  lime  are  much 
more  readily  and  profoundly  changed  than  those  which  are. made 
up  almost  entirely  of  silica.  The  distance  to  which  the  zone  of 
change  extends  is  wider  when  the  intrusive  mass  cuts  across  the 
strata  than  when  it  follows  the  bedding-planes,  so  that  a  dyke 
or  stock  is  more  effective  than  a  sill. 


408  METAMORPHISM 

We  may  now  consider  some  examples  of  contact  metamor- 
phism,  and,  for  this  purpose,  shall  select  only  the  changes  of  sedi- 
mentary rocks;  for  those  of  the  other  classes  require  a  treatment  too 
minute  and  refined  for  an  elementary  work.  We  may  note,  in 
passing,  however,  that  some  of  the  veins  given  off  from  granite 
stocks,  which  have  invaded  other  igneous  rocks,  are  probably  of  a 
metamorphic  nature  and  due  to  the  penetration  of  vapours. 

In  a  series  of  strata  which  have  been  invaded  by  an  igneous 
magma,  we  find  a  gradual  change  from  the  unmodified  rock  which 
lies  beyond  the  reach  of  the  transforming  agencies,  to  that  at  the 
actual  contact  with  the  igneous  mass.  Along  this  line  of  contact 
the  strata  are  so  thoroughly  reconstructed  that  often  only  a  micro- 
scopical examination  will  distinguish  the  changed  sediment  from 
the  igneous  rock.  A  siliceous  sandstone  or  conglomerate  develops 
no  new  minerals  in  the  change,  or  only  in  insignificant  quantity 
from  the  impurities  present.  The  bulk  of  the  material  simply 
crystallizes  and  forms  the  white  rock,  quartzite.  Clay  rocks 
undergo  more  radical  change  and  are  usually  divisible  into  distinct 
zones;  the  outermost  zone  is  unchanged;  in  the  intermediate 
one  the  shale  is  changed  to  a  dense  slate  spotted  with  biotite, 
magnetite,  or  other  dark  minerals.  The  spotted  slate  passes 
gradually  into  mica  schist,  a  rock  made  up  of  flakes  of  mica, 
with  some  quartz  and  felspar,  arranged  in  rudely  parallel  planes. 
At  the  contact  the  rock  is  converted  into  hornfels,1  which  is  a  very 
dense  substance,  looking  like  trap,  and  filled  with  numerous  sili- 
cated  minerals,  such  as  hornblende,  felspar,  and  many  others  which 
were  not  enumerated  in  the  chapter  on  the  rock-forming  minerals. 

Limestones  are  crystallized  by  the  heat  into  marble,  a  dense  ag- 
gregation of  crystalline  grains  of  calcite,  usually  with  obliteration 
of  the  bedding-planes  and  of  any  fossils  which  the  rock  may  have 
originally  contained.  Pure  limestone  gives  rise  to  white  marble, 
but  as  most  limestones  contain  impurities,  they  develop,  when 
metamorphosed,  a  large  variety  of  minerals,  such  as  biotite,  gar- 

1  Also  called  hornstone,  but  as  this  term  is  used  for  flint,  it  is  better  to 
retain  it  in  the  latter  sense  only. 


REGIONAL  OR  DYNAMIC  METAMORPHISM  409 

net,  amphiboles,  pyroxenes,  etc.  Beds  of  bituminous  coal  are 
baked  into  a  natural  coke,  as  in  Virginia  and  North  Carolina,  or 
changed  to  anthracite,  as  in  Colorado,  or  even  to  graphite  in  the  con- 
tact zone,  and  limonite  is  converted  into  magnetite. 

Among  investigators  of  the  subject  there  is  much  difference  of 
opinion  as  to  how  far  there  is  an  actual  migration  of  material  from 
the  plutonic  magma  into  the  enclosing  rock  walls.  When  there 
is  shattering  along  the  contact,  or  fissures  and  crevices  are  opened 
in  the  country  rock,  material,  both  in  solution  and  in  a  state  of 
fusion,  is  introduced.  Cementation  is  the  deposition  of  mineral 
matters  from  solution  in  the  interstices  between  the  granules  of  the 
rock.  Quartz,  calcite,  iron  oxides,  felspars,  mica,  augite,  and  other 
minerals  maybe  thus  introduced,  and  sometimes  the  quantity  of  new 
material  brought  into  the  rock  is  very  large.  Injection  is  the  pene- 
tration of  a  rock  by  molten  substances  which  may  fill  up  all 
the  minute  crevices.  The  distinction  between  cementation  and 
injection  is  not  a  very  sharply  marked  one,  because  superheated 
water  and  molten  magmas  appear  to  mix  in  all  proportions.  The 
difference  between  the  two  processes  seems  thus  to  be  largely  a 
question  of  the  quantity  of  water  present.  In  some  examples 
even  into  the  unruptured  walls  fluorine  and  boron  have  penetrated, 
and  probably  the  escaping  hydro-fluosilicic  acid  has  introduced 
silica  and  some  bases  for  a  short  distance  from  the  contact. 

Contact  metamorphism,  as  its  name  implies,  is  a  local  phenom- 
enon, but  a  widely  ramifying  and  complex  system  of  igneous 
intrusions  may  change  large  areas  of  sedimentary  rocks. 


II.   REGIONAL  OR  DYNAMIC  METAMORPHISM 

This  term  applies  to  the  reconstruction  of  rocks  upon  a  great 
scale,  in  areas  covering,  it  may  be,  thousands  of  square  miles,  and 
evidently  other  processes  in  addition  to  those  of  contact  meta- 
morphism are  needed  to  explain  such  widespread  changes.  Re- 
gionally metamorphic  rocks  are,  with  the  exception  of  the  slates, 


4io 


METAMORPHISM 


thoroughly  crystalline  and  usually  have  lost  all  trace  of  whatever 
fossils  and  stratification  planes  they  may  originally  have  had. 

The  first  step  in  metamorphism  consists  in  a  mere  hardening 
of  the  rock,  accompanied  with  the  loss  of  water  and  other  vola- 
tile substances.  In  the  second  stage  the  component  minerals 
already  present  are  crystallized,  but  new  compounds  are  sparingly 
formed.  This  .stage  is  frequently  accompanied  by  cleavage, 
which,  to  distinguish  it  from  that  of  minerals,  is  often  called 
slaty  cleavage. 


FlG.  223.  —  Oblique  synclinal  fold  in  slate,  showing  cleavage  planes  at  all  angles  to 
the  bedding-planes.     (U.  S.  G.  S.) 

Cleavage  and  Fissility.  —  Cleavage  is  "  a  capacity  present  in 
some  rocks  to  break  in  certain  directions  more  easily  than  in  others," 
while  fissility  is  a  "  structure  in  some  rocks,  by  virtue  of  which 
they  are  already  separated  into  parallel  laminae  in  a  state  of  nature. 
The  term  fissility  thus  complements  cleavage,  and  the  two  are 
included  under  cleavage  as  ordinarily  defined."  (Van  Hise.) 

Many  unmodified  igneous  rocks  have  a  marked  cleavage,  which  is 


CLEAVAGE  AND   FISSILITY 


occasioned  by  the  arrangement  of  the  constituent  mineral  grains 
with  their  long  axes  parallel,  or  by  a  parallelism  in  the  cleavage- 
planes  of  these  minerals,  or  by  both  factors  combined.  In  cleaved 
sedimentary  rocks  the  cleavage-planes  may  coincide  with  the  planes 
of  stratification.  Much  more  commonly,  however,  they  intersect 
the  latter  at  all  possible  angles,  keeping  a  constant  direction  for 
long  distances  (parallel  to  the  axes  of  the  folds  in  which  they 


FIG.  224.  — Fissile  quartzite,  California.     (U.  S.  G.  S.) 

occur),  while  the  bedding-planes  change  with  the  dip  from  point 
to  point.  Ordinary  roofing  slate  is  one  of  the  best  possible  exam- 
ples of  a  cleaved  rock  and  in  beds  of  slate  interstratified  with  other 
rocks,  the  cleavage  is  usually  quite  perfect  in  the  former,  absent  or 
but  partially  developed  in  the  latter. 

It  is  very  generally  agreed  among  geologists  that  slaty  cleavage 
is  a  result  of  compression;  for,  disregarding  certain  igneous  masses 


412  METAMORPHISM 

it  occurs  only  in  rocks  which  show  other  evidences  of  having  been 
subjected  to  compression.  On  the  other  hand,  the  mechanics  of 
the  problem  are  somewhat  obscure  and  have  given  rise  to  differ- 
ences of  opinion.  The  most  probable  view  seems  to  be  that  the 
cleavage-planes  are  developed  at  right  angles  to  the  compressing 
force,  and  are  due  to  the  arrangement  of  the  constituent  mineral 
particles  of  the  rock  with  their  longest  diameters,  their  cleavage- 
planes,  or  both,  in  parallel  directions.  Further,  that  "  this  arrange- 
ment is  caused,  first  and  most  important,  by  parallel  development 
of  new  minerals  ;  second,  by  the  flattening  and  parallel  rotation  of 
old  and  new  mineral  particles ;  and  third,  and  of  least  importance, 
by  the  rotation  into  approximately  parallel  positions  of  random 
original  particles."  (Van  Hise.)  Fissility  is  also  due  to  com- 
pression, the  rocks  giving  way  along  the  shearing-planes,  which  are 
inclined  to  the  direction  of  the  pressure.  Slaty  cleavage  is  brought 
about  in  the  softer  rocks  and  fissility  in  the  more  rigid  by  similar 
compression. 

A  more  advanced  degree  of  metamorphism  is  characterized  by 
the  schistosity  or  foliation  of  the  rocks,  as  is  also  true  of  contact 
metamorphism  when  such  a  rock  as  mica  schist  is  formed.  Schis- 
tosity or  foliation  is  the  arrangement  of  the  component  mineral 
particles  of  a  rock  into  rudely  parallel  planes  or  undulating  surfaces, 
in  consequence  of  which  the  rock  parts  most  readily  along  those 
planes  or  surfaces,  and  has  a  banded  appearance.  In  the  schis- 
tosity which  is  developed  in  the  contact  metamorphism  of  a  sedi- 
ment, the  foliation  appears  to  be  determined  by  the  stratification 
planes,  but  in  regionally  metamorphosed  rocks  this  is  generally 
not  the  case.  Here  the  foliation,  like  cleavage  and  fissility,  with 
which  the  former  is  connected  by  all  grades  of  transition,  as  a  rule, 
is  independent  of  previous  structures,  and  is  determined  by  the 
direction  of  the  compressing  force.  The  intergradations  between 
cleavage  and  fissility,  on  the  one  hand,  and  schistosity  on  the 
other,  make  it  appear  that  all  those  structures  are  due  to  the 
same  agency  operating  with  different  degrees  of  power  under 
somewhat  different  circumstances. 


THE   CAUSES  OF   METAMORPHISM  413 

The  shearing  and  crushing  of  the  rocks  frequently  change  the 
component  minerals  into  paramorphic  forms,  i.e.  those  which 
have  the  same  chemical  composition,  but  different  crystal  forms; 
for  example,  aragonite  is  thus  converted  into  calcite  and  augite 
into  hornblende.  In  the  more  complete  stages  of  metamorphism 
an  entire  chemical  reorganization  is  made,  and  new  minerals  are 
abundantly  generated.  Inasmuch  as  great  areas  of  metamorphic 
rocks  are  almost  invariably  those  which  have  been  intensely 
and  violently  compressed,  and  moderately  folded  sedimentary 
rocks  may  sometimes  be  traced  directly  into  intensely  plicated 
metamorphic  rocks,  we  are  justified  in  concluding  that  the  com- 
pression is  the  cause  of  the  reconstruction,  especially  as  the  excep- 
tions are  more  apparent  than  real.  If  this  conclusion  is  well 
founded,  it  leads  to  the  highly  interesting  and  important  generaliza- 
tion first  clearly  stated  by  President  Van  Hise,  that  the  structures 
impressed  on  the  stratified  rocks  after  their  first  formation,  folds, 
faults,  thrusts,  joints,  cleavage,  fissility,  and  foliation  are  all  due  to. 
lateral  compression,  acting  with  different  degrees  of  intensity  and  at 
different  depths,  depth  and  overlying  load  being  controlling  factors 
of  the  first  importance. 

There  is  some  difference  of  opinion  as  to  the  relative  importance 
of  contact  and  dynamic  metamorphism,  though  it  is  not  disputed 
that  large  areas  may  be  metamorphosed  by  frequent  and  extensive 
igneous  intrusions,  nor  that  such  intrusions  may  aid  very  materially 
in  the  transformations  made  by  intense  compression. 

Igneous  masses,  when  subjected  to  the  same  processes,  give  rise 
to  rocks  entirely  similar  to  those  made  by  the  metamorphism  of 
sediments,  so  that  it  is  sometimes  impossible  to  distinguish  between 
them.  The  compression  may  have  been  applied  while  the  magmas 
were  still  pasty,  or  long  after  they  had  cooled  and  solidified. 
Certain  rocks,  like  the  gneiss  of  Manhattan  Island,  are  believed  to 
have  been  formed  both  from  the  metamorphism  of  sediments 
and  the  injection  of  igneous  material,  and  thus  to  have  had  a  highly 
complex  origin. 

The  Causes  of  Metamorphism  have  already  been  suggested  in  the 


414  METAMORPHISM 

preceding  paragraphs,  but  it  will  be  well  to  summarize  them,  though 
it  should  be  borne  in  mind  that  the  metamorphic  processes  are  by 
no  means  completely  understood. 

1.  Heat  is  evidently  a  very  important  factor  of  change,  as  is 
shown  by  the  phenomena  of  contact  metamorphism  and  by  numer- 
ous experiments  by  which  the  process  has  been  imitated  success- 
fully.    In  contact  metamorphism  the  heat  is  derived  from  the  igne- 
ous magmas,  and  in  dynamic  it  is  in  part  mechanically  generated, 
in  part  due  to  the  interior  heat  of  the  earth  invading  deeply  buried 
masses. . 

2.  Compression  is  believed  to  be  the  great  agent  ol  dynamic 
metamorphism,  and  the  amount  of  the  change  depends  upon  the 
intensity  of  compression  and  the  depth  at  which  it  operates.     Hence 
the  varying  results,  ranging  from  gentle  folding,  at  one  end  of  the 
series,  through  violent  folding  to  complete  reconstruction,  crystalli- 
zation, and  foliation,  at  the  other. 

3.  Moisture  is  another  potent  agent  of  reconstruction.     Super- 
heated water  under  pressure  is  able  to  attack  and  dissolve  the  most 
refractory  substances  and  to  build  them  up  into  new  combinations. 
Many  minerals,  such  as  the  felspars,  which  have  never  been  arti- 
ficially crystallized  by  dry  heat  alone  will  crystallize  readily  in  the 
presence  of  superheated  water,  and  the  water  lowers  the  tempera- 
ture necessary  for  metamorphism.     Rocks  which  melt  at  2500°  F. 
dry  heat,  become  pasty  at  750°  F.  in  the  presence  of  water.     In 
contact  metamorphism,  steam  is  a  very  important  factor  of  change, 
but  other  vapours  and  gases  play  an  efficient  part. 

4.  Pressure,  as  distinguished  from  active  compression,  is  a  necess- 
ity for  any  extensive  metamorphic  action,  whether  contact  or  dy- 
namic.    It  is  the  difference  of  pressure  which  is  responsible  for  the 
different  effects  of  surface  flows  of  lava  and  of  subterranean  intru- 
sions and  which  gives  to  depth  its  importance  as  a  controlling  factor. 
The  dead-weight  pressure  of  overlying  rocks  prevents  the  rapid  es- 
cape of  the  mineralizing  vapours  ..and,  when  sufficiently  great,  causes 
the  rock  to  shear  and  "flow  without  fracture.     Limestone  heated  at 
the  pressure  of  the  atmosphere,  in  a  lime-kiln  or  an  open  vessel, 


THE   METAMORPHIC   ROCKS  415 

becomes  quicklime  (CaO)  through  the  expulsion  of  CO2,  but  heated 
under  pressure,  so  that  the  gas  cannot  escape,  it  -crystallizes  into 
marble.  Such  pressure,  also,  is  an  essential  factor  in  dynamic 
metamorphism  as  a  precondition  in  enabling  the  rock  to  behave 
more  or  less  plastically  under  active  compression  and  without 
shattering.  Dynamic  metamorphism  must  therefore  take  place 
at  considerable  depths  below  the  surface. 

It  is  believed  by  many  geologists  that  metamorphism  may  pro- 
ceed so  far  as  completely  to  melt  a  sedimentary  rock,  producing 
a  magma  which  is  indistinguishable  from  a  typically  igneous  one. 
Such  extreme  metamorphism  has  not  been  demonstrated  for  any 
considerable  body  of  rocks,  but  may  be  true,  nevertheless,  and  if  so, 
we  should  then  have  the  cycle  of  rock  transformation  complete, 
from  igneous  rock,  through  sedimentary  and  metamorphic,  back 
to  igneous.  Be  this  as  it  may,  certain  metamorphic  rocks  do  un- 
doubtedly form  a  common  meeting  place  for  the  sedimentary  and 
igneous  classes. 

THE  METAMORPHIC    ROCKS 

In  the  scheme  of  classification  it  is  not  yet  practicable  to  separate 
the  metamorphic  rocks  of  igneous  origin  from  those  which  are  trans- 
formed sediments,  for  it  is  often  impossible  to  distinguish  one  from 
the  other. 

A.   NON-FOLIATED  ROCKS 

These  represent  the  less  advanced  stages  of  metamorphism,  in 
which  the  forces  of  compression  may  have  produced  cleavage  or 
fissility,  but  not  foliation.  The  more  important  rocks  of  this 
class  are  of  sedimentary  origin,  and  it  will  be  unnecessary  for  us 
to  consider  the  igneous  rocks  which  have  been  changed,  though 
not  to  the  extent  of  producing  foliation. 

Quartzite  is  derived  from  the  metamorphosis  of  sandstone,  and 
between  the  two  kinds  of  rock  are  found  such  complete  transitions, 


4 1 6  METAMORPHIC   ROCKS 

that  the  separation  of  them  seems  almost  arbitrary.  In  a  typical 
quartzite  the  rock  is  crystalline,  and  the  quartz  deposited  around 
the  sand-grains  is  in  crystalline  continuity  with  those  grains,  though 
the  microscope  still  reveals  the  original  fragmental  nature  of  the 
rock.  Quartzites  also  result  from  the  metamorphism  of  conglom- 
erates, and  the  pebbles  are  sometimes  much  flattened  by  compres- 
sion. If  the  sandstone  or  conglomerate  contained  impurities,  other 
minerals  besides  quartz  are  generated;  if  any  considerable  quantity 
of  clay  was  present,  mica  will  be  produced  and,  it  may  be,  in  such 
abundance  that  the  rock  passes  into  mica  schist  (see  below). 

Quartzites  are  formed  both  in  contact  and  regional  metamor- 
phism, but  the  change  is  principally  due  to  cementation,  large 
amounts  of  silica  (estimated  as  one-sixth  of  the  original  quantity 
present  in  the  sandstone)  being  brought  in  and  deposited  from  solu- 
tion, though  this  cementation  may  be  effected  by  ordinary  perco- 
lating waters  bearing  SiO2  in  solution,  so  that  some  quartzites 
should  hardly  be  regarded  as  metamorphic.  Many  quartzites 
do  not  appear  to  have  been  subjected  to  great  compression,  while 
others  are  cleaved  or  fissile  (Fig.  224). 

Slate  and  Phyllite.  —  Slate  is  a  fine-grained,  dense,  and  hard 
rock,  which,  when  metamorphosed  by  compression,  is  cleaved.  It 
results  from  the  transformation  of  clay  shales,  fine  arkose,  and 
sometimes  of  volcanic  tuffs.  Crushed  fragments  of  felspar  change 
into  interlocking  crystals  of  quartz  and  felspar,  or  quartz  and  mica. 
The  mineral  particles,  both  original  and  newly  developed,  have 
a  parallel  arrangement  of  their  long  axes  and  cleavage  planes, 
which  determines  the  cleavage  of  the  rock.  In  colour,  slates  are 
usually  drab,  or  dull  dark  blue,  but  they  may  be  brick-red,  green, 
or  purple.  When  fine-grained  and  regularly  cleaved,  they  are  ex- 
tensively quarried  for  roofing  purposes.  Great  areas  of  them  occur 
in  Vermont,  eastern  Pennsylvania,  Virginia,  and  Georgia,  south  of 
Lake  Superior,  and  on  the  western  flank  of  the  Sierra  Nevada. 

Phyllite  is  slate  in  a  more  advanced  stage  of  metamorphosis,  in 
which  the  mica  spangles  are  more  abundant,  and  visible  to  the 
naked  eye,  giving  lustrous  surfaces  to  the  cleavage-planes.  Like 


MARBLE  417 

micaceous  quartzite,  phyllite  may  often  be  traced  into  mica 
schist. 

Marble  is  a  metamorphic  limestone,  in  which  the  fragments  and 
particles  of  organic  origin  have  been  converted  into  crystalline 
calcite.  Magnesian  limestones  yield  crystalline  dolomites,  which 
are  likewise  included  under  marble.  In  the  process  of  recon- 
struction, the  fossils  and  even  the  bedding- planes  of  the  original 
limestone  are  usually  entirely  obliterated.  The  grain  of  the  rock 
varies  much,  from  the  fine,  dense,  loaf-sugar-like  statuary  marble 
to  a  very  coarse  texture  of  large  crystals.  Pure  limestone  gives 
rise  to  a  white  marble,  but  the  presence  of  organic  matter  is  be- 
trayed by  veins  of  graphite,  which  may  indicate  the  lines  of  mash- 
ing and  flow,  along  which  the  rock  yielded  to  the  compressing 
force.  Iron  and  organic  matters  present  in  the  limestone  produce 
a  great  variety  of  coloured  and  variegated  marbles,  some  of  which 
are  of  extraordinary  beauty.  The  sand  and  clay  present  in  many 
limestones  will,  on  metamorphosis,  give  rise  to  a  variety  of  silicated 
minerals.  Not  all  crystalline  limestones  are  to  be  called  marbles, 
for  crystallization  may  be  the  work  of  surface  waters  at  ordinary 
temperature,  and  even  modern  coral-rocks  may  be  crystalline. 
Such  non-metamorphic  crystalline  limestones  differ  from  marbles 
in  being  less  hard  and  in  retaining  the  fossils  and  stratification 
planes  which  they  originally  had.  Other  crystalline  limestones, 
like  stalagmite  and  travertine  (see  p.  307),  were  deposited  from 
solution. 

Marble  is  an  exceptional  case  of  a  completely  crystalline  rock 
derived  from  sediments  by  dynamic  metamorphism,  which  is  not 
foliated  or  schistose.  This  is  believed  to  be  due  to  the  capacity 
of  calcite  to  recrystallize  freely  after  it  has  been  subjected  to  com- 
pression and  mashing. 

The  economic  value  cf  the  marbles  makes  them  largely  sought 
after;  in  this  country  they  are  extensively  developed  along  the 
Appalachian  region,  from  Vermont  to  Georgia,  in  the  Rocky  Moun- 
tains, and  the  Sierra  Nevada. 

The  Ophicalcites  are  crystalline  magnesian  limestones  and  dolo- 


41 8  METAMORPHIC  ROCKS 

mites,  with  varying  amounts  of  included  serpentine,  which  gives 
them  a  mottled  appearance.  They  are  not  thoroughly  understood, 
and  it  appears  that  they  may  be  formed  in  various  ways.  Some 
ophicalcites  are  almost  certainly  marbles,  in  which  inclusions  of 
olivine,  pyroxene,  or  hornblende  have  been  formed  and  afterward 
altered  into  serpentine  (see  p.  19).  Others  would  appear  to  be 
broken  and  fissured  serpentines,  having  the  crevices  filled  up  with 
calcite  deposited  from  solution. 

Anthracite  is  usually  regarded  as  a  metamorphic  form  of  coal, 
and,  as  we  have  seen  in  a  preceding  paragraph  of  this  chapter,  it 
is  formed  from  bituminous  coal  by  contact  metamorphism.  On  a 
large  scale  it  occurs  chiefly  in -areas  of  folded  and  disturbed  rocks, 
though  not  invariably  so.  A  more  intense  metamorphism  of  car- 
bonaceous material  gives  rise  to  graphite  (or  black  lead),  a  semi- 
crystalline  form  of  carbon,  which,  however,  is  a  mineral  rather 
than  a  rock. 


B.    FOLIATED  ROCKS 

The  foliated  or  schistose  rocks  are  those  which  are  divided  into 
rudely  parallel  planes,  with  rough  or  undulating  surfaces,  due  to 
the  flakes  and  spangles  of  some  mineral.  The  planes  of  foliation 
may  coincide  with  the  original  bedding-planes  or  they  may  inter- 
sect the  latter  at  any  angle,  just  as  do  the  planes  of  cleavage  and 
fissility.  The  foliated  rocks  represent  the  most  advanced  stage  of 
what  we  can  confidently  call  metamorphism,  and  may  be  derived 
from  either  sedimentary  or  igneous  originals;  it  is  not  always  pos- 
sible to  say  which. 

Gneiss  is  a  term  of  wide  significance,  which  includes  a  number 
of  rocks  of  different  modes  of  origin  and  different  mineralogical 
composition.  It  is  "  a  laminated  metamorphic  rock  that  usually 
corresponds  in  mineralogy  to  some  one  of  the  plutonic  types." 
(Kemp.)  The  varieties  of  gneiss  are  ordinarily  named  in  accord- 
ance with  the  most  conspicuous  dark  silicate  present,  as  Uotite 
gneiss,  hornblende  gneiss,  etc.;  but  this  system  of  nomenclature 


GNEISS 


419 


gives  an  imperfect  notion  of  the  character  of  the  rock.  A  better 
method  has  been  suggested  by  Dr.  C.  H.  Gordon  and  adopted  by 
Professor  Kemp,  though  the  older  scheme  is  still  in  general  use. 
This  is  to  name  the  varieties  in  accordance  with  the  igneous  rocks 
to  which  they  correspond  in  mineralogical  composition;  as  granitic 
gneiss,  syenitic  gneiss,  dioritic  gneiss,  etc.  The  commonest  variety 
is  granitic  gneiss,  with  mica  or  hornblende;  the  orthoclase  and 
quartz  are  mingled  together,  with  conspicuous  laminae  and  folia 
of  the  dark  mineral. 


FIG.  225.—  Plicated  gneiss,  Montgomery  Cotinty,  Pa.     (U.  S.  G.  S.) 
clearly  displays  the  characteristic  foliation 


This  figure 


Most  gneisses  were  generated  by  the  dynamic  metamorphism  of 
granite,  either  before  its  consolidation  or  after  it  had  cooled  and 
hardened.  Some  authorities  deny  that  gneiss  has  ever  been  formed 
from  sedimentary  rocks,  but  there  is  good  reason  to  believe  that 
it  sometimes  has  such  an  origin,  and  in  certain  instances  the  crushed 
pebbles*  of  the  parent  conglomerate  are  still  distinctly  visible, 
especially  on  a  weathered  surface.  Still  another  series  of  these  rocks 


420 


METAMORPHIC   ROCKS 


are  of  complex  origin,  granitic  magmas  being  injected  along  the 
foliation  planes  and  into  all  the  crevices  of  metamorphosed  sedi- 
ments. 

Gneisses  are  widely  spread  in  ancient  formations,  especially  in 
the  most  ancient  of  all,  and  they  cover  vast  areas  in  the  northern 
part  of  North  America. 

The  Crystalline  Schists  are  more  finely  foliated  than  gneiss,  into 
which  they  often  grade  imperceptibly,  having  very  similar  miner- 


FlG.  226.  —  Boulder  of  gneiss,  displaying  its  conglomeratic  nature  on  weathered 
surface.     (International  Boundary  Survey) 


alogical  composition.  They  have  very  diverse  modes  of  origin 
arising  from  both  sedimentary  and  igneous  rocks.  Slates,  impure 
sandstones  and  limestones,  as  well  as  felsites,  andesites,  diabases, 
tuffs,  etc.,  may  all  give  rise  to  crystalline  schists  by  contact  or 
dynamic  metamorphism.  The  varieties  are  named  from  their 
most  important  ferro-magnesian  mineral. 

Quartz  Schist  is  a  foliated  quartzite  in  which  cleavage  or  fissility 
has  developed  into  schistosity.  The  mashing  and  cementation  of 
the  original  sandstone  may  take  place  at  the  same  time,  or  the 


HORNBLENDE  SCHIST  421 

quartzite  may  be  produced  by  the  latter  process  and  subsequently 
converted  into  schist  by  compression. 

Mica  Schist  is  principally  composed  of  quartz,  muscovite,  and 
biotite,  with  more  or  less  felspar.  By  an  increase  in  the  quantity 
of  felspar  present,  and  a  coarser  foliation,  it  grades  into  gneiss, 
and  by  an  increase  of  quartz  it  may  pass  into  quartzite  and  thence 
to  sandstone.  Through  the  phyllites  mica  schists  are  connected 


FlG.  227.  —  Mica  schist  with  garnets.     Nearly  natural  size 

with  the  slates,  and  in  another  direction,  by  increase  of  lime  they 
pass  into  argillaceous  limestones.  Mica  schists  are  very  largely 
exposed  in  New  England  and  southward  along  the  eastern  flank 
of  the  Appalachian  Mountain  system. 

Hornblende  Schist  is  a  foliated  rock,  consisting  of  hornblende 
with  a  varying  proportion  of  felspar  and  less  quartz.  The  horn- 
blende schists  are,  for  the  most  part,  derived  from  the  dynamic 
metamorphism  of  various  basic  igneous  rocks,  the  augite  being 


422  METAMORPHIC  ROCKS 

readily  converted  into  hornblende  by  crushing,  but  in  rare  instances 
they  are  believed  to  have  bad  a  sedimentary  origin.  The  horn- 
blende schists  occur  as  belts  or  bosses  in  metamorphic  areas  and 
are  largely  exposed  around  Lake  Superior.  The  schists  already 
described  are  much  the  most  abundant  members  of  the  group, 
but  there  are  several  others.  Thus,  we  have  talc  and  chlorite 
schists,  both  of  which  are  due  to  alteration,  chiefly  of  hornblende 
schist,  and  graphite  schist,  which  has  quantities  of  that  carbon 
mineral  along  its  foliation  planes. 


CHAPTER  XVII 
MINERAL  VEINS  AND  ORE  DEPOSITS 

MINERAL  veins  and  ore  deposits  are  of  the  greatest  economic 
importance  and  have  therefore  received  a  great  deal  of  attention, 
and  a  very  extensive  literature  has  grown  up  concerning  them. 
Obviously,  but  a  meagre  outline  of  the  subject  can  be  attempted 
in  this  place,  and  the  treatment  of  the  much-disputed  questions  of 
the  modes  of  formation  cannot  be  given  adequately  or  at  length. 

I.  MINERAL  VEINS 

The  crevices,  fissures,  and  faults  which  traverse  hard  rocks 
generally  remain  open  for  a  time  and  are  then  frequently  filled  up 
by  the  deposition  of  material  which  is  quite  different  from  the  coun- 
try rock  of  the  walls.  Fissures  thus  filled  by  crystalline  deposits 
are  mineral  veins,  and  these  vary  greatly  in  dimensions,  from  a  few 
inches  to  many  miles  in  length.  The  minute  veins  are  filled  with 
material  derived  from  the  walls  by  solution  and  redeposited  in  the 
crevices,  such  as  the  veins  of  crystallized  calcite  in  limestone. 
Great  fissure  veins,  on  the  other  hand,  which  may  run  unchanged 
for  many  miles  and  penetrate  to  depths  beyond  the  reach  of  min- 
ing, are  "  characterized  by  regular,  straight  walls,  by  a  fairly 
constant  width,  and  by  a  definite  direction  of  both  strike  and  dip." 
(Spurr.)  Such  veins  are  usually  very  distinctly  marked  off  from 
the  wall  of  country  rock,  and  may  be  either  simple  or  banded, 
with  the  bands  in  general  parallelism  with  the  walls  of  the 
fissure.  In  a  simple  vein  the  mineral  contents  are  deposited  irreg- 
ularly without  any  definite  arrangement,  or  in  a  solid,  homogeneous 

423 


424  MINERAL  VEINS 

mass,  while  the  banded  structure  is  produced  in  several  differ- 
ent ways.  One  of  the  commonest  of  these  ways  is  by  the  deposi- 
tion of  minerals  on  the  walls  of  an  open  fissure,  for  the  more  perfect 
ends  of  the  crystals  project  from  the  walls  toward  the  middle  of  the 
vein,  and  the  bands  are  arranged  usually  in  symmetrical  pairs  from 
the  walls  inward.  In  many  instances  the  symmetrical  arrange- 
ment is  departed  from,  because  a  fissure  once  filled  with  crystalline 
minerals  may  be  again  opened  by  renewed  diastrophic  movements 
and  a  renewed  deposition  take  place,  the  older  vein  forming  the 
walls  of  the  newer  one.  The  parallel  bands  may  be  of  the  same 
mineral,  or  each  pair  may  be  of  a  different  mineral.  Banded  struc- 
ture may  also  be  brought  about  by  movements  of  the  rock  subse- 
quent to  the  filling  of  the  vein,  and  frequently  both  factors  cooper- 
ate to  produce  the  result  in  the  same  vein. 

As  we  have  already  seen  (p.  341),  a  fault  zone  is  often  a  mass  of 
shattered  and  sheared  rock,  consequently  it  is  not  surprising  that 
many  mineral  veins  should  be  highly  complex,  branching  and 
anastomosing  around  the  broken  pieces  of  country  rock.  In  such 
cases  it  is  evident  that  the  deposition  of  the  minerals  has  taken  place 
in  a  broader  or  narrower  zone  of  fault  rock.  The  nature  of  the 
country  rock  itself  often  determines  whether  a  vein  shall  be  simple 
or  complex,  and  the  same  vein  may  be  simple  in  one  part  of  its 
course  and  complex  in  another,  as  the  country  rock  changes  from 
point  to  point,  either  vertically  or  horizontally.  Before  the  deposi- 
tion of  the  mineral  contents,  the  fissure  was  open  in  part  of  its  course, 
where  the  rocks  yielded  easily  to  tension,  while  in  other  parts  the 
crack  was  represented  by  a  mass  of  shattered  rock,  yet  with  abun- 
dant narrow  openings,  through  which  water  could  circulate. 

A  third  class  of  mineral  veins  is  composed  of  the  veins  of  replace- 
ment, in  which  the  circulating  waters  have  not  merely  deposited 
minerals  in  an  open  fissure,  but  have  gradually  substituted  one 
substance  for  another,  by  dissolving  out  the  latter  and  replacing 
it  with  the  former,  it  may  be  molecule  by  molecule,  so.  that  the  re- 
placing minerals  are  pseudomorphs  after  the  older  series  (see  p.  1 1) , 
retaining  the  crystal  form,  sometimes  the  cleavage  of  the  latter. 


MINERAL  VEINS  425 

In  this  way  fossils  may  be  produced  in  newer  minerals,  even  metals. 
A  replacement  vein  represents  a  water  channel  of  some  kind,  and 
so  it  has  a  more  or  less  definite  direction,  but  it  seldom  has  sharply 
defined  walls,  for  the  new  deposits  impregnate  the  country  rock 
and  fade  away  into  it.  Sometimes,  however,  the  replacement  has 
been  so  complete  that  a  vein  results  which  is  at  first  sight  hardly 
distinguishable  from  a  true  fissure  vein,  and  even  a  banded  struc- 
ture may  occur  in  such  veins,  due  to  a  previous  banding  in  the 
rock  which  is  replaced.  This  banding  of  the  rock  may  be  occa- 
sioned by  a  succession  of  shear-planes,  along  which  the  first 
deposition  takes  place,  followed  by  the  replacement  of  the  rock  in- 
cluded between  the  shear-planes,  or  by  the  occurrence  of  bands  of 
more  and  less  soluble  material,  replaced  in  the  order  of  solubility. 

Replacement  veins  are  most  commonly  found  in  limestones, 
since  those  are  the  most  readily  soluble  rocks,  but  they  also 
occur  in  rocks  which  are  relatively  very  insoluble,  such  as  sand- 
stones, and  in  igneous  rocks  like  granite.  The  processes  of  mole- 
cular substitution,  which  are  carried  on  very  slowly,  may  take  place 
where  there  is  a  very  small  amount  of  soluble  material  present. 

Mineral  veins  are  especially  characteristic  of  disturbed,  frac- 
tured, and  dislocated  rocks,  and  are  practically  absent  from  regions 
of  undisturbed  strata.  This  association  is  what  we  should  expect 
to  find,  for  deep  fissures  are  to  be  found  only  among  rocks  which 
have  been  more  or  less  violently  shifted  by  diastrophism  or  by 
igneous  intrusions.  Such  intrusions  are  very  favourable  to  the 
formation  of  mineral  veins,  and  many  veins  may  be  traced  directly 
into  plutonic  bodies,  and  others  are  clearly  results  of  contact 
metamorphism. 

The  substances  which  are  found  in  mineral  veins  vary  widely, 
in  accordance  with  the  mode  of  formation,  and  in  the  same  vein 
may  differ  greatly  from  point  to  point.  Sometimes,  though  not 
always,  the  character  of  the  country  rock  exercises  a  controlling 
influence  upon  the  contents  of  the  vein,  which  change  as  the  rock 
traversed  changes.  Among  the  commonest  and  most  widely 
disseminated  of  vein  minerals  are  quartz,  calcite,  and  barite  (heavy 


426  MINERAL   VEINS 

spar,  BaSO4).  Frequently  the  ores  of  the  commercially  valuable 
metals  are  found  in  mineral  veins,  which  then  are  called  metal- 
liferous veins.  The  minerals  which  fill  up  most  of  the  vein,  such 
as  quartz,  calcite,  etc.,  form  what  is  called  the  vein  stu/,  organgue, 
and  in  the  latter  the  ores  may  be  disseminated  in  fine  particles,  or 
gathered  in  threads,  pockets,  or  nuggets,  sometimes  in  the  native, 
or  uncompounded  state,  but  much  more  frequently  as  sulphides, 


FIG.  228.— Dykes  of  sandstone  in  shales,  Northern  California.     (U.  S.  G.  S.) 

oxides,  carbonates,  and  other  combinations.  Mineral  veins  may 
thus  be  regarded  as  a  special  case  of  ore  deposits,  and  the  mode  of 
their  formation  can  most  conveniently  be  discussed  in  connection 
with  the  latter. 

Sediment-filled  Veins,  though  belonging  in  an  entirely  different 
category  from  true  mineral  veins,  may  be  briefly  mentioned  here. 


ORE   DEPOSITS  42; 

Vertical  fissures  are  sometimes  filled  up  by  sediment  washed  in 
from  above,  but  more  remarkable  are  the  instances  where  the  fis- 
sure was  evidently  filled  from  below  with  sediment  different  from 
the  walls.  In  Fig.  229  is  seen  an  example  from  northern  California: 
the  fissures  which  traverse  the  shale  have  been  filled  with  sand, 
which  has  consolidated  into  firm  sandstones  and,  as  they  resist 
weathering  better  than  the  enclosing  soft  shales,  they  stand  out  in 
relief.  These  are  called  sandstone  dykes,  though  they  are  not  true 
dykes,  which  are  always  of  igneous  rocks.  The  explanation  of 
these  curious  structures  is  given  by  many  modern  earthquakes, 
notably  the  great- Indian  quake  of  1897  (see  p.  42).  It  will  be 
remembered  that  on  that  great  disturbance  the  ground  opened 
in  innumerable  fissures,  through  which  "  astounding  quantities  " 
of  sand  and  water  were  discharged.  Not  all  the  fissures  communi- 
cate with  the  surface,  and  if  the  superficial  rocks  rest  upon  uncon- 
solidated  beds  of  sand,  the  sand  will  be  forced  upward  into  any 
cracks  that  may  be  formed,  as  bore-holes  are  sometimes  clogged 
at  considerable  depths  with  clay  squeezed  into  them  by  the  pressure 
of  the  overlying  rock. 

II.    ORE  DEPOSITS 

The  term  ore  implies  an  economic  conception  and  means  a 
source  of  supply  of  a  metal  which  can  be  profitably  worked,  hence 
the  proportion  of  the  metallic  constituent  which  must  be  present 
for  profitable  working  depends  very  largely  upon  the  price  of  the 
metal.  Iron  ore,  ready  for  the  blast-furnace,  must  have  at  least 
35  %  of  the  metal,  while  a  3  %  ore  of  copper  may  be  employed. 
The  table  of  the  elements  which  chiefly  make  up  the  accessible 
parts  of  the  earth's  crust  (see  p.  6)  shows  that  the  only  commer- 
cially important  metals  which  are  among  the  first  eight  elements 
are  aluminium  and  iron,  while  the  other  metals  form  but  an  ex- 
cessively small  proportion  of  the  crust.  It  has  been  estimated  that 
lead,  tin,  and  zinc  form  some  hundred-thousandths  of  a  per  cent 
each,  copper  is  in  the  hundred-thousandths  or  millionths  of  a  per 


428  ORE  DEPOSITS 

cent,  silver  a  tenth  or  a  hundredth  as  much  as  copper,  and  gold 
one  tenth  as  much  as  silver.  (Vogt.)  Infinitesimal  as  these  pro- 
portions seem,  the  metals  are  very  widely  disseminated  in  the  rocks, 
and  the  processes  of  ore  deposition  are  therefore,  above  all,  pro- 
cesses of  concentration,  by  which  the  scattered  particles  of  the  metal- 
lic compounds  are  brought  together  in  relatively  large  quantity. 

The  variety  of  ore  deposits,  regarded  from  the  standpoint  either 
of  their  contents,  their  mode  of  formation,  or  the  rocks  in  which 
they  are  found,  is  excessively  great,  and  no  classification  of  them 
is  satisfactory.  All  that  can  be  attempted  here  is  a  description  of 
some  of  the  commoner  and  more  typical  kinds  of  ore  deposits, 
with  a  brief  discussion  of  the  problems  concerning  their  origin, 
problems  which  are  still  very  far  from  definitive  solution. 

Stratified  Ore  Deposits  are  usually  of  iron,  or  less  commonly 
manganese,  and  occur  in  beds  interstratified  with  other  rocks. - 
The  ores  themselves  may  be  found  in  continuous  sheets,  thick  beds, 
or  scattered  nodules,  and  were  evidently  deposited  from  solution 
in  water,  like  the  bog  and  lake  ores  which  are  now  in  process 
of  formation.  Very  frequently  bedded  ores  of  iron  are  found 
among  highly  metamorphic  rocks,  especially  the  crystalline 
schists.  Placers  are  river  gravels  which  contain  grains,  or  nug- 
gets of  heavy  metals,  such  as  gold,  platinum,  or  tin  oxide  (stream 
tin).  They  are  due  to  the  concentration  of  the  metallic  particles, 
originally  scattered  through  veins  or  rocks  by  erosion  and  stream 
transportation,  and  owing  to  their  high  specific  gravity  the  metallic 
particles  are  thrown  down  where  gravel  is  deposited.  The  strati- 
fied ore  deposits  thus  offer  no  particular  difficulty  of  explanation. 

Ores  due  to  Magmatic  Segregation. — In  our  study  of  the  igneous 
rocks,  we  learned  that  deep-seated  molten  masses  in  the  slow  process 
of  cooling  and  consolidation  frequently  undergo  differentiation, 
so  that  different  parts  of  the  same  continuous  magma  consolidate 
into  rocks  of  very  different  composition  (see  p.  290).  Many  basic 
rocks  contain  considerable  quantities  of  metals,  and  there  is  good 
reason  to  believe  that  by  segregation  these  metallic  constituents  may 
be  so  concentrated  as  to  form  ore  bodies.  The  commonest  ores 


ORES  DUE  TO  CONTACT  METAMORPHISM  429 

which  are  referred  to  this  mode  of  origin  are  magnetic  iron  oxide, 
generally  containing  titanium,  such  as  those  of  the  Adirondack 
Mountains,  New  York,  and  many  other  regions.  Iron  sulphides 
containing  nickel  in  paying  quantities  occur  in  Pennsylvania  and 
Canada,  and  nickeliferous  olivines  in  Oregon,  all  of  which  are  re- 
garded as  due  to  magmatic  segregation.  Chromite,  the  oxide  of 
iron  and  chromium,  also  forms  ore  bodies  of  probably  similar 
origin,  and  the  great  body  of  zinc  ores  at  Franklin  Furnace,  New 
Jersey,  has  been  referred  to  the  same  category. 

Ores  due  to  Contact  Metamorphism.  —  When  the  country  rock, 
which  is  invaded  by  a  plutonic  mass,  is  of  a  kind  that  permits  ex- 
tensive penetration  by  the  magmatic  vapours  and  gases,  metamor- 
phism  may  result  for  a  considerable  distance  from  the  intruding 
igneous  rocks.  Among  the  new  minerals  which  are  generated 
along  the  contact  zone,  metallic  ores  may  occur  in  sufficient  quan- 
tity to  be  economically  important.  The  minerals  in  question  may 
be  deposited  in  the  interstices  of  the  wall  rock,  or  may  replace  that 
rock  bodily  for  a  greater  or  less  distance  from  the  actual  contact. 
Ore  bodies  formed  in  this  manner  are  usually  characterized  by 
the  presence  of  garnets,  oxides  and  sulphides  of  iron  in  association, 
and  by  fluorite  and  other  minerals  containing  fluorine  and  boron. 
As  we  have  seen  (p.  409)  there  is  a  difference  of  opinion  among 
geologists  as  to  how  far  new  mineral  substances  can  be  carried 
into  the  walls  of  country  rock,  but  such  a  case  as  the  Dolcoath 
mine  in  Montana,  more  than  half  a  mile  from  the  contact  with  the 
granite,  which  has  been  the  chief  agent  in  metamorphosing  the 
district,  is  highy  suggestive.  "The  ore-bearing  stratum  of  the 
mine  was  originally  a  bed  of  impure  limestone,  which  has  been 
metamorphosed  to  garnet  and  pyroxene,  with  spots  of  calcite. 
Associated  with  these  gangue  minerals  are  sulphide  and  telluride 
of  bismuth,  containing  gold."  (Spurr.) 

Metalliferous  Veins  (or  Lodes).  — These  are  particular  varieties 
of  mineral  veins,  the  principal  characters  of  which  have  been  given 
in  the  preceding  section.  Metalliferous  veins  are  no  exception  to 
the  rule  that  subterranean  activities  are  not  well  understood, 


43O  ORE  DEPOSITS 

and  among  students  of  the  subject  there  are  many  and  strong  differ- 
ences of  opinion  concerning  the  mode  of  formation  of  such  veins. 
However,  there  is  general  agreement  that  the  contents  of  veins,  both 
gangue  and  ores,  have  been  deposited  from  solution  in  thermal  waters 
and  vapours,  just  as  certain  existing  hot  springs  are  making  similar 
deposits  now  (see  p.  194).  The  first  requisite  for  the  formation  of  a 
lode  is  a  water  channel,  because  the  metals  are  present  in  minute 
quantities,  and  immense  quantities  of  water  must  pass  before  any 
considerable  deposit  can  be  accumulated.  Hence,  ore  deposits 
are  found  in  fissures,  shattered  rock-masses,  in  joints,  in  porous 
and  soluble  strata,  where  water  may  pass  with  comparative  free- 
dom, and  further  these  waterways  must,  directly  or  indirectly, 
communicate  with  great  depths,  or  with  highly  heated  rocks, 
permitting  supplies  of  hot  water  to  reach  them. 

There  is  no  general  agreement  as  to  the  source  of  the  waters 
that  have  filled  the  veins  with  gangue  and  ores.  Perhaps  the 
majority  of  geologists  incline  to  the  opinion  that  such  waters  are 
meteoric,  i.e.  of  atmospheric  origin,  and  that  the  waters  descending 
through  the  rocks  dissolve  the  metallic  and  other  minerals  and  pene- 
trate to  great  depths  until  they  become  highly  heated  and  rise  again 
through  fissures.  As  the  waters,  thus  charged  with  ore  and  gangue 
minerals  in  solution,  ascend  to  the  cooler  layers  nearer  the  surface, 
they  are  chilled  and  precipitate  the  greater  part  of  the  dissolved 
substances  along  the  waterways.  An  alternative  view,  which  seems 
to  be  better  founded,  is  that  the  solvent  hot  waters  are  largely  of 
magmatic  origin ;  that  is  to  say,  that  they  are  derived  from  the 
immense  quantities  of  superheated  steam  which  impregnate  the 
igneous  magmas.  How  vast  is  the  amount  of  this  water,  is  shown 
us  by  every  great  volcanic  eruption,  but  the  slowly  solidifying  plu- 
tonic  bodies  must  give  off  their  steam  much  more  gradually.  With 
the  highly  heated  ascending  magmatic  waters  are  doubtless  mingled 
a  greater  or  less  proportion  of  meteoric  waters,  varying  in  amount 
according  to  local  circumstances. 

The  views  held  concerning  the  origin  of  the  ore  substances 
themselves  are  similarly  divergent.  The  hypothesis  that  the  solvent 


METALLIFEROUS   VEINS  431 

waters  are  mainly  of  meteoric  origin  seems  to  involve  the  conclusion 
that  the  metallic  minerals  are  dissolved  out  of  the  rocks  through 
which  the  waters  descend,  while  the  magmatic  hypothesis  finds  the 
source  of  the  metals  in  the  plutonic  masses.  For  lack  'of  space,  it 
is  impracticable  to  present  here  the  evidence  for  and  against  these 
conflicting  opinions;  it  must  suffice  to  point  out  that  the  exceptional 
occurrence  of  the  metalliferous  veins  and  the  nearly  or  quite  uni- 
versal association  of  igneous  rocks  with  such  veins,  seem,  in  the 
present  state  of  knowledge,  to  lend  greater  probability  to  the  mag- 
matic hypothesis.  Obviously,  however,  no  definitive  conclusion 
is  yet  possible. 

Secondary  Enrichment  of  Veins.  —  The  outcrop  of  a  mineral 
vein  is  much  altered  by  weathering ;  the  depth  to  which  this  altera- 
tion penetrates  is  determined  by  the  level  of  the  ground  water. 
For  example,  in  the  deeper  portion  of  many  gold-bearing  veins  the 
gold  is  contained  in  crystals  of  pyrite,  while  above  the  ground- 
water  level,  in  the  shell  of  weathering,  the  gold  is  scattered  in 
minute  threads  and  grains  of  native  metal  through  a  mass  of  more 
or  less  shattered  quartz,  which  is  stained  rusty  red  or  brown,  and  the 
pyrite  has  disappeared.  Pyrite,  when  exposed  to  air  and  water, 
is  slowly  converted  into  the  soluble  ferrous  sulphate  (FeSO4), 
which  in  turn  is  oxidized  into  limonite,  with  liberation  of  sulphuric 
acid.  Iron  is  an  important  constituent  of  most  ore  bodies,  and  its 
concentration  and  deposition  below  the  surface  as  limonite  or 
haematite  forms  the  gossan,  or  iron  hat  of  mining  phraseology. 

In  many  veins  the  process  of  weathering  results  in  the  formation 
of  a  zone  of  secondarily  enriched  sulphides.  The  unaltered  ores 
in  the  depths  of  the  vein  are  sulphides,  but  from  the  surface  to  the 
ground-water  level  they  are  oxidized,  and  below  the  zone  of  oxida- 
tion is  found  that  of  the  secondary  sulphides,  which,  when  present, 
is  apt  to  be  much  richer  than  the  deeper  portions  of  the  vein,  be- 
cause it  represents  an  additional  stage  of  concentration.  The 
metals  are  dissolved  in  the  oxidized  portion  of  the  vein  by  percolat- 
ing waters,  carried  downward  and  substituted  for  part  of  the 
iron  in  the  original  sulphides  below. 


432  ORE  DEPOSITS 

Ore  Deposits  formed  by  Surface  Waters.  —  The  ore  bodies  of  this 
class  are  formed  by  the  concentration  of  the  metals  disseminated 
in  the  rocks,  through  solution  and  deposition  by  surface  waters. 
Such  deposits  are  made  not  far  from  the  surface,  to  which  they  show 
a  definite  relation,  and  disappear  downward.  The  most  abundant 
ores  of  this  class  are  those  of  iron  and  are  exemplified  in  the  famous 
Lake  Superior  region. 

Many  of  the  ores  of  lead  and  zinc,  which  also  occur  in  veins, 
seem  to  be  referable  to  this  class,  though  the  bodies  have  no  definite 
relation  in  form  to  the  surface.  Such  ores  occur  in  limestones, 
in  crevices,  along  joint  or  bedding-planes,  in  cavities,  or  by  replace- 
ment of  the  country  rock,  and  appear  to  have  no  connection  with 
any  fissures  rising  from  great  depths,  nor  with  intrusive  masses  of 
igneous  rock.  The  mode  of  formation  of  these  ore  bodies  has  been 
the  subject  of  much  discussion  and  is  not  yet  entirely  clear,  but  in 
the  case  of  the  upper  Mississippi  Valley,  for  example,  it  is  very 
generally  believed  that  the  deposition  has  been  accomplished  by  de- 
scending and  circulating  waters  from  the  surface  which  have  dis- 
solved and  concentrated  the  metallic  sulphides  originally  dissemi- 
nated thinly  through  the  limestones.  The  disseminated  sulphides 
are  supposed  to  have  been  deposited  in  the  limestones  at  the  time 
the  latter  were  accumulating  in  a  great  inland  sea,  being  brought 
in  solution  from  the  land.  There  is  ground  for  believing  that  lead 
is  but  one  member  of  a  series  of  radio-active  elements,  and,  if  this 
is  true,  we  shall  be  unable  to  determine  which  of  these  elements  was 
the  one  actually  deposited  in  that  ancient  sea. 

Summary  of  Structural  Geology.  —  Structural  geology  brings 
vividly  before  us  the  innumerable  changes  through  which  the 
earth's  surface  has  passed,  and  which  are  recorded  in  the  rocks. 
The  sedimentary  rocks,  originally  laid  down  under  water  in  approxi- 
mately horizontal  positions,  have  been  upheaved  into  land  surfaces, 
either  without  losing  that  horizontality,  or  being  tilted,  folded,  com- 
pressed, or  even  violently  overturned.  Or,  they  may  be  fractured 
and  dislocated  in  great  faults  and  thrusts.  These  movements 


SUMMARY  OF  STRUCTURAL  GEOLOGY  433 

we  have  found  to  be  due  to  enormous  lateral  compression  set  up 
within  the  crust  of  the  earth,  a  compression  generated  in  some 
manner  not  yet  clearly  understood.  Whether  folding  or  faulting 
shall  result  from  a  given  compression  depends  upon  the  rigidity 
of  the  strata,  upon  the  load  which  overlies  them,  and  the  sudden  or 
gradual  way  in  which  compression  is  applied.  The  results  of  com- 
pression on  a  large  scale  are  accompanied  by  certain  minor  changes 
not  less  characteristic.  Compressed  rocks  are  cleaved,  fissile, 
or  schistose,  according  to  the  intensity  of  the  action,  and  whether 
the  rocks  affected  are  in  the  shell  of  flowage  or  of  fracture.  These 
changes  may  go  so  far  as  completely  to  reconstruct  the  minerals  of 
the  rocks,  destroying  the  old,  generating  new,  and  obliterating  the 
original  character  of  the  strata.  Thus,  displacements,  dislocations, 
cleavage,  fissility,  and  dynamic  metamorphism  are  but  the  varying 
results  of  lateral  compression,  acting  under  different  conditions  and 
at  varying  depths. 

Another  class  of  rocks  —  the  igneous,  massive,  or  unstratified  — 
we  found  to  have  penetrated  and  overflowed  the  strata,  and  to  have 
consolidated  in  the  fissures  and  cavities  which  they  have  made  for 
themselves,  or  to  have  been  poured  out  freely  on  the  surface. 
According  to  the  circumstances  under  which  these  masses  have 
cooled,  the  resulting  rock  is  of  glassy,  porphyritic,  finely  or  coarsely 
crystalline  texture.  When  solidified  as  sheets  or  dykes,  the  igneous 
rocks  may  be  folded,  faulted,  cleaved,  or  metamorphosed  like  the 
strata,  and  when  a  region  has  been  long  and  repeatedly  subjected 
to  compression,  its  structure  may  become  excessively  complex, 
and  the  metamorphosis  of  its  rocks  so  complete  that  not  even  the 
most  careful  examination  will  suffice  to  distinguish  those  rocks 
which  were  originally  sedimentary  from  those  which  were  igneous. 

Highly  heated  waters  circulating  through  fissures  and  along  the 
joint-planes  of  the  rocks  deposit  the  substances  which  form  the 
mineral  and  metalliferous  veins,  though  concerning  the  source  of 
these  substances  and  of  the  solvent  waters  there  is  much  difference 
of  opinion. 

Our  study  has  taught  us  that  many  of  these  processes  go  on 


434  ORE  DEPOSITS 

deep  within  the  earth's  crust,  and  hence  cannot  be  directly  ob- 
served, but  must  be  inferred  from  their  results.  Encouraging 
progress  has  already  been  made  in  this  work,  but  very  much  more 
remains  to  be  done  before  our  knowledge  of  structure  and  its  full 
meaning  shall  be  even  approximately  complete. 


PART    III 

GEOMORPHOLOG  Y 

CHAPTER  XVIII 
THE   GEOGRAPHICAL   CYCLE 

GEOMORPHOLOGY,  or  physiography,  is  the  study  of  the  topo- 
graphical features  of  the  earth,  and  of  the  means  by  which,  and  the 
manner  in  which,  they  have  been  produced.  In  this  country  the 
term  physiography,  or  physiographical  geology,  is  firmly  established 
and  very  widely  used.  This  is  unfortunate,  because  the  term  was 
originally  proposed  and  still  continues  to  be  employed  in  a  very 
different  sense.  It  would  be  an  advantage  in  clearness  and  pre- 
cision of  nomenclature,  if  Geomorphogeny,  which  is  extensively 
made  use  of  in  Germany,  could  be  substituted. 

This  subject  is  primarily  a  department  of  physical  geography, 
but  is  of  value  to  the  geologist  for  the  light  which  it  throws  upon 
the  historical  development  of  the  land  surfaces,  and  upon  features 
of  the  past  which  are  not  recorded  in  the  processes  of  sedimenta- 
tion. The  geographer  endeavours  to  explain  the  topographical 
forms  of  the  land,  and,  in  order  to  do  this,  he  must  show  how 
those  forms  have  originated.  The  geologist,  on  the  other  hand, 
makes  use  of  the  topography  to  determine  what  changes  have 
passed  over  the  land,  and  in  what  order  those  changes  have  oc- 
curred. The  old  method  of  reading  geological  history  concerned 
itself  merely  with  the  sedimentary  accumulations  and  igneous 
intrusions.  This  method  has  the  defect  of  leaving  us  without 
information  regarding  the  changes  of  land  surfaces  (except  where 

435 


436  THE  GEOGRAPHICAL  CYCLE 

transgressions  of  the  sea  are  recorded  in  unconformities)  and  the 
details  of  mountain-making.  The  physiographical  method  sup- 
plements this  by  adding,  in  part,  the  required  information  con- 
cerning the  land  surfaces.  Each  method  is  improved  and  strength- 
ened when  we  use  both  of  them  together,  and  when  we  are  able 
to  correlate  the  accumulations  of  sediments  with  the  denuding 
processes  which  furnished  the  material. 

The  topography  of  any  land  area  may  be  considered  as  the 
outcome  of  a  struggle  between  two  opposing  sets  of  agencies: 
(i)  those  which  tend  to  upheave  the  region  and  thus  increase  its 
elevation;  (2)  those  which  tend  to  cut  down  the  land  in  one  place 
and  build  it  up  in  another.  The  latter  comprise  the  agencies 
of  degradation  and  aggradation  respectively,  while  the  former  are 
the  diastrophic  agencies. 

The  details  of  topography  are,  in  large  degree,  controlled  by 
still  a  third  class  of  factors,  which,  however,  are  passive  rather 
than  active;  namely,  the  character,  arrangement,  and  attitude  of 
the  rock  masses.  A  partially  degraded  region  in  which  the  rocks 
are  homogeneous  will  have  a  very  different  kind  of  relief  from  one 
in  which  the  rocks  are  heterogeneous  and  differ  materially  in  their 
powers  of  resistance  to  the  denuding  agents.  A  region  of  hori- 
zontal strata  will  give  rise  to  very  different  topographical  forms 
from  those  which  are  developed  in  areas  of  folded  or  tilted  strata. 
We  must  further  distinguish  between  regions  whose  topography  is, 
in  the  main,  due  to  constructive  processes  and  those  in  which 
denudation  has  prevailed.  Examples  of  such  constructive  forms 
are  volcanic  mountains,  and  plains  or  plateaus  formed  by  widely 
extended  lava  flows,  plains  newly  deserted  by  the  sea  and  due  to 
sedimentation,  alluvial  plains  of  rivers,  and  the  mounds,  ridges, 
or  sheets  of  drift  spread  out  by  the  action  of  glaciers  and  of  the 
waters  derived  from  their  melting.  Still  another  important  kind 
of  topography  is  the  tectonic,  in  which  the  main  features  have  been 
determined  by  tectonic  processes,  more  or  less  modified  by  sub- 
sequent denudation;  the  ridges  are  anticlines  and  the  valleys 
synclines,  while  fault  scarps  may  form  long  lines  of  cliff. 


THE  GEOGRAPHICAL  CYCLE  437 

The  topography  of  any  region  is,  as  we  have  seen,  the  resultant 
of  the  very  complex  interaction  of  many  different  kinds  of  factors, 
and  is  subject  to  continual  change  according  to  definite  laws. 
Let  us  suppose,  in  the  first  instance,  a  region  newly  upheaved 
from  beneath  the  sea  into  dry  land.  The  topography  of  such  an 
area  will  be  constructional,  due  entirely  to  the  processes  of  dia- 
strophism  and  accumulation,  and  characterized  by  the  absence  of 


FlG.  229.  —  Volcanic  topography,  northern  Arizona.     (U.  S.  G.  S.) 

a  highly  developed  system  of  drainage  by  streams.  The  coastal 
plain  of  the  middle  and  southern  Atlantic  States  is  an  example  of 
such  topography  but  slightly  modified. 

Next,  the  processes  of  denudation  begin  their  work  upon  the 
region.  The  sea  attacks  the  coast-line  by  cutting  it  back  in  one 
place  and  building  it  out  in  another,  until  a  condition  of  equilib- 
rium is  attained.  Rivers  are  established,  adjusting  themselves  to 
the  structure  of  the  underlying  rocks,  and  cutting  deep,  trench-like 


438  THE  GEOGRAPHICAL  CYCLE 

valleys,  while  the  atmospheric  agencies  widen  out  the  valleys, 
slowly  wearing  down  and  washing  away  the  sides  and  tops  of  the 
hills.  This  is  the  stage  in  which  we  find  the  greatest  degree  and 
variety  of  relief,  and  it  may  be  called  the  stage  of  maturity,  as 
contrasted  with  the  first,  which  is  a  stage  of  youth.  The  continu- 
ance of  the  degrading  operations  will,  if  uninterrupted,  eventually 
wear  down  the  region  to  a  nearly  plane  surface,  through  which 


FlG.  230.  —  Glacial  topography,  eastern  Washington.     (U.  S.  G.  S.) 

sluggish  streams  meander,  the  featureless  condition  of  old  age. 
When  the  process  is  complete,  the  country  is  said  to  be  base- 
levelled'. 

The  conception  of  the  cycle  of  topographical  development  is 
essential  in  geomorphological  reasoning.  Each  cycle  begins  with 
the  uplift  of  an  area  approximately  at  base  level,  the  processes 
of  denudation  working  with  minimum  efficiency  and  extreme 
slowness.  The  movement  of  upheaval  revivifies  the  destructive 


THE  GEOGRAPHICAL  CYCLE  439 

agencies,  and  the  work  of  carving  out  a  surface  of  relief  begins 
afresh,  only  to  terminate,  unless  interrupted  by  renewed  elevation, 
in  once  more  base-levelling  the  region.  A  complete  cycle  is  thus 
from  base  level  back  to  base  level,  though,  as  it  is  a  cycle,  a  be- 
ginning may  be  selected  at  any  part  of  it. 

The  details  of  the  cycle  differ  widely  under  different  climatic 
conditions.  If  we  take  the  successive  stages  as  they  are  developed 
in  a  pluvial  climate,  with  all  basins  filled,  abundant  rivers  running 
to  the  sea,  and  all  the  snow  of  winter  melting  in  summer,  to  con- 
stitute the  normal  cycle,  we  shall  find  that  the  arid  cycle  of  desert 
climates  deviates  from  the  normal  in  very  important  ways. 

The  term  age  as  applied  to  topographical  features  does  not 
mean  the  length  of  time  required  for  their  formation,  but  merely 
the  stage  of  development  within  the  cycle  which  they  have  at- 
tained. The  length  of  time  required  to  reach  a  given  stage  of  such 
development  will  vary  greatly  in  different  regions,  in  accordance 
with  climatic  conditions,  the  resistance  of  the  rocks,  their  altitude 
above  sea-level,  and  similar  factors.  An  area  of  resistant  rocks 
in  an  arid  climate  will  be  hardly  at  all  affected  in  the  time  that  a 
mass  of  soft  rocks  exposed  to  a  heavy  rainfall  will  be  cut  down  to 
base-level. 

It  seldom,  if  ever,  happens  that  the  topographical  development 
of  a  region  proceeds  uninterruptedly  through  the  stages  of  youth, 
maturity,  and  old  age.  Oscillations  of  level  introduce  new  con- 
ditions and  cause  the  work  of  denudation  to  start  afresh  with  re- 
newed energy,  or,  if  the  movement  be  one  of  depression,  it  will 
check  the  work  already  in  progress.  The  cycles  of  development 
are  thus  partial  rather  than  complete,  and  a  given  region  may 
display  topographical  forms  dating  from  very  different  and  widely 
separated  cycles.  The  more  resistant  rocks  retain  the  features 
acquired  in  an  earlier  cycle,  while  the  weaker  and  more  destructible 
rocks  have  already  taken  on  the  forms  due  to  a  later  cycle.  A 
landscape  thus  often  includes  features  of  different  geological  dates, 
and  it  is  in  the  identification  of  these  that  the  value  of  the  physio 
graphical  method  to  historical  geology  consists. 


440  THE  GEOGRAPHICAL  CYCLE 

In  the  production  of  new  topographical  forms,  old  ones  are 
more  or  less  completely  destroyed,  and  thus,-  the  farther  back  in 
time  we  go,  the  fewer  subdivisions  are  recognizable,  and  only  the 
outlines  of  the  great  cycles  can  be  followed.  Very  ancient  features 
would  be  quite  obliterated  in  the  successive  cycles  of  develop- 
ment, were  they  not  sometimes  buried  under  the  sediments  of  an 
encroaching  sea.  A  subsequent  reelevation  of  the  area  into  land, 
and  a  stripping  away  of  the  covering  of  newer  sediments  by  the 
agencies  of  denudation,  will  again  bring  to  light  the  ancient  land 
surface  which  had  been  buried  for  ages.  An  interesting  example 
of  this  is  presented  by  the  Charnwood  Forest  in  England,  where 
an  extremely  ancient  landscape  is  slowly  coming  to  light,  as  the 
covering  of  soft  rocks,  which  has  so  long  preserved  it,  is  removed 
by  denudation. 

In  Part  I  we  have  already  studied  the  agencies  of  denudation, 
but  there  we  concerned  ourselves  principally  with  their  modes  of 
operation  and  their  efficiency  in  destroying  old  rocks  and  in  fur- 
nishing material  for  the  construction  of  new.  We  have  now  to 
consider  these  agencies  from  a  somewhat  different  point  of  view; 
to  determine  the  characteristic  forms  of  land  sculpture  which  they 
produce  at  the  various  stages  of  their  work. 

The  Sea.  —  The  work  of  the  sea  is  confined  to  the  coast-line, 
which  it  cuts  back  by  the  impact  of  its  waves  and  currents.  Speak- 
ing broadly,  the  waves  do  but  little  effective  work  below  the  limits 
of  low  tide,  and  advance  by  undermining  and  cutting  down  the 
cliffs  which  form  the  coast.  The  result  of  the  work  is  to  form 
a  platform  covered  by  shallow  water,  which  is  called  a  plain  of 
marine  denudation.  As  observed  in  actual  cases,  these  platforms 
are  narrow;  for  so  long  as  the  sea-level  remains  constant  with 
reference  to  the  land,  there  is  a  limit  to  the  effective  assault  of 
the  waves  upon  the  shore.  The  water  covering  the  platform  is 
very  shallow,  and  only  in  exceptional  cases  do  the  waves  have 
sufficient  power  to  overcome  the  friction  of  a  wide  platform.  The 
materials  removed  from  the  land  are  piled  up  at  the  seaward  foot 
of  the  platform  and  extend  it  in  that  direction. 


THE  SEA  441 

An  example  of  a  plain  of  marine  denudation  is  found  on  the 
north  coast  of  Spain,  where  there  is  a  broad  platform  between  the 
mountains  and  the  sea,  almost  perfectly  flat.  This  plain  has  been 
uplifted  above  the  sea-level  and  has  been  but  little  dissected  by  the 
subaerial  agents.  Narrower  platforms,  still  in  process  of  exten- 
sion, may  be  observed  on  most  rocky  and  precipitous  coasts,  as 
those  of  Scotland,  Ireland,  and  France.  Along  a  slowly  sinking 
coast  the  platforms  may  be  cut  back  much  farther,  for  the  deep- 
ening water  prevents  the  loss  of  wave  power  by  the  friction  on  a 
shoal  bottom.  If,  on  the  other  hand,  the  coast  rises  at  intervals, 
a  series  of  terrace-like  platforms  will  be  cut. 

As  we  shall  see  in  the  following  section,  plains  may  be  produced 
by  the  work  of  the  subaerial  agencies,  and  it  is  often  important  to 
distinguish  between  the  plains  of  submarine  and  those  of  subaerial 
origin.  This  distinction  cannot  always  be  made  with  certainty, 
but  not  unfrequently  the  plain  shows  unmistakable  signs  of  the 
manner  in  which  it  was  made.  In  the  plain  of  marine  denudation 
the  sediments  formed  from  the  waste  of  the  land  will  be  deposited 
upon  the  seaward  portion  of  the  platform,  or  upon  a  lower  level 
of  previous  formation.  Further,  this  sediment  will  show  by  its 
character  that  it  actually  was  derived  from  the  material  cut  away 
by  smoothing  the  plain,  and  the  whole  of  it,  even  its  bottom 
layers,  will  be  of  marine  origin.  In  such  a  plain  the  advancing 
sea  must  have  obliterated  the  stream  valleys  which  had  been 
excavated  when  the  region  was  land.  This  obliteration  will  be 
performed  partly  by  shaving  down  the  divides,  or  watersheds, 
between  the  streams  and  partly  by  filling  up  the  valleys  with 
sediment. 

When  the  region  is  once  more  uplifted  above  the  level  of  the 
sea,  an  entirely  new  system  of  drainage  will  be  established  upon 
it,  determined  by  the  slopes  of  the  overlying  cover  of  newly  de- 
posited sediments,  and  having  no  reference  to  the  structure  and 
arrangement  of  the  underlying  older  rocks.  These  newly  estab- 
lished streams  may,  if  the  upheaval  of  the  country  gives  them 
sufficient  fall,  cut  down  through  the  newer  sediments.  Indeed, 


442  THE  GEOGRAPHICAL  CYCLE 

the  latter  may  eventually  be  swept  away  entirely  by  the  various 
subaerial  agencies,  but  the  stream  courses,  which  were  determined 
originally  by  the  slopes  of  that  newer  sediment,  will  show  little  or 
no  adjustment  to  the  structure  of  the  underlying  older  rocks. 

These  criteria  are  useful  in  identifying  those  plains  which  were 
smoothed  by  the  action  of  the  sea;  but  when  the  processes  of  sub- 
aerial  denudation  have  completely  dissected  the  elevated  area,  all 
such  evidences  may  be  removed  and  the  origin  of  the  plain  may 
become  quite  indeterminable. 

The  Subaerial  Agents  are  those  which  operate  over  the  entire 
surface  of  the  land.  Their  tendency  is,  in  the  first  instance,  to 
carve  out  valleys  and  leave  relative  eminences  standing,  and  thus 
to  increase  the  irregularity,  or  relief,  of  the  land.  This,  however, 
is  merely  a  temporary  stage,  and  if  time  enough  be  granted,  these 
agencies  will  sweep  away  the  irregularities  and  plane  the  entire 
region  down  to  base-level. 

Rivers  cut  down  and. deepen  their  channels  so  long  as  their 
beds  have  sufficient  slope  and  fall.  The  banks  also  are  under- 
mined, as  the  current  swings  from  side  to  side,  and  frequently  fall, 
thus  widening  the  channel.  The  sides  of  the  trench,  unless  re- 
moved by  other  agencies,  will  be  as  steep  as  the  nature  of  the 
rock  material  will  allow.  Unassisted  river  action  will,  therefore, 
cut  nearly  vertical  trenches,  which  are  continually  deepened,  until 
the  base-level  is  reached.  Examples  of  such  river-cut  trenches 
are  the  Au  Sable  Chasm  (see  Fig.  58,  p.  142)  and  the  inner  gorge 
of  the  Grand  Canon  of  the  Colorado. 

The  trench-like  valley,  with  nearly  vertical  sides,  is,  however, 
not  the  usual  form  of  river  valley.  The  atmospheric  agencies,  the 
undermining  and  sapping  of  springs,  landslips,  and  the  like,  are 
continually  wearing  away  the  sides  of  -the  excavation,  the  waste 
thus  produced  being  readily  carried  away  by  the  stream.  As  the 
upper  part  of  each  hillside  and  cliff  is  that  which  has  been  longest 
exposed  to  the  denuding  agencies,  the  valley  will  be  widened 
at  the  top  more  than  at  the  bottom,  and  will  gradually  become 


THE   SUBAERIAL  AGENTS  443 

widely  open,  unless  the  alternation  of  hard  and  soft  strata  be  such 
as  to  favour  the  retention  of  the  cliff-like  form  by  undermining. 
A  system  of  river  valleys  is  normally  accordant,  the  tributaries 
entering  the  main  stream  at  grade,  and  each  valley  is  winding, 
with  projecting  spurs  from  the  sides,  and  of  V-shaped  cross- 
section. 

The  rapidity  with  which  the  deep  and  narrow  trench  is  widened 
into  the  broad,  gently  sloping  valley  will  depend  upon  two  sets  of 
conditions,  (i)  Upon  the  climate,  which  is  as  much  as  to  say 
the  intensity  with  which  the  denuding  forces  operate.  Canons 
and  narrow  gorges  are  much  more  frequent  in  arid  regions  than 
in  those  of  abundant  rainfall.  (2)  Upon  the  resistant  power  of 
the  rocks.  If  the  valley  sides  are  composed  of  rocks  which  yield 
readily  to  weathering,  the  trench  will  be  speedily  broadened,  while 
if  the  rocks  offer  great  resistance  to  chemical  and  mechanical  dis- 
integration, the  gorge-like  form  will  be  retained  very  much  longer. 
This  is  illustrated  by  almost  any  considerable  stream,  such  as  the 
Delaware  or  the  Potomac.  In  certain  places  the  valley  is  widely 
open,  while  in  other  parts  of  the  course  are  deep  gorges,  as  at  the 
Delaware  Water  Gap  and  Harper's  Ferry.  The  gorges  occur  in 
the  places  where  the  stream  cuts  across  hard,  resistant  rocks,  and 
the  open  valleys  are  found  where  it  intersects  softer  and  more 
destructible  rocks. 

Rivers  also  produce  changes  in  topography  by  constructional 
processes,  as  in  their  flood  plains  and  terraces,  processes  which 
are  most  notable  in  the  lower  parts  of  the  course,  and  which  gain 
increased  efficiency  through  a  subsidence  of  the  region. 

Degradation  is  most  rapid  on  the  hillsides  which  border  river 
valleys,  because  of  the  removal  of  waste  by  the  rivers.  Away 
from  the  streams  the  denudation  of  the  country  is  much  slower, 
because  the  waste  is  less  readily  removed.  Those  points  will 
longest  remain  standing  above  the  general  level  which  are  com- 
posed of  the  hardest  rocks  and  are  farthest  removed  from  the 
principal  lines  of  drainage. 

A  glaciated  region   has   a   topography  marked   by  rounded, 


444  THE  GEOGRAPHICAL  CYCLE 

flowing  outlines,  with  smoothed,  polished  and  striated  rocks  in  the 
central  zone,  where  erosion  was  most  active,  and  with  lines  of 
moraine,  sheets  of  drift  and  overwash  plains,  eskers  and  drumlins 
in  the  peripheral  zone,  where  denudation  was  feeblest  and  deposi- 
tion more  important.  Glacially  excavated  valleys  are  over  deep- 
ened, U-shaped  in  section,  with  the  projecting  spurs  truncated, 
or  entirely  removed,  and  the  tributary  valleys  are  not  graded  to 
the  main  trunk,  but  left  hanging  on  the  retreat  of  the  ice.  Great 
numbers  of  lakes  are  characteristic  of  such  regions. 

The  subaerial  agencies  act  with  the  greater  efficiency  the  more 
elevated  the  region  upon  which  they  operate.  Consequently,  so 
long  as  the  region  be  not  again  elevated,  denudation  operates  at 
a  continually  diminishing  rate.  The  strong  relief  of  hill  and 
valley  is  carved  out  with  comparative  rapidity,  but  the  more 
nearly  the  country  is  reduced  to  base-level,  the  more  slowly  does 
degradation  proceed,,  and  the  final  stages  of  base-levelling  must  be 
exceedingly  slow.  Nevertheless,  if  no  renewed  upheaval  takes 
place,  the  loftiest  and  most  rugged  land  surface  must  be  eventu- 
ally cut  down  to  that  level.  The  universal  and  permanent  base- 
level  is,  of  course,  the  sea;  but  other  local  and  temporary  base- 
levels  may  for  a  time  control  the  development  of  certain  areas. 
Tributaries  cannot  cut  below  the  main  stream  into  which  they 
flow;  a  lake  forms  the  base-level  for  the  streams  which  supply 
it,  until  the  lake  is  removed  by  draining  away  or  being  filled  with 
sediment.  Regions,  like  the  Great  Basin,  whose  drainage  finds 
no  outlet,  may  have  base-levels  either  above  or  below  the  level  of 
the  sea;  e.g.  the  surface  of  the  Dead  Sea  of  Palestine  is  1308  feet 
below  the  Mediterranean. 

It  is  perhaps  a  question  whether  any  large  region  has  ever 
remained  stationary  for  a  sufficiently  long  time  to  be  absolutely 
base-levelled.  On  the  other  hand,  there  is  abundant  evidence 
to  show  that  such  areas  have  been  worn  down  to  a  low-lying, 
featureless  surface,  with  only  occasional  low  protuberances  rising 
above  the  general  level.  Such  a  surface  is  called  a  peneplain, 
and  represents  what  is  usually  the  final  stage  of  a  cycle  of  denuda- 


THE   SUBAERIAL  AGENTS 


445 


tion.  Here  and  there  an  isolated  peak  may  remain  high  enough  to 
deserve  the  name  of  mountain,  which  owes  its  preservation  to  the 
exceptionally  resistant  nature  of  the  rocks  of  which  it  is  composed, 
or  to  its  exceptionally  favourable  position  with  reference  to  the 
drainage  lines.  A  renewed  upheaval  of  the  peneplain  will  begin 
another  cycle  of  denudation,  revivifying  and  rejuvenating  all  the 
destructive  agencies,  and  valleys  and  hills  will  be  carved  out  of 
the  approximately  level  surface.  In  a  peneplain  dissected  by  the 
revived  streams  the  sky-line  of  the  ridges  is  notably  even,  and  all 


FIG.  231.  —  Peneplain,  with  residual  mountain,  Southern  California. 
H.  W.  Fairbanks) 


(Photograph  by 


the  heights  rise  to  nearly  the  same  level.  Differences  of  level  are, 
however,  frequently  produced  by  a  warping  process,  which  may 
accompany  the  upheaval,  raising  some  portions  of  the  peneplain 
to  greater  heights  than  others.  Excellent  examples  of  reelevated 
and  subsequently  dissected  peneplains  are  the  uplands  of  southern 
New  England  and  the  highlands  of  New  Jersey. 

In  topography  climatic  differences  are  very  obvious,  because 
in  each  climate  the  dominant  subaerial  agents  are  characteristic, 
and  the  modifying  effects  of  vegetation  are  likewise  dependent 


446  THE  GEOGRAPHICAL  CYCLE 

upon  climatic  factors.  In  the  polar  lands  destructive  work  is 
accomplished  chiefly  by  the  activities  of  frost  and  ice,  while  in 
temperate  lands  with  normal  rainfall  rivers  and  rain  are  the 
principal  agents.  In  arid  regions  changes  of  temperature  and 
wind  are  the  most  active  processes,  though  the  scanty  vegetation 
gives  to  the  rare  but  violent  rains  an  unusual  effectiveness.  All 
of  these  climatic  differences  are  reflected  in  characteristic  topo- 
graphical forms. 

THE  ARID  CYCLE 

"The  essential  features  of  the  arid  climate  ...  are:  so  small 
a  rainfall  that  plant  growth  is  scanty,  that  no  basins  of  initial 
deformation  are  filled  to  overflowing,  that  no  large  trunk  rivers 
are  formed,  and  hence  that  the  drainage  does  not  reach  the  sea." 
(Davis.) 

The  peculiarities  of  erosion  in  arid  climates  have  already  been 
described;  it  remains  to  point  out  the  characteristic  features 
of  the  geographical  cycle  under  arid  conditions,  as  these  are 
defined  in  the  preceding  paragraph.  The  successive  steps  of 
the  cycle  are  much  affected  by  the  topography  at  the  beginning, 
but  it  would  lead  us  too  far  to  take  into  consideration  all  the 
various  cases,  and  only  a  general  outline  can  be  attempted.  We 
need  only  assume  the  elevation  of  a  large  area,  with  more  or  less 
of  deformation.  The  drainage  will  be  consequent  on  the  newly 
formed  slopes,  and  the  lowest  part  of  each  basin  will  form  the 
local  base-level,  for  the  streams  of  each  basin  of  deformation 
are  confined  to  that  basin  and  die  away  in  the  floor  without 
uniting  into  a  permanent  trunk  stream.  Occasionally  or  peri- 
odically a  playa  lake  may  form,  into  which  all  the  streams  may 
flow,  but  as  a  rule  they  are  disconnected  fragments  of  a  drainage 
system. 

In  the  youth  of  the  cycle  the  highlands  are  slowly  eroded, 
and  deposition  takes  place  on  the  slopes  and  floor  of  each  basin, 
diminishing  the  relief  and  raising  the  local  base-level,  a  strong 
contrast  to  the  corresponding  stage  of  the  normal  cycle,  in  which 


THE  ARID   CYCLE 


447 


relief  is  increased  by  the  excavation  of  stream  valleys.  Even 
in  arid  regions,  however,  valleys  are  cut  on  the  highland  slopes, 
while  the  basin  floor  is  made  nearly  level  by  deposition.  This 
stage  is  exemplified  by  the  Great  Basin  and  its  mountains.  Water 
is  the  chief  agent  of  erosion  and  deposition  during  the  period 
of  youth,  but  the  wind  is  also  important  in  eroding  the  bare 
rocks  and  in  distributing  the  finer  waste,  part  of  which  it 
carries  outside  of  the  arid  region  altogether.  Extremely  slow 
as  this  process  of  complete  removal  of  the  finer  debris  by  the 


FIG.  232.  —The  Mohave  Desert,  California.     (Photograph  by  H.  W.  Fairbanks) 

wind  undoubtedly  is,  yet  it  is  the  only  agency  which  actually 
lowers  the  average  altitude  of  the  region,  for  no  water  flows  out 
of  the  area  we  are  considering. 

Maturity  of  development  is  attained  by  the  connection  of 
the  separate  initial  basins  into  a  continuous  whole.  Erosion 
of  the  highlands  and  deposition  on  the  basin  floors  may  result 
in  the  formation  of  a  continous  slope  from  a  higher  basin  to  a 
lower  one,  so  that,  even  in  the  absence  of  any  permanent  stream 
connecting  the  two,  the  rain  will  wash  material  to  the  lower 
basin  and  a  new  and  lower  base-level  will  be  established  for  the 


448  THE  GEOGRAPHICAL  CYCLE 

higher  one,  while  the  lower  basin  floor  will  be  built  up  by  the 
transfer  to  it  of  the  waste  accumulated  in  that  which  lies  at  the 
higher  level.  "  As  the  coalescence  of  basins  and  the  integration 
of  stream  systems  progress,  the  changes  of  local  base-levels 
will  be  fewer  and  slower,  and  the  obliteration  of  the  uplands, 
the  development  of  graded  piedmont  slopes  and  the  aggradation 
of  the  chief  basins  will  be  more  and  more  extensive."  (Davis.) 
In  maturity  the  relative  efficiency  of  the  eroding  agents  is 
not  the  same  as  in  the  earlier  stages.  Large  areas  at  the  foot 
of  the  mountains  and  highlands  have  already  been  graded  to  an 
even  slope  by  the  torrents  formed  by  the  rare  but  violent  rains. 
When  vertical  trenching  can  be  carried  no  farther,  lateral  erosion 
removes  the  divides  between  the  streams,  which  thus  have  no 
definite  channels,  and  the  water  flows  down  the  graded  slope 
in  a  thin  sheet.  The  wasting  away  of  the  highlands  diminishes 
the  scanty  rainfall,  and  the  quantity  and  efficiency  of  the  water 
decrease  with  the  diminution  of  relief,  but  the  activity  of  the 
wind  is  not  affected,  and  hence  its  relative  importance  is  increased. 

The  continuance  through  vast  periods  of  time  of  the  slow 
processes  above  outlined  will  result  in  the  destruction  of  the  origi- 
nal relief,  its  place  being  taken  by  large  plains  of  bare  rock, 
sloping  to  plains  of  accumulation.  If  the  strata  have  been  dis- 
turbed by  folding  or  tilting,  the  plain  surface  cuts  across  their 
structure  to  an  even  slope.  The  work  is  done  without  reference 
to  the  sea  as  a  base-level  and  the  altitude  of  the  plain  is  deter- 
mined by  the  loss  of  material  through  transportation  by  the  wind. 

The  period  of  old  age  begins  with  the  breaking  up  of  the  unified 
drainage  system  through  the  excavation  of  wind-made  hollows 
in  the  softer  rocks,  a  process  unlike  anything  which  we  have 
found  in  pluvial  climates,  where  a  drainage  system  once  established 
is  not  disintegrated  during  the  progress  of  a  normal  cycle.  If 
the  desert  were  strictly  rainless  and  erosion  carried  on  by  the 
wind  alone,  there  would  apparently  be  no  limit  to  the  excavation 
of  such  hollows,  but  no  such  instance  is  known,  and  the  occasional 
heavy  rains  suffice  to  counteract  it  by  filling  up  the  hollows,  but 


THE  ARID   CYCLE  449 

the  drainage  system  is  effectively  disintegrated,  and  the  waste, 
which  in  the  stage  of  maturity  gathered  on  the  lower  parts  of  the 
region,  is  washed  about  irregularly.  The  surface  worn  down 
by  the  wind  may  have  no  slope  in  any  particular  direction,  for 
the  wind  has  no  base-level  and  is  not  affected  by  the  inclination 
of  the  ground,  but  this  surface  may  be  reduced  to  an  essentially 
plain-like  character.  Harder  masses  of  rock,  which  have  success- 
fully resisted  wear,  may  rise  above  the  plain  as  residual  moun: 
tains,  just  as  they  sometimes  do  in  the  peneplain  of  the  normal 
cycle. 

The  deserts  of  North  America  have  not  reached  the  condition 
of  old  age,  but  it  is  realized  in  South  Africa,  where  the  desert 
plains  have  lately  been  studied  by  Dr.  Passarge,  whose  con- 
clusions are  thus  summarized  by  Professor  Davis:  "  Passarge 
states  that  these  desert  plains  are  not  undulating  with  low  hills, 
but  true  plains  of  great  extent,  from  which  the  isolated  residual 
mountains  rise  like  islands  from  the  sea.  The  residuals  may  be 
low  mounds,  only  a  few  metres  high,  or  lofty  mountain  masses, 
rising  several  thousand  metres  above  the  plains.  The  plain 
surrounds  the  steep  slope  of  the  mountains  with  a  table-like 
evenness;  there  is  no  transitional  belt  of  piedmont  hills,  and  no 
intermediate  slope.  The  mountains  consist  of  resistant  rocks, 
such  as  granite,  diorite,  gabbro,  quartzite,  etc.,  granite  being 
the  most  frequent;  the  plains  are  of  more  easily  eroded  rocks, 
such  as  gneiss,  schists,  slates,  sandstones,  and  limestones.  The 
bedding  of  the  rocks  is  not  flat,  but  disturbed;  the  plain  therefore 
truncates  the  rock  structures.  .  .  .  The  products  of  weathering 
are  usually  spread  as  a  thin  veneer  on  the  plain;  the  waste  does 
not  lie  in  place,  on  the  rocks  from  which  it  was  weathered,  but 
has  been  drifted  about  by  wind  and  flood  and  has  gathered  in 
slight  depressions.  .  .  .  These  rock-floored  plains  are  not  up- 
lifted peneplains,  but  are  the  product  of  desert  erosion  unrelated 
to  normal  base-level,  in  which  occasional  water-action  has  co- 
operated with  persistent  wind-action." 

We  have  definite  evidence  that  the  earth  has  undergone  many 


450  THE  GEOGRAPHICAL  CYCLE 

climatic  changes  and  that  among  these  changes  the  alternation 
of  arid  and  pluvial  conditions  in  the  same  region  have  not  in- 
frequently occurred.  Hence,  in  deciphering  the  history  of  any 
high-level  plain  the  distinction  between  the  normal  and  the  arid 
cycle  must  always  be  borne  in  mind,  and,  if  possible,  evidence 
obtained  which  will  enable  the  observer  to  determine  whether 
the  plateau  is  a  reelevated  peneplain,  or  the  product  of  an  arid 
climate. 


CHAPTER   XIX 
LAND   SCULPTURE 

WHILE  the  final  effect  of  the  subaerial  denuding  agencies  is  to 
sweep  away  all  relief,  and  to  cut  the  land  surface  down  to  low- 
lying  base-levels  or  peneplains,  yet  in  the  process  great  irregu- 
larities are  produced  by  the  more  rapid  removal  of  some  parts 
than  of  others.  The  topographical  forms  generated  by  this  dif- 
ferential erosion  vary  much  according  to  circumstances.  We 
have  already  considered  some  of  these  differences  with  regard 
to  the  agencies  which  have  produced  them.  Now  we  have  to 
examine  the  differences  with  a  view  of  learning  how  topographi- 
cal forms  are  determined  by  the  character  and  arrangement  of 
the  rocks  which  are  undergoing  degradation. 

Forms  in  Horizontal  Strata.  —  When  a  peneplain  or  plain  of 
marine  denudation  is  lifted  high  above  sea-level,  without  folding 
or  steep  tilting  of  the  strata,  streams  are  soon  established  upon 
the  new  land,  and  proceed  to  cut  deep  trenches  across  the  plateau, 
which  are  gradually  widened  out  under  the  influence  of  weather- 
ing, and  the  arrangement  of  hard  and  soft  rocks  finds  expression 
in  the  resulting  forms.  If  the  surface  layers  resist  weathering, 
the  plateau  will  be  gradually  dissected  into  flat-topped  mesas 
and  table-mountains,  which  in  the  progress  of  denudation  are 
converted  into  pyramidal  shapes ;  while  if  the  whole  mass  of  rocks 
be  easily  destructible,  they  weather  down  into  dome-shaped  and 
rounded  hills,  which  are  smallest  at  the  top,  the  part  longest 
exposed  to  weathering.  The  wild  and  grotesque  scenery  of  the 
Western  bad  lands,  with  their  chaos  of  peaks,  ridges,  mesas, 
and  buttes,  is  merely  the  result  of  the  differential  weathering  of 
horizontal  strata,  some  beds  and  parts  of  beds  yielding  more 


452  LAND   SCULPTURE 

readily  than  others.  The  bad  lands  are  carved  out  of  soft  and 
scarcely  indurated  rocks,  but  firm  rocks  in  climates  of  similar 
aridity  give  rise  to  many  vertical-sided  mesas  (see  Frontispiece), 
as  is  so  notably  the  case  in  the  sandstones  of  New  Mexico.  In 
pluvial  climates  the  slopes  are  gentler. 

If  a  series  of  more  resistant  beds  underlies  a  mass  of  softer 
strata,  a  change  in  the  topographical  forms  will  occur  when  the  un- 
derlying harder  rocks  are  partially  exposed.  In  the  soft  rocks 
the  valley  sides  have  gentle  slopes,  but  when  the  harder  mass  is 
penetrated,  the  slopes  become  steep,  or  even  vertical.  When 
hard  and  soft  strata  alternate  in  a  valley  wall,  the  harder  beds 
form  cliffs.  This  is  accomplished  by  cutting  away  the  softer 
beds  and  thus  undermining  the  harder  ones,  until  the  latter  can 
no  longer  support  their  own  weight,  and  masses  fall  from  the 
face  of  the  cliff,  thus  maintaining  the  verticality.  The  talus 
blocks  form  a  slope,  connecting  the  successive  cliffs  by  gentler 
inclines.  The  Uinta  Mountains  in  northern  Utah  are  formed 
by  a  great  anticlinal  arch,  so  broad  and  gently  curved  that  in 
a  given  section  the  strata  appear  almost  horizontal.  Out  of  these 
immensely  thick  and  nearly  level  masses  the  subaerial  de- 
nuding agencies  have  carved  an  infinite  and  most  picturesque 
variety  of  peaks,  pinnacles,  columns,  and  amphitheatres,  while 
the  streams  have  cut  profound  and  gloomy  canons.  Vast  talus 
slopes  remain  to  indicate  the  amount  of  destruction 

Forms  in  Inclined  Strata.  —  Inclined  or  tilted  strata  give  rise 
to  a  different  class  of  topographical  forms.  If,  as  is  generally 
the  case,  harder  and  softer  strata  alternate,  the  latter  will  be  swept 
away  more  rapidly  than  the  former,  which  are  left  standing  as 
ridges  or  cliffs,  the  height  and  steepness  of  which  are  determined 
by  the  thickness  and  inclination  of  the  more  resistant  rocks. 
In  case  the  strata  are  steeply  inclined,  a  succession  of  hard  beds 
alternating  with  soft  will  give  rise  to  a  series  of  ridges  and  valleys, 
the  slopes  of  which  depend  upon  the  angle  of  dip.  If  the  beds 
are  standing  in  a  vertical  position,  the  two  slopes  of  each  ridge 
will  be  nearly  equal,  the  hard  strata  forming  the  backbone  of 


FORMS   IN  INCLINED   STRATA 


453 


the  ridge  and  the  softer  ones  the  sloping  sides.  Often  narrow 
ridges,  more  or  less  closely  resembling  dykes  of  igneous  rocks, 
are  formed  by  the  isolation  of  hard  vertical  strata,  the  softer  beds 
on  each  side  being  removed  by  erosion.  As  the  inclination  departs 
from  verticality,  the  more  unequal  do  the  two  slopes  of  each 
ridge  become,  the  longer  and  gentler  one  being  in  the  direction 
of  the  dip.  Ridges  and  valleys  of  this  class  are  beautifully  ex- 
emplified in  the  Appalachian  Mountains.  Figure  40  (p.  115)  shows 


FIG.  233.  —  Escarpments  and  dip  slopes,  Montana.     (U.  S.  G.  S.) 

Kittatinny  Mountain,  through  which  the  Delaware  River  has 
cut  the  famous  Water  Gap;  the  crest  of  the  ridge  is  formed 
by  very  hard  and  indestructible  sandstones  and  conglomerates, 
while  the  broad  valley  above  and  below  the  gap  is  in  slates  or 
other  destructible  rock. 

In  gently  inclined  strata  the  abruptly  truncated  and  cliff-like 
outcrops  of  the  hard  beds  are  called  escarpments,  and  follow, 
with  some  irregularities  and  sinuosities,  the  line  of  strike. 


454  LAND   SCULPTURE 

Whether  the  general  course  of  the  escarpment  shall  be  straight 
or  curved  will,  therefore,  be  determined  by  the  constancy  or 
change  in  the  direction  of  the  dip;  for,  as  we  have  already  learned, 
the  strike  changes  with  the  dip,  always  keeping  at  right  angles 
to  the  latter.  The  upper  surface  of  the  gently  inclined  hard 
stratum  may  be  completely  exposed  by  the  stripping  away  of  the 
softer  overlying  mass,  and  then  the  slope  of  the  ground  is  the 
same  as  that  of  the  resistant  stratum,  and  is  called  a  dip  slope. 
A  series  of  gently  inclined  strata,  made  up  of  alternating  harder 
and  softer  beds,  will  thus  give  rise  to  parallel  ridges  and  valleys, 
or  escarpments  and  dip  slopes,  according  to  the  completeness 
with  which  the  softer  beds  are  removed  and  the  harder  ones 
exposed.  A  magnificent  example  of  such  escarpments  and  slopes 
is  displayed  in  the  high  plateaus  of  Utah  and  Arizona,  where 
the  dip  slopes  are  from  20  to  60  miles  broad  and  the  escarpments 
1500  to  2000  feet  high.  The  amount  of  denudation  involved 
in  the  production  of  these  vast  amphitheatres  staggers  belief, 
though  there  is  no  escape  from  the  enormous  figures. 

Under  the  influence  of  denudation  escarpments  are  continually 
though  slowly  receding,  being  cut  back  in  the  direction  of  the 
dip.  Rain  and  frost  act  directly  upon  the  hard  beds,  but  work 
more  effectively  by  cutting  away  the  softer  beds  below  and  thus 
undermining  the  hard  strata,  causing  them  to  fall.  The  fallen 
masses  are  gradually  disintegrated  in  their  turn  and  washed 
away  into  the  water-courses.  The  escarpments  may  follow  a  rela- 
tively straight  or  a  very  sinuous  course.  Sinuosities,  when  present, 
are  commonly  due  to  the  action  of  springs,  which  undermine 
the  escarpments  and,  by  the  recession  of  their  heads,  excavate 
the  line  of  cliffs  into  bays  and  amphitheatres.  A  sinuous  es- 
carpment is  more  rapidly  cut  back  than  a  straight  one,  because, 
in  addition  to  the  cooperation  of  the  springs,  it  offers  a  larger 
surface  to  the  attack  of  the  destructive  agencies.  Every  step  in 
the  recession  of  an  escarpment  lowers  the  ridge  and  brings  it 
nearer  to  base-level,  because  the  direction  of  retreat  follows  the 
line  of  dip,  which  carries  the  beds  down  to  base-level  with  a 


FORMS   IN  INCLINED   STRATA  455 

rapidity  determined  by  the  angle  of  dip.  A  steeply  inclined  bed 
needs  to  be  cut  back  only  a  short  distance,  when  it  will  be  reduced 
to  base-level,  whereas  a  bed  dipping  very  gently  remains  above 
base-level  for  long  distances.  Of  course  the  general  elevation 
of  the  whole  region-  above  base-level  is  also  an  important  factor 
in  determining  the  amount  of  work  to  be  done. 

For  reasons  that  will  appear  later,  we  assume  that  when  denu- 
dation began  its  work  upon  a  region  of  inclined  strata,  that  region 
was  a  sloping  plain,  or  peneplain,  formed  by  the  outcropping 
edges  of  the  strata.  The  first  lines  of  drainage  established  would 
necessarily  follow  this  slope,  and  the  first  valley  or  valleys  cut 
would  be  across  the  strike  of  the  beds,  trenching  both  hard  and 
soft  beds.  Such  valleys  are  called  transverse,  or  dip  valleys,  and 
the  streams  which  flow  in  them,  transverse  streams.  Transverse 
streams  cut  steep-sided,  canon-like  valleys,  the  rocks  giving 
way  along  the  joints  and  making  the  two  valley-walls  alike,  as 
though  the  valley  were  cut  in  horizontal  strata.  A  second  series 
of  valleys  will  be  excavated  along  the  strike  of  the  softer  beds, 
giving  longitudinal,  or  strike,  valleys  and  streams.  In  such  a 
longitudinal  valley,  following  the  strike  of  a  mass  of  soft  strata, 
the  stream  which  occupies  it  will  tend  to  flow  along  the  foot  of 
the  escarpment  formed  by  the  outcrop  of  hard  strata,  and  to 
shift  its  course  laterally  in  the  direction  of  the  dip,  cutting  away 
the  soft  beds  in  which  it  flows,  and  undermining  the  hard  escarp- 
ment. Longitudinal  or  strike  valleys  tend  to  have  one  steep  or 
vertical,  and  one  gently  sloping  side.  The  strata  dip  across  the 
stream  and  hence  on  one  side  are  inclined  toward. the  valley  and 
on  the  other  away  from  it.  The  former  is  the  weaker  structure, 
because  the  loosened  joint-blocks  glide  into  the  stream,  and  the 
ground-water,  following  the  stratification  planes,  forms  springs 
on  that  side  of  the  valley.  The  side  on  which  the  dip  is  away 
from  the  stream  is  attacked  chiefly  by  the  undermining  action 
of  the  stream  and  thus  kept  vertical.  Such  a  stream  is  a  potent 
agent  in  causing  the  recession  of  the  escarpment  and  may  remove 
large  areas  of  both  hard  and  soft  strata. 


456 


LAND   SCULPTURE 


The  steep  ridges,  or  "  hog-backs,"  which  occur  among  the 
foothills  of  the  Rocky  Mountains,  show  interesting  examples  of 
streams  flowing  along  the  strike  of  inclined  strata,  though  the 
ridges  are  themselves  not  formed  quite  in  the  way  already  de- 
scribed. They  are  composed  of  the  steeply  dipping  limbs  of 
monodinal  folds,  of  which  the  upper  horizontal  limbs  have  been 
removed  by  denudation  (Fig.  234). 


FIG.  234.—  Hog-backs,  east  side  of  Laramie  Mts.,  Wyo.     (U.  S.  G.  S.y 

Forms  in  Folded  Strata.  —  A  region  of  folded  strata  is,  in  the 
first  instance,  thrown  into  a  series  of  ridges  and  valleys,  the  ridges 
formed  by  anticlines  and  the  valleys  by  synclines;  in  other  words, 
the  topography  is  tectonic  in  character  and  determined  by  dias- 
trophic  movements.  If  the  folding  be  of  moderate  degree,  so 
as  to  produce  undulations  of  sweeping  and  gentle  curves,  the 
tendency  of  denudation  is  to  reverse  the  original  topography 


FORMS  IN  FOLDED   STRATA  457 

and  convert  the  anticlines  into  valleys  and  the  synclines  into 
ridges.  This  apparently  paradoxical  result  is  found,  when 
examined,  to  be  natural  and  simple  enough.  The  crests  of 
newly  formed  anticlines  have  been  subjected  to  tensile  stresses 
which  open  the  joints  in  the  strata  and  render  them  an  easy 
prey  to  the  denuding  agents.  The  surface  of  the  synclines,  on 
the  contrary,  has  been  tightly  compressed,  and  their  joints  are 


FIG.  235.  —  Anticlinal  ridge,  Big  Horn  Mts.,  Wyo.,  hard  beds  in  relief.    (U.  S.  G.  S.) 

closed  by  crowding.  Aside  from  this,  another  factor  tends  to 
produce  the  same  result.  In  a  folded  series  of  alternating  harder 
and  softer  beds  denudation  is  most  rapid  on  the  exposed  anti- 
clines, and  in  them  the  hard  strata  are  first  reached  and  cut 
through.  When  an  underlying  mass  of  soft  strata  is  reached, 
it  is  rapidly  trenched  into  valleys  which  may  soon  be  excavated 
below  the  level  of  the  synclinal  troughs. 


458 


LAND   SCULPTURE 


If  the  folds  originally  made  by  the  force  of  lateral  compression 
be  steep  and  high,  as  in  mountain  ranges,  the  anticlines  persist 
longer  as  ridges,  but  the  wearing  away  of  their  summits  gives 
rise  to  subordinate  ridges  and  valleys  within  the  limits  of  each 
anticlinal  arch.  Here  also  the  ridges  are  the  outcropping  harder 
beds,  and  the  valleys  are  cut  in  the  softer  ones.  Even  in  mountain 
ranges  denudation  may  reverse  the  original  structural  topography 
and  give  rise  to  anticlinal  valleys  and  synclinal  mountains. 


FIG.  236. — Truncated  anticlinal  ridge,  Montana.     (U.  S.  G.  S.) 

If  a  region  of  folded  rocks  has  once  been  planed  down  to  base- 
level  or  to  a  peneplain,  and  then  reelevated  and  subjected  to 
denudation,  the  resulting  topography  will  be  determined  by 
the  same  laws.  Indeed,  this  is  a  frequent  method  in  which 
regions  of  tilted  or  inclined  strata  are  produced,  for,  as  we  saw 
in  Chapter  XII  (p.  327),  inclined  beds  are  very  often  parts  of 
truncated  folds.  In .  such  regions  drainage  is  first  in  accordance 
with  the  slopes  of  the  planed  and  tilted  surface,  but  as  denuda- 
tion proceeds,  the  structure  and  arrangement  of  the  rocks  make 


FORMS   IN  VOLCANIC  ROCKS  459 

themselves  felt,  and  bring  about  changes  and  adjustments  of  the 
drainage  to  the  structure,  as  will  be  more  fully  explained  in  a 
subsequent  chapter. 

Forms  in  Volcanic  Rocks.  —  Volcanic  topography  is  primarily 
constructive.  The  cones  vary  in  form  and  height  in  accordance 
with  the  amount  and  character  of  the  material  of  which  they  are 
composed,  and  the  nature  of  the  eruptions.  Thus',  we  find  lofty, 
steep-sided  cinder  cones,  like  those  of  the  Pacific  coast,  or  very 
gently  sloping  lava  cones,  like  those  of  the  Sandwich  Islands, 
or  truncated  cones  and  crater-rings,  due  to  violent  explosions  and 
to  remelting  and  engulfment  of  the  upper  part  of  the  cone.  Lava 
flows  may  take  the  form  of  long,  narrow  streams,  or  great  floods 
poured  out  one  upon  another,  until  immense  volcanic  plateaus 
are  built  up,  like  those  of  Oregon  and  Washington. 

The  progress  of  denudation  sculptures  these  volcanic  masses 
in  characteristic  ways,  already  described  in  Chapter  XV.  Cones 
are  first  furrowed  with  ravines  and  valleys  and  then  gradually 
degraded  into  necks,  or  into  low  hills  of  volcanic  agglomerate. 
The  infinite  variety  of  combinations  of  soft  tuffs,  loose  masses  of 
scoriae  and  ash,  hard  sheets,  streams,  pipes  and  dykes  of  lava, 
give  rise  to  the  manifold  forms  of  volcanic  mountains  and  islands 
in  the  course  of  denudation,  the  harder  elements  resisting  longer 
and  standing  in  relief.  Lava  flows  are  generally  harder  than  the 
stratified  rocks  upon  which  they  rest,  and  therefore,  aside  from 
their  original  form,  their  topographical  effect  is  much  the  same 
as  that  of  an  exceptionally  hard  stratum  among  softer  beds.  A 
surface  stream  may  be,  indeed  eventually  must  be,  cut  up  by 
erosion  into  isolated  masses,  which  protect  the  underlying  softer 
rocks  and  thus  form  flat-topped  table  mountains  and  mesas, 
the  lava  cap  with  nearly  vertical  sides,  the  stratified  rocks  below 
with  gentler  slopes,  especially  in  pluvial  climates.  A  lava  plateau 
is  dissected  by  streams,  first  trenching  steep  canons  and  then  at- 
mospheric erosion  widens  the  canons  and  narrows  the  divides,  just 
as  in  ordinary  plateaus  of  stratified  rocks,  but  the  great  hardness 
of  the  volcanic  masses  renders  the  process  very  slow. 


460  LAND   SCULPTURE 

Forms  in  Plutonic  Rocks.  —  It  is  exceptional  that  topographica\ 
features  can  be  definitely  referred  to  the  constructive  or  tectonic 
effect  of  intrusive  plutonic  bodies,  for  the  obvious  reason  that  the 
presence  of  a  plutonic  mass  at  a  given  point  can  rarely  be  determined 
until  the  covering  of  overlying  strata  has  been  removed.  An 
exception  to  this  is  given  by  many  laccolithic  hills  and  mountains, 
in  which  the  covering  of  strata  is  more  or  less  completely  retained. 
In  Little  Sun-Dance  Hill  (Fig.  219)  we  have  a  dome-like  hill,  the 
strata  of  which  are  almost  intact  and  the  presence  of  the  plutonic 
body  is  only  inferred,  not  absolutely  certain.  A  second  stage  of 
denudation  is  found  in  Bear  Butte  (Fig.  220)  of  the  same  region,  the 
covering  of  strata  being  removed,  except  where  they  are  upturned 
around  the  base  of  the  butte,  and  finally,  in  Mato  Tepee,  we  have 
only  the  central  core  of  the  laccolith  preserved  (Fig.  221).  The 
Henry  Mountains  of  southern  Utah  and  several  of  the  Colorado 
ranges  display  laccoliths  in  all  stages  of  dissection. 

The  plutonic  bodies  are  exposed  by  denudation,  and  since  they 
are,  as  a  rule,  more  resistant  than  the  invaded  rocks,  they  generally 
form  prominences  corresponding  to  the  form  of  the  intrusive  mass. 
Sometimes,  however,  the  intrusive  body  is  less  resistant  than  the 
enclosing  rocks  and  then  is  marked  by  a  depression.  Dykes, 
when  exposed  by  denudation,  stand  out  in  relief  as  long  walls, 
the  height  of  which  is  determined  largely  by  the  thickness  of  the 
dyke  and  by  its  resistance  to  destruction.  In  certain  cases,  as  in 
North  Carolina,  the  dyke-rock  disintegrates  more  rapidly  than  the 
enclosing  rock,  and  hence  long  trenches  indicate  the  position  of  the 
dykes.  Sills,  so  far  as  their  effect  upon  topography  is  concerned, 
may  be  regarded  simply  as  hard  strata,  but  some  sills  are  much 
thicker  than  strata  often  are. 

Stocks,  which  increase  in  diameter  downward,  project  as  small 
hills  when  first  exposed,  but  when  they  are  slowly  denuded  and  the 
country-rock  is  rapidly  worn  away,  they  become  larger  and  rela- 
tively higher,  as  the  surrounding  area  is  lowered  by  denudation.  If 
the  region  where  the  stock  is  found  is  sufficiently  above  base- 
level,  a  very  high  hill  and  even  a  mountain  may  thus  be  formed. 


FORMS   IN   PLUTONIC   ROCKS 


461 


FlG.  237.  —  Palisade-sill,  Fort  Lee,  N.  J.     (Photograph  by  van  Ingen) 


462  LAND   SCULPTURE 

Batholiths,  like  stocks,  increase  in  size  downward,  lying  upon  no 
floor  of  country-rock.  Hence,  they  give  rise  to  great  ridges,  or 
irregular  masses,  often  of  enormous  size,  when  laid  bare  by  erosion. 
Many  mountain  ranges  are  composed  of  granite  batholiths,  from 
which  the  covering  of  strata  has  been  stripped  away  and  which  are 
themselves  deeply  dissected  into  peaks  and  crags.  On  the  other 
hand,  such  batholiths  are  no  exception  to  the  rule  that  plutonic 
bodies  may  sometimes  wear  away  more  rapidly  than  the  rocks 
which  enclose  them.  When  this  occurs,  the  batholith  will  be 
found  as  a  plain,  or  depression,  with  the  more  resistant  rocks 
rising  above  it. 

In  brief,  the  controlling  factor  in  a  region  of  mature  topography 
is  the  arrangement  of  the  rock  masses,  prominences  being  due  to  the 
more  resistant  rocks,  whatever  their  nature. 


CHAPTER   XX 
TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

NOT  uncommonly  faults  have  no  direct  effect  upon  topography, 
or  whatever  influence  they  may  originally  have  exerted  has  been 
lost,  denudation  having  worn  down  the  two  sides  to  the  same  level 
or  to  a  continuous  slope,  so  that  no  evidence  of  the  fault  appears  in. 
the  surface  forms,  but  must  be  indirectly  obtained.  When  newly 
formed,  a  fault  is  accompanied  by  the  scarp,  as  a  long  line  of  cliff 
or  bluff.  As  modern  faults  teach  us,  displacements  of  great 
throw  are  probably  produced  by  repeated  movements  along  the 
same  plane  of  fracture,  and  thus  require  long  periods  for  their  com- 
pletion ;  and  during  this  process  of  dislocation  erosion  is  actively 
at  work,  especially  upon  the  scarp,  which  is  most  exposed  to  attack. 
The  length  of  time  during  which  the  scarp  persists,  as  in  the  case 
of  topographical  features  generally,  depends  upon  the  activity  of 
weathering  and  the  resistant  power  of  the  rock.  Well-preserved 
and  lofty  fault -scarps  are  therefore  very  much  more  frequent  in  arid 
than  in  pluvial  climates.  Long  before  its  removal  the  scarp  is 
deeply  dissected  and  made  rough  and  craggy,  so  as  to  resemble 
a  mountain  range.  Examples  of  this  kind  of  topography  are  to  be 
seen  on  a  grand  scale  in  the  arid  parts  of  the  West,  their  preser- 
vation being  due  in  part  to  the  geologically  late  date  of  the  dis- 
locations, and  in  part  to  the  aridity  of  the  climate  which  makes 
denudation  very  slow.  The  imposing  lines  of  cliff  which  demar- 
cate the  plateau  of  the  Colorado  on  the  western  side  are  fault 
scarps,  and  the  plateau  itself  is  crossed  by  many  minor  faults 
which  still  form  prominent  surface  features,  though  much  modi- 
fied by  erosion  and  some  of  them  quite  disguised. 

463 


464    TOPOGRAPHY  AS  DETERMINED  BY   FAULTS  AND  JOINTS 

The  Great  Basin,  which  is  enclosed  between  the  Wasatch 
Mountains  on  the  east  and  the  Sierra  Nevada  on  the  west,  and  com- 
prises nearly  all  of  Utah  and  Nevada,  is  bounded  by  the  enormous 
fault-scarps  of  those  ranges.  The  Sierra  Nevada  escarpment  has 
a  throw  estimated  at  15,000  feet,  though  this  has  been  much  reduced 
in  height,  and  is  deeply  dissected  by  erosion,  so  that  its  true  nature 
is  not  immediately  apparent.  On  the  eastern  side  the  Wasatch 
escarpment  is  similarly  dissected,  but  there  have  been  additional 
very  recent  movements  along  the  old  fault-planes,  and  the  lately 
formed  scarps,  though  low,  are  remarkably  fresh  and  unchanged 


*•**•• -5^  »-sl — -  '       *"^*     --"       '   ••"•— 


FIG.  238.  —  Abert  Lake,  Oregon.    The  line  of  cliffs  is  a  fault-scarp.     (Russell) 

even  in  incoherent  materials.  The  Basin  itself  is  traversed  by 
many  north  and  south  faults,  and  the  blocks  included  between 
these  parallel  lines  of  dislocation  are  tilted  and  form  great  scarps, 
but  with  gently  inclined  top.  Each  block  thus  has  an  abrupt 
side,  the  rugged  and  worn  scarp,  and  a  long,  gently  sloping  side, 
which  gradually  inclines  to  the  foot  of  the  next  parallel  scarp. 
Denudation  has  carved  these  tilted  fault-blocks  into  ridges  and 
peaks,  giving  them  the  appearance  of  an  ordinary  mountain 
range,  when  seen  from  the  escarpment  side,  but  formed  in  an 
entirely  different  manner  from  true  mountains  of  folding,  and 
known  as  block  mountains.  In  southern  Oregon  the  plateau  of 


TOPOGRAPHY  AS  DETERMINED  BY  FAULTS 


465 


basalt  has  been  fractured  into  a  series  of  blocks,  which  are 
tilted,  with  very  gentle  ascent  on  one  side,  ending  in  an  abrupt 
scarp  on  the  other.  These  have  been  so  little  affected  by  erosion 
that  their  true  character  is  immediately  apparent. 

The  Adirondack"  Mountains  display  a  topography  which  is 
strongly  dominated  by  systems  of  faults;  their  general  character 
is  thus  described  by  Professor  Kemp:  "The  Adirondack  region 
...  is  mountainous  in  its  eastern  half  and  has  its  highest  peaks 
near  its  centre,  but  on  the  west  the  mountains  disappear  and  the 


FlG.  239.  — Sierra  Nevada  fault-scarp,  Mono  Lake,  Cal.     (U.  S.  G.  S.) 

area  becomes  a  plateau  ranging  from  2000  feet  above  tide  gradually 
downward  to  the  west  until  it  is  but  slightly  higher  than  Lake 
Ontario  and  the  St.  Lawrence.  .  .  .  The  mountains  are  arranged 
in  visible  northeast  and  southwest  lines,  and  are  often  very  steep 
if  not  positively  precipitous  in  the  portions  that  look  to  the  south- 
east or  northwest.  There  are  also  other  steep  faces  nearly  at 
right  angles  with  the  above,  but  they  are  less  pronounced."  These 
features  are  due  to  three  intersecting  systems  of  faults,  the  principal 
one  of  which  is  northeast  and  southwest  and  determines  the  general 
trend  of  the  mountains.  The  second  system,  which  trends  north- 
an 


466    TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

east  at  right  angles  to  the  first,  is  less  important,  and  acts  chiefly 
in  cutting  the  ridges  into  separate  blocks.  The  third  series, 
with  north  and  south  trends,  is  quite  subordinate  in  its  effects. 
The  faults  extend  southward  for  many  miles  from  the  mountains 
into  the  plain,  and  some  of  them  still  retain  their  scarps. 

In  Chapter  XIII  (p.  347)  we  learned  that  a  block  isolated  by 
faults  hading  away  from  it  on  all  sides,  so  that  the  block  is  on  the 
upthrow  side  with  reference  to  the  area  all  around  it,  is  called  a 
Horst,  a  name  adopted  from  the  German  for  lack  of  an  English 
term.  The  Horst  structure  is  thus  much  like  that  of  the  block 
mountains  of  the  Great  Basin  and  the  Adirondacks,  except  for  the 
direction  of  slope  in  the  fault-planes  and  for  the  fact  that  there  is 
usually  no  relation  between  the  strike  of  the  beds  and  that  of  the 
faults.  Several  examples  of  Horst  mountains  occur  in  Europe, 
such  as  the  central  plateau  of  France,  the  Vosges,  the  Black  and 
Thuringian  Forests,  and  the  Hartz  Mountains.  These  uplifts  are 
composed  chiefly  of  ancient,  very  strongly  folded  and  hard  rocks, 
from  which  most  of  the  covering  strata  have  been  removed  by 
denudation,  and  rise  quite  abruptly  from  the  comparatively  un- 
disturbed strata  of  the  surrounding  lowlands. 

Along  the  entire  eastern  coast  of  Asia  the  topography  is  con- 
trolled by  gigantic  systems  of  faults.  The  zigzag  mountain  ranges 
which  bound  the  coastal  plains  on  the  west  have  a  general  trencl 
approximately  parallel  to  the  coast,  and  mark  a  series  of  great  faults 
with  downthrows  to  the  east.  The  coastal  plain  itself  is  divided 
into  a  series  of  more  or  less  parallel  blocks  by  step  faults,  also 
with  downthrows  to  the  eastward,  resulting  in  a  system  of  terraces, 
which,  however,  have  not  flat,  but  inclined  tops,  because  of  the 
tilting  of  the  blocks,  much  as  in  the  block  mountains  of  southern 
Oregon,  though  on  a  vastly  larger  scale.  An  east-west  section 
through  northern  China  shows  three  such  enormous  blocks,  the 
Mongolian  block  on  the  west,  the  Japanese  block  on  the  east,  with 
the Manchurian  between;  the  Japanese  block  is  partly  submerged, 
making  the  outer  portion  an  island,  as  is  true  of  the  whole  chain 
of  islands  which  fringes  the  eastern  Asiatic  coast.  These  dis- 


TOPOGRAPHY  AS  DETERMINED   BY   FAULTS  467 

focations  may  be  followed  from  the  Tropic  of  Cancer  to  the  Arctic 
Circle  and  through  sixty  degrees  of  longitude. 

Not  only  are  the  grand  features  of  topography  in  eastern  Asia 
dominated  by  gigantic  fault  systems,  but  the  minor  features  indi- 
cate similar  control.  The  Korean  peninsula,  for  example,  has  its 
topography  determined  by  systems  of  intersecting  faults,  of  which 
the  principal  series  extends  from  N.N.W.  to  S.S.E.,  and  the  whole 
country  has  been  compared  to  a  chess-board  of  fault-blocks. 
Japan  is  divided  into  two  parts  by  a  great  transverse  fault,  the 
fossa  magna,  north  of  which  the  island  has  a  northerly  trend  and 
is  dominated  by  fault  lines,  while  south  of  the  fossa  the  island  turns 
westward  and  is  controlled  by  the  axes  of  folds. 

Faults  frequently  determine  the  location  of  valleys,  either  by 
affording  convenient  courses  which  are  taken  possession  of  and 
excavated  by  streams,  or  directly  by  trough  faulting,  that  is, 
parallel  faults  hading  toward  each  other,  including  an  elongate 
sunken  block.  A  single  fault  may  lead  to  the  formation  of  a  valley 
when  followed  by  a  stream.  Fault  valleys  are  very  common  in  the 
Sierra  Nevada  and  Great  Basin;  and  in  the  Coast  Range  of  Cali- 
fornia a  series  of  valleys,  together  more  than  400  miles  long,  has 
been  excavated  along  the  fault  line  which  is  the  seat  of  the  recent 
earthquake  disturbances.  The  lower  Hudson  flows  in  a  fault 
valley,  of  which  the  Palisades  form  the  scarp,  and  in  the  Adiron- 
dacks  the  drainage  is  so  largely  along  intersecting  fault  lines  that 
its  regularity  is  very  striking  and  has  occasioned  the  descriptive 
term  of  "  lattice  drainage  "  (Brigham) ;  the  zigzag  course  of  the 
Au  Sable  Chasm  is  determined  partly  by  faults  and  partly  by  joints. 
The  valley  of  the  Rhine  above  Strassburg  is  a  great  trough-fault 
included  between  the  Black  Forest  highlands  on  the  east  and  the 
Vosges  Mountains  on  the  west,  both  of  which,  as  we  have  seen,  are 
Horst  mountains.  The  trough  is  very  deep  and  is  nearly  filled  with 
river  deposits,  which  indicates  the  gradual  character  of  the  dislocat- 
ing movements.  Other  European  examples  of  such  trough  valleys 
are  the  basin  of  Lake  Garda  in  northern  Italy,  the  Christiania 
fjord  in  Norway,  and  the  Gulf  of  Patras  and  Straits  of  Corinth  in 


468     TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

Greece     Such  valleys  are  likewise  extensively  developed  in  Mon- 
golia and  southern  Siberia;  the  basin  of  Lake  Baikal  is  in  a  trough 


most  remarkable  known  instance  of  trough  faulting  is  the 


F,G    =40  -  Normal  fault,  Au  Sable  Chasm,  N.V.      (Photograph  by  van  Ingen.) 
At  this  point  the  stream  leaves  the  line  of  fault  to  follow  a  jomt-plane 


TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  469 

great  Rift  Valley  of  eastern  Africa,  which  begins  about  15°  S. 
lat.,  and  after  a  short  course  sends  off  to  the  northwest  the 
Central  African  branch,  in  which  are  situated  lakes  Tanganyika, 
Albert  Edward,  and  Albert;  both  branches  are  also  the  seat  of 
extensive  volcanic  action.  The  main  valley  continues  northward 
from  Lake  Nyassa  and  contains  Lake  Rudolph  and  several  smaller 
lakes;  passing  between  the  plateaus  of  Abyssinia  and  Somaliland, 
it  extends  to  the  Red  Sea,  which  is  itself  regarded  as  being  but  a 
very  deep  part  of  the  same  great  downthrow  inundated  by  the  sea. 
At  the  northern  end  of  the  Red  Sea,  the  Gulf  of  Akabah,  on  the 
east  side  of  the  Sinai  Peninsula,  is  an  extension  of  the  trough  which, 
growing  shallower,  rises  to  the  land  and  keeps  its  northerly  direc- 
tion for  a  long  distance.  Growing  deeper  again,  it  becomes  the 
basin  of  the  Dead  Sea,  the  surface  of  which  is  1300  feet  below  sea- 
level,  and  the  valley  of  the  Jordan,  and  north  of  that,  the  valley  of 
the  Orontes,  reaching  nearly  to  Antioch.  The  valley  of  the  Jordan 
and  the  Dead  Sea  is  enclosed  between  parallel  faults  and  mono- 
clinal  folds,  the  descent  being  very  abrupt  on  the  eastern  side, 
while  on  the  west  it  is  more  gradual  and  by  means  of  several 
terraces  and  scarps.  In  Africa  the  great  Rift  Valley  is  not  bor- 
dered by  mountain  ranges,  but  cuts  through  high  plateaus,  and  the 
form  of  the  valley,  with  its  constantly  rising  and  falling  bottom, 
shows  that  it  is  not  a  valley  of  erosion,  but  one  of  tectonic  de- 
pression. 

If  this  interpretation,  which  we  owe  to  Suess,  is  correct,  this  vast 
dislocation  extends  through  fifty  degrees  of  latitude,  and  is  one  of 
the  most  striking  structural  features  of  the  earth's  surface. 

Faults,  especially  those  of  large  throw,  frequently  bring  rocks  of 
very  different  degrees  of  hardness  into  close  juxtaposition.  If  the 
harder  rocks  are  on  the  upthrow  side,  then  the  scarp  may  persist 
for  a  very  long  period,  because  they  are  so  much  more  slowly  worn 
away  than  the  softer  rocks  of  the  downthrow.  If  the  latter  is 
composed  of  the  harder  rock,  the  scarp  is  first  eroded  away,  and 
then  continued  denudation,  removing  the  softer  rocks  more 
rapidly,  forms  a  scarp  on  the  downthrow  side,  a  reversed  scarp, 


4/0    TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

so  to  speak,  due  entirely  to  differential  erosion.  In  other  words, 
the  hard  rocks  will  eventually  be  left  standing  at  a  higher  level 
than  the  softer  ones,  whether  they  are  on  the  upthrow  or  down- 
throw side,  and  thus  conform  to  the  general  rule  of  topographic 
development,  provided  the  region  stand  sufficiently  high  above 
base-level. 

Under  these  circumstances,  faults  of  very  high  geological  antiq- 
uity may  continue  to  dominate  the  topography  of  the  faulted 
region.  A  classical  example  of  this  is  given  by  the  Central  Low- 
lands of  Scotland,  which  lie  between  the  Highlands  on  the  north 
and  the  southern  Uplands,  and  consist  chiefly  of  quite  soft  and 
easily  destructible  rocks,  while  the  higher  areas  which  bound  them 
on  each  side  are  made  up  of  much  more  resistant  rocks.  When  the 
rocks  on  the  two  sides  of  the  fault-plane  differ  notably  in  hardness, 
then  the  transition  from  higher  to  lower  ground  is  well-defined  and 
abrupt,  but  the  transition  is  gradual  and  by  gentle  slopes  when 
the  difference  in  hardness  is  small.  The  great  faults,  with  throws 
sometimes  amounting  to  6000  feet,  which  demarcate  the  Low- 
lands, are  very  ancient,  but  they  still  control  the  main  features  of 
topography. 

When  the  faults  pass  through  strata  of  approximately  uniform 
durability,  the  scarps  are  removed  with  a  rapidity  which  is  chiefly 
dependent  upon  the  climate,  because  the  upthrow  side  is  exposed 
to  attack  and  the  downthrow  is  an  area  of  accumulation.  In 
such  rocks  all  surface  evidences  of  the  fault  are  removed,  and  the 
very  existence  of  the  dislocation  is  often  very  difficult  to  detect. 
Unquestionably,  countless  undetected  faults  remain  to  be  dis- 
covered even  in  well-known  regions. 

Unless  interrupted  by  diastrophic  movements,  the  progress  of 
denudation  must  eventually  remove  all  surface  indications  of  faults, 
however  great  the  difference  in  hardness  between  the  rocks  of  the 
upthrow  and  downthrow  sides.  When  the  region  is  base-levelled, 
or  reduced  to  a  peneplain,  no  fault-scarps  will  be  apparent.  On  the 
other  hand,  the  beginning  of  a  new  cycle  of  denudation,  through  the 
upheaval  of  the  faulted  area  and  the  rejuvenation  of  the  erosive 


TOPOGRAPHY  AS  DETERMINED  BY  FAULTS 


471 


forces,  may  result  in  the  reappearance  of  the  scarps,  provided  the 
rocks  on  the  two  sides  of  the  fault  differ  notably  in  hardness.  In 
this  case  the  harder  will  stand  above  the  level  of  the  softer,  whether 
they  are  on  the  upthrow  or  the  downthrow  side.  If  the  disloca- 
tions have  not  brought  together  rocks  of  different  resisting  powers, 
then  reelevation,  not  accompanied  by  renewed  displacements  along 
the  old  fault-planes,  will  not  bring  the  scarps  again  into  promi- 
nence, though  newly  established  streams  may  take  advantage  of 
these  lines  of  weakness  in  trenching  their  channels. 


FIG.  241.  —  Great  thrust,  near  Highgate  Springs,  Vt.  The  upthrow  side  has  been 
denuded  away  and  the  hammer  spans  the  thrust-plane,  connecting  beds  which 
are  stratigraphically  many  thousands  of  feet  apart.  (U.  S.  G.  S.) 

From  what  has  been  said  it  is  sufficiently  evident  that  faulting 
plays  a  very  important  part  in  the  formation  of  topographical  as 
well  as  of  structural  features,  and  to  faults  may  be  added  thrusts, 
though  the  importance  of  the  latter  is  relatively  less,  for  thrusts 
occur  among  violently  compressed  and  folded  rocks,  while  faults 
are  found  chiefly  in  strata  which  otherwise  are  not  greatly  dis- 
turbed. 


4/2    TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

THE  TOPOGRAPHICAL  INFLUENCE  OF  JOINTS 

As  we  have  repeatedly  had  occasion  to  observe,  all  firm  and  co 
herent  rocks  are  divided  by  sets  of  joints  into  blocks  of  varying 
sizes  and  shapes,  each  kind  of  rock  tending  to  display  a  certain 
degree  of  characteristic  uniformity  in  the  shape  and  size  of  its  joint- 
blocks.  There  is,  however,  much  latitude  in  this  respect,  for 
jointing  is  largely  determined  by  the  stresses  of  compression, 
tension,  or  torsion,  to  which  the  rock  has  been  subjected  after  its 
formation.  In  whatever  manner  they  may  have  been  formed, 
joints  exercise  a  very  important  control  in  the  development  of 
topographical  details.  The  reason  for  this  becomes  obvious  when 
we  reflect  that  joints  are  lines  of  weakness,  along  which  the  rocks 
are  especially  liable  to  attack,  and  that  master  joints  are  important 
structural  planes,  akin  to  faults,  which  persist  for  long  distances 
and  often  penetrate  to  very  considerable  depths  from  the  surface. 

One  of  the  most  important  kinds  of  control  exerted  by  joints  is 
in  conditioning  the  drainage  lines  of  a  given  region.  The  general 
direction  in  which  the  drainage  of  any  region  is  carried  is  deter- 
mined by  the  prevailing  direction  of  slope  existing  when  the  river 
system  was  established,  but  the  flow  of  minor  streams  is  very  often 
dependent  upon  the  direction  of  the  joints  in  the  rocks  through 
which  they  flow.  In  several  regions  where  the  matter  has  been 
carefully  examined,  as  in  France,  Connecticut,  and  Wisconsin, 
it  is  found  that  the  network  of  streams  closely  coincides  with 
the  network  of  .dip,  strike,  and  diagonal  joints,  which  have 
a  constant  trend  over  wide  areas.  The  Au  Sable  Chasm,  so 
frequently  referred  to  in  preceding  pages,  pursues  a  zigzag  course 
through  the  eastern  part  of  the  Adirondack  uplift;  in  part  this 
course  is  along  a  fault-plane,  which  is  again  forsaken  to  follow 
a  line  of  master-joint. 

An  especially  notable  case  of  joint-control  is  seen  in  the  gorge 
and  Victoria  Falls  of  the  Zambesi  in  South  Africa,  cut  through 
an  enormously  thick  volcanic  plateau  of  basaltic  lava,  an  account 
of  which  is  given  on  page  144  and  need  not  be  repeated  here. 


THE  TOPOGRAPHICAL   INFLUENCE  OF  JOINTS          4/3 

Suffice  it  to  say  that  the  course  of  the  gorge  has  been  determined 
by  the  master-joints,  the  river  endeavouring  to  keep  a  southerly 
course,  but  repeatedly  deflected  by  the  joint-planes.  The  great 
chasm  is  cut  along  a  line  of  fault,  which  yielded  readily  to  abra- 
sion, and  may  be  traced  into  the  sloping  cliff  at  each  end.  At 
the  present  time  the  western  cataract  of  the  Victoria  Falls,  called 
Leaping  Water,  has  discovered  another  line  of  weakness  and  has 
cut  its  bed  fifteen  feet  below  the  level  of  the  other  falls,  manifestly 
preparing  the  way  for  the  excavation  of  a  new  zigzag. 


FIG.  242.  —  Gorge  of  the  Zambesi  below  Victoria  Falls,  South  Africa 

Another  respect  in  which  joints  are  important  is  in  controlling 
the  details  of  topography,  for  the  obvious  reason  that  it  is  along 
the  joint-planes  that  rocks  yield  to  the  attack  of  the  denuding 
agencies,  most  of  which  loosen  and  detach  the  joint-blocks.  Evi- 
dently, the  form  of  surface  left  after  the  detachment  of  the  blocks 
is  conditioned  by  the  joint-planes.  In  the  plutonic  igneous 


474    TOPOGRAPHY  AS  DETERMINED  BY  FAULTS   AND  JOINTS 

rocks  there  is  great  variety  of  form  and  size  in  the  joint-blocks. 
Granite  is  often  divided  by  three  sets  of  nearly  rectangular  joints, 
one  of  which  is  horizontal  or  gently  inclined,  and  widely  spaced 
so  as  to  form  large  rectangular  blocks.  When  this  is  the  case, 
weathered  granite  cliffs  look  like  old  masonry  and  rise  in  step-like 
terraces.  When  the  joints  are  very  close  together,  the  rock  breaks 
up  into  small  angular  blocks.  In  other  examples,  only  the  vertical 


FIG.  243.  —The  Chasm,  Victoria  Falls  of  the  Zambesi 

joints  are  strongly  pronounced,  and  then  wild  and  craggy  cliffs  and 
needle-like  summits  result  from  weathering.  The  characteristic 
granite  domes  so  frequently  found,  to  which  reference  has  already 
been  made,  are  due  to  exfoliation  of  granites  in  which  the  joints 
are  few  and  widely  spaced. 

Columnar  jointing  in  sheets  is  chiefly  vertical,  and  hence  pro- 
duces vertical  faces,  which  change  and  follow  any  curvature  that 
may  develop  in  the  columns.  Most  lavas  are  closely  and  very 


THE  TOPOGRAPHICAL   INFLUENCE   OF  JOINTS          475 

irregularly  jointed,  so  that  they  are  rapidly  broken  up  by  frost. 
Such  rocks,  when  exposed  in  mountain  tops,  give  rise  to  sharp 
ridges,  jagged,  irregular  peaks,  and  very  rough  slopes,  portions  of 
the  rock  where  the  joints  are  more  widely  spaced  and  the  blocks 
larger,  yielding  less  rapidly  to  destruction,  and  projecting  as  ridges 
and  buttresses.  Aside  from  rectangularly  jointed  granite,  with 
well-defined  horizontal  division  planes,  masses  of  igneous  rock 


FlG.  244.  —  Granite  dome,  Yosemite  Valley,  Cal.     (Photograph  by  Sinclair) 

yield  rough,  irregular,  and  craggy  surfaces.  On  the  other  hand, 
the  direct  influence  of  joints  is  often  masked  by  the  effects  of  chemi- 
cal decomposition  produced  by  water  descending  along  the  joint- 
planes,  in  consequence  of  which  a  freshly  exposed  surface  is  often 
already  rotted  and  ready  to  yield  to  the  rain  and  wind.  Thus, 
with  general  characteristic  features,  igneous  rocks  give  an  infinite 
variety  of  details  in  form. 


476    TOPOGRAPHY  AS  DETERMINED  BY  FAULTS  AND  JOINTS 

In  stratified  rocks  the  joints  are  usually  vertical  to  the  bedding- 
planes,  and  when  the  strata  are  hard  and  do  not  crumble  easily  on 
weathering,  the  surfaces  are  vertical  in  horizontal  strata,  which 
thus  give  rise  to  flat-topped,  vertical -sided  mesas,  and  pyramidal 
mountains.  Inclination  of  the  strata  changes  the  position  of  the 
joint-planes,  resulting  in  escarpment,  dip  slopes,  and  the  other 
classes  of  forms  already  described.  The  fact  to  be  emphasized 
here  is  the  share  which  the  joints  have  in  producing  these  forms. 


FIG.  245.  —  Limestone  cliffs,  Black  Hills,  S.  D.     (U.  S.  G.  S.) 

The  crystalline  schists  are,  for  the  most  part,  jointed  in  a  highly 
irregular  manner,  though  gneiss  sometimes  has  rectangular  blocks, 
like  those  of  granite.  As  a  result  of  this  confused  jointing,  the 
schists  give  rise  to  forms  which  display  a  maximum  of  irregularity. 

Under  given  climatic  conditions,  each  kind  of  rock  weathers  in 
a  characteristic  manner,  and  this  manner  is,  in  large  degree,  de- 
termined by  the  joint-blocks  into  which  it  is  divided. 


CHAPTER    XXI 
ADJUSTMENT  OF  RIVERS 

RIVERS  are  among  the  most  powerful  of  the  agents  of  topo- 
graphical development,  and  it  is  important  to  understand  some- 
thing of  their  modes  of  change  and  adjustment.  These  changes 
are  sometimes  exceedingly  complex  and  puzzling,  for  rivers  do  the 


FIG.  246.  — Stream  cutting  through  a  ridge,  Middle  Park,  Col.     (U.  S.  G.  S.) 

most  unexpected  things  in  what  seems  an  utterly  capricious  and 
whimsical  way.  We  often  see  rivers  breaching  hills  and  even  vast 
mountain  ranges,  cutting  their  way  through  enormous  obstacles, 
which  a  slight  deviation  from  their  course  would,  seemingly,  have 
enabled  them  to  avoid.  They  apparently  choose  the  difficult  and 

477 


478  ADJUSTMENT  OF   RIVERS 

shun  the  easy  path.  The  general  explanation  of  these  paradoxical 
results  is,  that  the  river  began  its  flow  when  the  topography  was 
entirely  different  from  its  present  state  of  development.  It  is  this 
fact  which  renders  the  rivers  such  valuable  aids  to  the  geologist 
in  his  attempts  to  reconstruct  the  past,  for  the  apparent  whims  and 
caprices  are  really  the  necessary  results  of  law. 

Consequent  Rivers.  —  A  river  has  its  stages  of  development, 
youth,  maturity,  and  old  age,  just  as  has  a  land  surface,  each  stage 
displaying  its  characteristic  marks.  When  an  entirely  new  land 
surface  is  upheaved  from  the  sea,  it  has  no  rivers,  and  its  drainage 
must  consist  merely  of  the  surface  rain  wash  following  the  initial 
slopes  of  the  new  land.  No  instance  of  any  considerable  area  of 
newly  uplifted  land  has  ever  been  observed,  but  the  sequence  of 
events  may  be  readily  inferred  from  known  facts.  Since  the 
slopes  cannot  be  absolutely  plane  nor  the  material  entirely  homoge- 
neous, there  must  be  slight  depressions  along  which  the  rain  water 
will  gather  into  rills,  and  these  will  wear  out  little  trenches.  The 
more  favourably  situated  trenches  will  receive  more  water  and 
be  more  rapidly  deepened  and  enlarged  into  ravines.  In  this 
early  stage  of  drainage  development  there  will  be  many  ravines, 
more  or  less  parallel,  which  are  dry  except  after  rains.  Those 
ravines  which  are  most  rapidly  deepened  will  be  cut  down  to  the 
level  of  the  ground  water  and  will  there  be  fed  by  springs  and 
become  permanent  streams  when  a  level  is  reached  bjlow  which 
the  ground  water  does  not  sink  in  the  driest  seasons.  If  the  new 
land  is  not  simply  sloping,  but  folded,  valleys  for  drainage  are 
afforded  by  the  synclines.  The  principal  valleys  are  thus  longi- 
tudinal, the  main  streams  flowing  in  the  synclinal  troughs  and 
passing  from  one  syncline  to  another  at  the  points  where  the  anti- 
clines are  lowest,  owing  to  the  descending  pitch  of  the  folds. 
Such  a  drainage  system  is  exemplified  in  the  Jura  Mountains  of 
Switzerland.  Thus,  in  a  newly  upheaved  or  newly  folded  land 
the  streams  are  determined  entirely  by  the  slopes  of  the  new  sur- 
face and  are  called  consequent  streams.  In  its  earliest  stages  a 
river  can  drain  its  territory  or  basin  in  only  imperfect  fashion,  and 


CONSEQUENT   RIVERS 


479 


FIG.  247.  —  Two  very  voung  gulches,  Colorado.     (U.  S.  G.  S.) 


480  ADJUSTMENT  OF  RIVERS 

whatever  depressions  exist  in  the  surface  of  the  new  land  are  filled 
up  with  water  and  form  lakes.  Tributaries  are  much  fewer  than 
in  later  stages  of  development;  the  divides  between  the  tributaries 
are  obscurely  marked,  and  in  plains  these  divides  are  broad  areas, 
not  lines.  The  Red  River  of  the  North  is  an  example  of  a  stream 
in  a  very  youthful  stage,  which  flows  across  the  level  floor  of  an 
abandoned  lake.  In  this  plain  the  divides  between  the  streams 
are  so  wide  and  flat  that  water  gathers  on  them  after  heavy  rains, 
having  no  reason  to  flow  in  one  direction  rather  than  another.  In 
northern  Minnesota  is  the  watershed  or  divide  between  the  Mis- 
sissippi, St.  Lawrence,  and  Hudson's  Bay  drainage  systems,  which 
is  hardly  visible,  the  sluggish  streams  wandering  over  an  almost 
flat  surface,  which  has  countless  marshes  and  lakes. 

As  the  river  system  becomes  somewhat  older,  the  stream  chan- 
nels are  deepened,  the  larger  ones  being  cut  down  to  base-level, 
and  if  the  region  be  one  of  considerable  elevation,  deep  gorges 
and  canons  are  excavated.  If  the  streams  flow  across  strata  of 
different  hardness,  waterfalls  result  where  a  hard  ridge  crosses 
them,  but  in  the  main  stream  these  cascades  and  rapids  are 
ephemeral  and  soon  removed  by  the  stream's  wearing  down  the 
obstacle.  On  the  head-waters  of  streams,  however,  waterfalls  may 
persist  for  a  long  period.  The  river  valleys  are  widened  out  by 
atmospheric  denudation,  and  channels  are  formed  on  their  sloping 
sides,  which  gradually  grow  into  side  valleys.  The  lakes  are  for 
the  most  part  drained  or  silted  up,  only  the  more  important  and 
deeper  ones  remaining,  while  the  system  of  tributary  streams  and 
rills  is  greatly  expanded.  A  mature  river  system  is  characterized 
by  the  complete  development  of  its  tributaries  and  drainage,  so 
that  every  part  of  its  basin  is  reached  by  the  ramifying  channels, 
and  rivers  of  the  same  grade  tend  to  be  separated  by  nearly  equal 
interspaces.  The  waterfalls  have  disappeared,  except  near  the 
stream-heads,  and  the  stream-channels  have  sought  out  and 
utilized  every  weakness  in  the  strata,  adjusting  themselves  to  the 
structure  of  the  rocks  and  the  alternations  of  hard  and  soft  beds. 

Valley  floors  are  broadened  and  deposition  begins  upon  them, 


CONSEQUENT   RIVERS  481 

and  the  streams,  reaching  a  condition  of  equilibrium  between  ero- 
sion and  deposition,  are  said  to  be  graded.  In  graded  streams  the 
slope  attained  varies  greatly;  a  small  stream  or  one  loaded  with 
sediment  requires  a  steeper  slope  than  a  large  one,  or  one  carry- 
ing but  a  small  load.  Thus,  the  lower  Mississippi  and  its  tributa- 
ries are  graded,  but  while  the  great  river  flows  in  a  valley  with 
hardly  any  slope,  the  valleys  of  the  smaller  streams  are  still  quite 
steep.  In  the  process  of  development  the  stream  gradients  are 
continually  readjusted,  with  the  general  result  of  diminishing 
the  slope.  When  the  stream  has  reached  base-level  and  no 
longer  erodes  vertically,  save  in  seasons  of  flood,  it  continues  to 
cut  laterally  and  to  receive  and  transport  the  material  washed  into 
it  by  rains,  and  thus  the  divides  are  worn  away. 

The  complete  network  of  streams  has  enlarged  the  valley 
surfaces,  which  increases  the  rate  of  destruction  and  brings  to 
the  river  a  greater  load  of  sediment  to  carry.  In  maturity  the 
river  receives  its  maximum  load,  sometimes  so  great  that  the  lower 
reaches  of  the  main  stream  are  unable  to  transport  it  all,  and 
spread  the  excess  out  over  the  flood-plain.  The  channel  of  an 
overloaded  stream  may  be  so  raised  and  banked  in  by  its  own 
deposits  that  some  of  the  tributaries  are  deflected  and  made  to 
run  for  some  distance  parallel  to  the  main  stream,  perhaps  even 
reaching  the  sea  independently.  An  example  of  this  is  the  Loup 
Fork  of  the  Platte  in  Nebraska.  "  The  Platte  flows  there  upon  a 
ridge  of  its  own  creation.  The  Loup  comes  down  into  its  valley 
and  flows  parallel  with  it  for  many  miles."  (Gannett.) 

The  final  stages  of  river  development  are  reached  when  the 
base-level  is  attained,  and  the  drainage  basin  reduced  to  a  pene- 
plain by  the  combined  action  of  the  streams  and  weathering.  The 
flood-plain  deposits  may  now  be  partially  or  completely  removed, 
for  the  main  trunk  no  longer  receives  an  excessive  load,  and 
hence  it  is  able  to  carry  away  some  of  that  sediment  which  it  had 
previously  deposited.  With  its  drainage  basin  smoothed  down 
into  a  peneplain,  the  river's  work  is  done;  it  has  reached  old  age. 

The  course  of  river  evolution  above  described  is  the  ideal  cycle 


482  ADJUSTMENT  OF   RIVERS 

of  development,  which,  however,  may  be  and  generally  is  inter- 
rupted by  diastrophic  movements.  An  elevation  of  the  region  may 
simply  rejuvenate  the  streams  and  start  them  afresh  upon  a  career 
of  wearing  down  the  land.  But  if  accompanied  by  extensive 
warping  or  folding  of  the  rocks,  the  drainage  system  of  the  entire 
region  may  be  revolutionized.  A  depression  of  the  region  will 
have  the  contrary  effect,  checking  or  stopping  the  work  in  which 
the  streams  were  engaged,  drowning  their  lower  reaches  and  con- 
verting them  into  estuaries.  A  lowered  land  surface  has  less 
material  to  lose  before  it  is  reduced  to  base-level,  but  the  work 
of  denudation  is  accomplished  more  slowly. 

When  it  was  first  suggested  that  rivers  had  cut  their  own  valleys 
and  had  not  merely  taken  possession  of  ready-made  trenches,  it 
was  objected  that  such  an  explanation  required  many  streams  to 
begin  their  course  by  flowing  uphill.  It  is  very  common  to  find 
a  stream  flowing  across  a  region,  cutting  its  way  through  ridge 
after  ridge,  instead  of  following  the  easy  path  of  the  longitudinal 
valleys.  This  is  just  what  the  principal  streams  of  the  northern 
Appalachians,  such  as  the  Delaware,  the  Susquehanna  and  the 
Potomac,  have  done,  and  at  first  sight,  their  course  is  very  difficult 
to  explain.  Without  going  very  far  back  into  the  history  of  these 
mountains,  we  may  simply  state  that  the  ridges  through  which  the 
rivers  named  have  cut  are  the  remnants  of  a  reelevated  and  dis- 
sected peneplain,  across  which  the  streams  flowed  to  the  sea,  cut- 
ting transverse  valleys  that  were  rapidly  deepened  into  gorges. 
On  the  soft  strata  longitudinal  valleys  were  opened  out  which, 
however,  were  formed  after  the  transverse  streams  and  could  not 
be  deepened  faster  than  they,  because  the  main  stream  flowing  in 
each  transverse  valley  gave  a  temporary  base-level  for  the  tribu- 
taries flowing  in  the  longitudinal  valleys.  The  hard  beds  were 
sawed  through  by  the-  descending  streams,  but  elsewheie  these 
beds  stood  up  as  ridges,  and  thus  the  ridges  are  also  younger  than 
the  streams.  The  mystery  disappears  at  once,  if  we  simply  remem- 
ber that  the  transverse  streams  began  their  flow  upon  a  sloping 
plain  above  which  the  present  ridges  did  not  project. 


ANTECEDENT   RIVERS  483 

Antecedent  Rivers.  —  Another  way  in  which  rivers  have  been 
enabled  to  cut  their  way  through  opposing  ranges  of  hills  and 
even  mountains  is  by  occupying  the  district  before  the  hills  or 
mountains  were  made.  Such  streams  are  called  antecedent  and 
are  denned  as  "  those  that  during  and  for  a  time  after  a  disturb- 
ance of  their  drainage  area  maintain  the  courses  that  they  had 
taken  before  the  disturbance."  (Davis.)  In  this  manner  a 
stream  originally  consequent  may  become  antecedent.  The  sim- 
plest case  of  antecedent  drainage  is  where  an  area  is  uplifted  with- 
out deformation  and  without  changing  the  direction  of  the  slopes. 
Under  such  circumstances  all  the  streams  retain  their  old  channels, 
and  simply  gain  renewed  power  to  cut  them  into  deeper  trenches, 
down  to  the  new  base-level.  Such  streams  are  said  to  be  revived. 
Revived  streams  which  had  begun  to  meander  may  be  held  in  these 
windings  and  trench  them  into  deep  gorges.  Even  if  the  upheaval 
be  accompanied  by  folding  or  deformation,  one  or  more  of  the 
streams  may  persist  in  its  ancient  course,  provided  the  folding  be 
very  slow  and  gradual,  so  that  the  river  is  able  to  cut  down  through 
the  obstacles  which  are  raised  athwart  its  course.  A  revolving 
saw  cuts  its  way  through  a  log  which  is  pushed  against  it,  so  the 
river  cuts  its  way  through  the  rising  barrier.  If  the  latter  be  raised 
faster  than  the  river  can  cut,  then  the  stream  will  be  dammed 
back  into  a  lake,  or  will  be  diverted  to  a  new  course.  Naturally, 
the  great  trunk  rivers  are  more  likely  to  hold  their  previous  courses 
than  the  smaller  streams. 

A  fine  example  of  an  antecedent  river  is  the  Columbia,  of  Wash- 
ington and  Oregon,  which  is  deflected  to  the  westward  by  the  vol- 
canic plateau  of  central  Washington  as  far  as  the  foot  of  the  Cascade 
Mountains,  where  it  turns  southward,  following  the  mountains 
for  some  distance,  then  it  once  more  turns  to  the  westward  and  cuts 
through  the  Cascades  in  a  great  canon.  This  course  the  river  has 
maintained  despite  a  differential  uplift  of  thousands  of  feet,  and 
probably  also  the  rising  of  the  Cascades  athwart  its  course.  The 
Snake  River,  a  tributary  of  the  Columbia,  has  cut  a  canon  6000  feet 
deep  through  lava  and  granite,  through  a  slowly  rising  upwarp. 


484  ADJUSTMENT  OF   RIVERS 

Several  rivers  in  the  Alps  and  Himalayas,  which  rise  in  the  innei 
part  of  the  ranges  and  cut  their  way  out  through  deep  chasms, 
are  believed  to  be  antecedent. 

Superimposed'  Rivers.  —  An  old  land-surface  with  well-defined 
topography  may  be  deeply  buried  under  newer  accumulations,  as  of 
lava  floods,  great  bodies  of  volcanic  ash  and  tuff,  sheets  of  glacial 
drift,  lake  deposits,  or,  after  depression  beneath  the  sea,  by  marine 
deposits.  In  each  of  these  cases,  the  new  surface  has  no  reference 
whatever  to  the  old ;  the  more  ancient  and  buried  rocks  may  and 
generally  do  have  an  entirely  different  character,  arrangement,  and 
structure  from  those  which  overlie  them.  The  drainage  system 
established  upon  the  new  surface  is  consequent  upon  the  initial 
slopes  of  the  latter,  and  when  the  streams  have  cut  through  the 
mantle  of  newer  rocks  and  reach  the  ancient  surface  below,  they 
are  entirely  out  of  adjustment  with  that  surface  and  its  rocks.  As 
the  streams  cut  their  trenches  through  the  overlying  mantle  of 
newer  strata,  they  encounter  the  older  rocks  below,  first  laying 
bare  the  higher  ridges  of  the  latter,  which  will  cause  waterfalls 
and  rapids.  The  upper  Mississippi  has  in  many  places  excavated 
its  channel  through  the  surface  sheet  of  glacial  drift  and  is  now 
engaged  in  eroding  the  ancient  crystalline  rocks  which  the  drift 
had  covered.  When  the  stream  has  everywhere  cut  through  the 
newer  rocks,  its  course  will  be  seen  to  have  no  relation  to  the 
structure  of  the  older  rocks  which  it  is  now  trenching.  If,  as  fre- 
quently has  happened,  denudation  has  stripped  away  almost  all 
the  newer  strata,  the  drainage  of  the  country  seems  to  be  quite 
inexplicable  and  to  be  arranged  without  any  reference  to  the  struc- 
ture of  the  rocks  across  which  the  streams  flow.  Such  a  system 
of  drainage  is  said  to  be  superimposed,  inherited,  or  epigenetic. 

Examples  of  superimposed  streams  may  be  found  in  great 
numbers  in  the  United  States.  Almost  all  of  the  minor  streams  in 
that  part  of  the  country  which  is  covered  with  glacial  drift  belong 
in  this  class,  as  do  also  the  rivers,  like  the  Columbia,  the  Snake, 
and  the  Des  Chutes,  which  trench  the  great  volcanic  plateau  of 
Idaho,  Oregon,  and  Washington.  An  especially  curious  and  in- 


ADJUSTMENT  OF   RIVERS  485 

teresting  case  is  found  in  western  Colorado  in  the  valleys  of  the 
Uncompahgre  and  Gunnison.  When  these  streams  were  first 
established,  the  region  was  a  plateau  with  westerly  slope  largely 
built  up  of  volcanic  ash,  beneath  which  an  old  topography  was 
buried.  The  Gunnison  flowed  over  a  concealed  mountain  of 
granite,  and  when  its  valley  had  been  cut  down  to  the  granite,  it 
was  compelled  to  hold  the  same  course,  and  has  trenched  a  canon 
2000  feet  deep  in  that  rock.  The  Uncompahgre,  which,  though  a 
tributary  of  the  Gunnison,  flows  parallel  with  it  for  a  considerable 
distance,  followed  a  course  which  took  it  over  an  old  valley  buried 
under  soft  materials  which  were  rapidly  removed.  At  the  present 
time  the  course  of  the  Gunnison  seems  to  be  quite  paradoxical, 
though  it  is  easily  explained  by  its  history. 

Subsequent  Streams.  —  As  a  river  system  approaches  maturity, 
and  as  the  drainage  of  the  area  becomes  more  complete,  it  will 
increase  the  number  of  its  branches.  Those  branches  which 
were  not  at  all  represented  in  the  youthful  stages  of  the  system, 
and  are  opened  out  along  lines  of  yielding  rocks,  are  called  subse- 
quent, and  all  streams  will  develop  more  or  fewer  of  such  branches 
as  they  advance  to  maturity. 

The  foregoing  classification  of  streams  does  not  involve  cate- 
gories which  are  entirely  exclusive  of  one  another.  Any  stream, 
whatever  its  mode  of  origin,  may  become  antecedent  through 
diastrophic  movements.  Most  superimposed  streams  are  also  con- 
sequent, but  by  no  means  are  all  consequent  streams  superimposed. 
The  Columbia  and  Snake  are  both  superimposed  and  antecedent 

Adjustment  of  Rivers.  —  However  the  streams  of  a  district 
may  have  been  established  in  the  first  instance,  whether  they 
were  consequent,  antecedent,  or  superimposed,  they  are  liable  to 
changes  more  or  less  profound  and  far-reaching.  These  changes) 
which  belong  to  the  normal  development  of  the  drainage  system 
and  are  not  dependent  upon  diastrophism,  are  due  to  adjustment 
of  the  streams  to  the  rock  structure  of  the  district,  the  streams 
searching  out  the  lines  of  weakness  and  least  resistance,  and 
everywhere  taking  the  easiest  path  to  their  destination.  The  up- 


486  ADJUSTMENT  OF   RIVERS 

stream  extension  of  branches  and  the  shifting  of  divides  result 
in  the  capture  of  streams,  or  parts  of  such,  by  others  more  favour- 
ably situated,  one  master  stream  gradually  absorbing  many  smaller 
ones  which  had  originally  been  independent. 

A  divide,  or  water-parting,  between  two  streams  is  gradually 
shifted  by  the  lengthening  of  the  more  favourably  situated  stream, 
or  of  one  of  its  subsequent  branches.  This  more  favourable  situa- 
tion may  be  because  it  has  a  shorter  course  and  greater  fall,  giving 
a  swifter  flow,  or  because  it  flows  at  a  lower  level,  giving  greater 
fall  to  its  tributaries,  or  because  its  course  is  through  soft  and  easily 
eroded  rocks,  while  its  rival  is  embarrassed  by  hard  rocks  and 
ledges.  Another  favourable  circumstance  which  may  decide  be- 
tween streams  otherwise  equal  is  given  by  the  attitude  of  the  strata. 
In  regions  of  inclined  strata,  as  we  have  already  learned,  the  es- 
carpments formed  by  outcropping  ledges  of  harder  rocks  tend 
to  migrate  in  the  direction  of  the  dip.  As  such  escarpments 
frequently  form  divides  between  minor  streams,  the  stream  toward 
which  the  escarpment  migrates  will  be  at  a  disadvantage.  This 
shifting  of  divides  is  a  very  slow  process,  but  after  a  long  time  of 
insidious  advance  the  actual  capture  and  diversion  of  part  of  a 
stream  may  be  quite  suddenly  effected. 

Stream  capture  may  be  effected  in  a  great  variety  of  ways,  but 
a  few  examples  must  suffice.  We  may,  in  the  first  place,  suppose 
two  neighbouring  streams  following  roughly  parallel  courses,  but 
owing  to  the  original  conformation  of  the  region,  flowing  at  dif- 
ferent levels.  The  stream  that  flows  at  the  lower  level  will  allow 
greater  fall  to  its  tributaries,  which  will  thus  work  upward  more 
rapidly.  One  of  these  tributaries  will  eventually  work  its  way 
through  the  divide  and  tap  the  rival  stream,  all  of  whose  waters 
above  the  point  of  tapping  will  be  diverted  to  the  main  stream 
which  flows  at  the  lower  level. 

The  same  mode  of  capture  may  be  effected  in  the  case  of  two 
streams  which  head  on  opposite  sides  of  the  same  divide,  one  of 
which  has  a  much  steeper  grade  than  the  other.  The  stream 
which  has  the  steeper  slope  will  work  headward  more  rapidly,  and 


ADJUSTMENT  OF  RIVERS  48? 

will  eventually  tap  the  head-waters  of  the  opposite  stream.  This 
method  is  illustrated  in  the  Catskill  Mountains,  which  are  carved 
out  of  a  table-land  sloping  gently  westward,  but  having  a  steep 
escarpment  on  the  eastern  face,  with  recesses  in  which  easterly 
flowing  streams  were  established.  At  the  same  time  westwardly 
flowing  streams  formed  on  the  gently  sloping  summit  level.  The 
eastern  streams,  especially  the  Kaaters  Kill  and  Plaaters  Kill,  have 
a  steep  descent  and  swift  flow  and  have  thus  been  able  to  extend 
their  heads  rapidly  and  to  capture  and  divert  many  tributaries  of 
Schoharie  Creek,  which  runs  westward.  These  captured  tributaries 
keep  their  original  course  in  the  direction  of  Schoharie  Creek,  but 
make  sharp  turns  and  reverses  to  join  the  capturing  streams.  The 
Yellowstone  Lake  once  discharged  southwestward  into  the  Snake 
River.  The  Yellowstone  River  was  then  a  small  stream,  but  was 
favourably  situated  for  establishing  a  steep  gradient  as  it  flowed 
over  a  plateau  of  comparatively  soft  lava.  Extending  upward,  the 
stream  tapped  the  lake  and  reversed  the  direction  of  its  discharge. 
Another  method  of  stream  capture  is  well  illustrated  by  the 
Delaware,  the  Potomac,  and  other  transverse  rivers  which  have 
cut  deep  gorges  through  the  Appalachian  ridges.  Suppose  two 
parallel  transverse  streams  flowing  across  a  gently  sloping  pene- 
plain which  is  composed  of  tilted  rocks  of  different  degrees  of 
hardness.  In  the  manner  already  explained  (p.  482)  these  streams 
cut  gorges  through  the  ridges  of  hard  rock,  while  longitudinal  val- 
leys are  worn  out  along  the  strike  of  softer  strata,  which  valleys  are 
occupied  by  tributaries  of  the  transverse  streams.  If  one  of  the 
two  transverse  streams  be  considerably  larger  than  the  other,  it  will 
saw  its  way  through  the  hard  ridges  at  a  correspondingly  faster  rate 
and  establish  a  lower  base-level  for  its  tributaries.  One  of  the 
tributaries  with  its  more  rapid  fall  will  be  thus  enabled  to  shift  its 
divide  at  the  expense  of  a  branch  of  the  rival  transverse  stream, 
capture  it,  and  by  reversing  the  direction  of  its  flow  draw  off  the 
waters  of  the  smaller  main  stream  above  the  point  where  its  cap- 
tured tributary  entered  it.  Or  a  tributary  of  the  larger  main 
stream  may  push  its  way  up  a  longitudinal  valley  until  it  taps  and 


488  ADJUSTMENT  OF   RIVERS 

diverts  the  smaller  transverse  stream  without  the  intermediation 
of  any  tributary  of  the  latter.  Examples  of  both  of  these  varieties 
of  capture  may  be  found  among  the  Appalachian  rivers;  an  ex- 
cellent illustration  of  the  latter  method  is  given  by  the  Potomac 
and  Shenandoah. 

When  the  Potomac  was  beginning  to  cut  its  gap  through  the 
Blue  Ridge  at  Harper's  Ferry,  a  smaller  stream,  Beaverdam 
Creek,  was  cutting  a  similar  gorge  through  the  same  ridge  a  few 
miles  to  the  south.  The  Shenandoah  was  then  a  young  and  short 
tributary  of  the  Potomac,  which  it  entered  from  the  south,  flowing 
through  the  longitudinal  valley  which  was  opening  along  the 
strike  of  the  softer  strata  to  the  west  of  the  Blue  Ridge.  As  the 
Potomac  is  much  larger  than  Beaverdam  Creek,  it  cut  its  gap 
much  more  rapidly,  thus  giving  a  steep  and  swift  course  to  the 
Shenandoah.  The  latter  pushed  its  way  up  the  longitudinal 
valley  until  it  tapped  Beaverdam  Creek  and  captured  its  upper 
course,  diverting  its  waters  to  the  Potomac.  Beaverdam  Creek 
no  longer  flowed  through  the  gorge  which  it  had  cut  in  the  Blue 
Ridge  and  which  was  thus  abandoned  and  became  a  "  wind-gap," 
the  beheaded  Beaverdam  now  rising  to  the  eastward  of  the 
abandoned  gorge.  This  gorge  is  known  as  Snickers  Gap.  The 
great  number  of  wind-gaps  in  the  Appalachian  ridges  show  how 
frequently  the  capture  and  diversion  of  smaller  streams  by  larger 
ones  has  been  accomplished  among  those  mountains. 

Figures  248  and  249  show  two  stages  in  the  evolution  of  a  river 
system.  Figure  248  represents  the  first  stage,  in  which  several 
transverse  streams,  a,  c,  e,  f,  g,  are  breaching  the  escarpments 
indicated  by  shaded  lines.  Of  these  streams,  c  carries  the  most 
water,  and  will  therefore  deepen  its  gorges  through  the  hard 
ridges  more  rapidly  than  the  others,  and  give  its  tributaries  the 
advantage  of  a  greater  fall.  In  the  second  stage  (Fig.  249),  c  has 
captured  the  upper  courses  of  all  the  other  streams  except  g, 
which  has  not  yet  been  reached.  The  branch  /  has  captured  a, 
beheading  it,  diverting  the  portion  a"  and  reversing  the  portion  a1 '. 
Similarly,  m  has  captured  and  divided  e,  n  has  done  the  same 


ADJUSTMENT  OF  RIVERS 


489 


with  b,  and  p  with  d,  while  g  must  eventually  suffer  the  same 
fate.  Wind-gaps  will  be  left  in  the  ridges  where  the  captured 
streams  once  crossed  them. 

In  regions  of  folded  rocks  thrown  into  a  series  of  parallel 
anticlines  and  synclines,  the  process  of  adjustment  may  become 
exceedingly  complicated.  Suppose  an  original  consequent  stream 
flowing  in  a  syncline  of  hard  rock  considerably  above  base-level, 
whose  subsequent  branches  have  opened  out  valleys  in  softer 
rocks  along  the  crests  of  the  anticlines,  where  the  harder  surface 


FlG.  248. —  Evolution  of  a  river  system, 
first  stage.  The  shaded  lines  repre- 
sent escarpments  of  hard  rock.  (De 
Lapparent) 


FlG.   249.  —  Evolution  of  a  river  sys- 
tem, second  stage.     (De  Lapparent) 


stratum  is  first  cut  through.  The  extension  and  junction  of  these 
subsequent  branches  may  offer  a  more  advantageous  course 
than  the  hard  syncline,  and  cause  the  latter  to  be  wholly  or 
partially  deserted.  The  streams  originally  flowing  in  the  synclinal 
troughs  may  gradually  be  shifted  to  the  degraded  anticlines  which, 
as  we  have  seen,  are  wasted  away  more  rapidly  and  this  trans- 
fer is  facilitated  by  the  fact  that  the  synclinal  troughs  have  very 
gentle  slopes  longitudinally,  giving  but  small  velocity  to  the  rivers 
which  flow  in  them. 


490  ADJUSTMENT  OF  RIVERS 

A  thoroughly  mature  drainage  system  is  characterized  by  a 
complete  adjustment  of  its  streams  to  the  structure  of  the  rocks. 
The  rivers  as  finally  established  are  thus  apt  to  be  a  patchwork 
of  streams  captured  and  diverted,  and  the  result  of  adjustment 
is  the  production  of  a  system  often  radically  different  from  the 
original  one.  Even  after  a  river  system  has  become  maturely 
adjusted,  a  reelevation  of  the  country  may  produce  a  new  and 
entirely  different  adjustment,  by  changing  the  relation  of  the 
folds  and  outcrops  of  hard  and  soft  strata  to  the  base-level.  A 
region  of  great  antiquity  which  has  repeatedly  been  worn  down 
and  reelevated  will  have  experienced  many  revolutions  of  its 
drainage  systems. 

Warping  of  the  surface  nearly  always  produces  extensive 
changes  in  the  drainage  systems  affected  by  diverting  the  course 
of  many  streams,  though  the  master  streams  may  excavate  with 
sufficient  rapidity  to  hold  their  channels  as  antecedent  rivers. 
As  will  be  shown  in  a  subsequent  chapter,  the  Appalachian 
Mountains  had,  by  the  close  of  the  Cretaceous  period,  been  worn 
•  away  to  a  peneplain,  across  which  the  Delaware,  Susquehanna, 
and  Potomac  flowed  in  transverse  valleys  to  the  Atlantic,  while 
the  New  River  in  Virginia  and  the  French  Broad  in  North  Carolina 
flowed  westward  to  the  Mississippi.  The  southern  part  of  the 
peneplain  was  drained  by  a  longitudinal  river  called  the  Ap- 
palachian River,  and  smaller  streams  running  westward  and 
southwestward  to  an  extension  of  the  Gulf  of  Mexico  drained 
the  southwestern  side  of  the  peneplain.  Next  followed  an  up- 
warping  of  the  peneplain  along  a  north-south  axis,  through  which 
the  northern  rivers  continued  in  their  old  courses,  but  one  of  the 
southwestern  streams  extended  headward  and  captured  the  head- 
waters of  the  Appalachian  River.  Still  another  upwarp  suc- 
ceeded, this  time  on  an  axis  running  nearly  east  and  west  through 
northern  Alabama  and  Mississippi,  in  consequence  of  which 
a  tributary  of  the  Ohio  extended  itself  southward  and  captured 
the  southwestern  stream  which  had  before  beheaded  the  Ap- 
palachian River.  Thus  arose  the  Tennessee,  which  enters  the 


ACCIDENTS  TO  RIVERS  49 1 

Ohio  after  such  a  curious  course  and  which  is  made  up  of  parts 
of  three  originally  independent  river  systems,  and  in  its  changes 
records  the  history  of  the  region  through  which  it  flows. 

Accidents  to  Rivers. — This  term  is  employed  to  express  the 
interruptions  which  hinder  or  prevent  the  normal  development  of 
a  river  system.  The  diastrophic  changes  and  their  effects  we  have 
already  considered,  but  there  are  others  which  should  be  men- 
tioned. A  change  of  climate  from  moist  to  arid  greatly  interferes 
with  the  development  and  adjustment  of  a  river  system.  Many 
stream  channels  are  abandoned  and  others  are  occupied  only  after 
rains,  while  the  reduced  flow  in  the  permanent  streams  diminishes 
their  erosive  powers.  Large  areas,  like  the  Great  Basin  region, 
may  have  no  outlet  to  the  sea,  because  the  mountain  streams  all 
lose  themselves  in  the  desert  sands.  Lake  Bonneville  (see  p.  219) 
had  an  outlet  until  the  increasing  dryness  of  the  climate  so  lowered 
its  waters  that  the  outlet  could  no  longer  be  reached,  evaporation 
exceeding  influx.  Great  lava  flows  may  obliterate  the  drainage 
system  of  a  region  and  compel  the  establishment  of  an  entirely 
new  one,  as  has  happened  in  southern  Idaho  and  southeastern 
Oregon,  a  region  of  exceedingly  immature  topography  and  drain- 
age. Extensive  ice-sheets,  by  spreading  a  thick  mantle  of  drift 
which  fills  up  the  valleys,  may  produce  the  same  effects  as  lava 
flows,  except  that  the  drift  is  more  easily  removed.  In  the  north- 
eastern United  States  many  streams  have  been  displaced  by  the 
sheets  of  glacial  drift,  and  forced  to  seek  new  channels  at  a  com- 
paratively recent  date;  they  still  preserve  all  the  signs  of  youth, 
such  as  deep,  trench-like  gorges  (see  Fig.  58),  waterfalls,  and 
rapids.  The  larger  rivers  have,  for  the  most  part,  been  able  to 
reoccupy  their  old  valleys,  but  the  smaller  streams  have  generally 
been  compelled  to  excavate  new  channels. 


CHAPTER   XXII 
SEA-COASTS 

THE  sea-coast  is  not  merely  a  line,  but  a  zone  of  varying  breadth, 
sloping  toward  the  sea,  and  with  a  subaerial  and  a  submarine 
portion.  The  submarine  portion  of  the  coast  frequently  con- 
tinues the  slope  of  the  subaerial  portion,  with  an  interruption 
formed  by  the  actual  beach,  upon  which  the  surf  breaks.  'On 
flat,  gently  sloping  coasts,  the  beach  is  generally  broad,  especially 
if  the  range  of  the  tides  is  great,  and  a  beach  wall  is  present 
(see  p.  247) ,  frequently  with  a  belt  of  sand-dunes  behind  it,  while 
on  steep,  rocky  coasts  the  beach  is  narrow  and  may  occasionally 
be  absent  altogether. 

The  coasts  of  the  different  parts  of  the  world  display  a  great 
variety  of  form  and  structure,  but  they  may  all  be  included  in 
a  small  number  of  classes.  An  obvious  primary  division  is  into 
(i)  regular,  (2)  irregular,  and  (3)  lobate  coast-lines. 

i.  Regular  Coasts  continue  "  for  great  distances  without 
notable  indentations  and,  for  the  most  part,  in  gentle  curves, 
convex  toward  the  land,  which  are  connected  by  curved  lines 
or  meet  at  obtuse  angles.  .  .  .  The  flatter  the  coast,  the  more 
perfectly  is  this  type  developed,  and  the  coast-line  runs  for  many 
kilometers  in  the  same  curve.  With  a  steep  slope  the  course 
is  regular  only  in  general;  in  detail  it  seems  as  though  drawn 
by  a  trembling  hand,  with  numerous  little  prominences,  which 
project  but  a  few  hundred  metres  beyond  the  general  coast-line 
and  separated  from  one  another  by  shallow,  curved  indentations." 
(Penck.)  Flat  coasts  in  most  cases  border  coastal  plains  and  are 
very  generally  regular,  while  the  regular  steep  coasts  are  marked 
by  lines  of  cliff,  which  abruptly  break  the  slope  of  the  land  toward 

492 


REGULAR   COASTS  493 

the  sea.  Certain  regular  coasts,  like  that  of  eastern  North  America, 
for  example,  are  uniformly  flat  or  steep  for  long  distances,  others 
are  alternately  flat  and  steep,  and  are  then  said  to  be  adjusted. 
Regular  coasts  have  few  islands. 

Coastal  plains  are  absent  from  desert  regions  and  are  built  up 
by  the  activity  of  rivers;  they  may  be  coalesced  deltas,  or  of 
littoral  origin,  or  submarine  with  the  river  sediments  distributed 
along  the  shore  by  currents;  subsequent  elevation  has  converted 
the  sea-bottom  into  land.  The  longest  known  coastal  plain, 
that  of  the  middle  and  south  Atlantic  States,  is  chiefly  of  this 
submarine  origin.  Other  coastal  plains,  like  that  of  Holland, 
are  of  mixed  origin,  subaerial,  littoral  and  marine  deposits  oc- 
curring near  and  upon  one  another.  Diastrophic  movements 
have  but  little  effect  in  changing  the  character  of  a  regular  coast 
bordering  a  coastal  plain;  elevation  merely  makes  a  new  coast- 
line along  what  was  before  the  flat  sea-bottom,  and  depression 
causes  the  sea  to  advance  over  the  very  gently  sloping  plain, 
in  either  case  without  changing  the  regularity  of  the  coast. 

Along  the  steep,  regular  coasts  the  line  of  cliffs,  though  pursuing 
a  very  uniform  course,  is  broken  by  small  bays,  giving  a  serrate 
coast-line.  It  is  on  such  coasts  that  the  destructive  work  of  the 
waves  is  most  advantageously  seen  (Figs.  74-77,  pp.  168-171); 
sea-caves,  isolated  pillars  and  stacks,  and  lines  of  rocky  ledges 
and  shoals  abound.  The  submarine  part  of  the  slope  is  usually 
gentler  than  the  subaerial,  and  descends  gradually  to  depths  of 
ten  to  twenty  fathoms.  Above  water  the  height  of  the  sea-cliff 
is  moderate,  seldom  more  than  300-400  feet  and,  consequently, 
this  kind  of  coast  is  most  typically  developed  in  rather  low  lands, 
mountainous  coasts  giving  rise  to  other  forms.  The  line  of  cliffs 
intersects  hill  and  valley  and  interrupts  the  system  of  connected 
valleys,  making  it  evident  that  land  has  been  lost  along  that  line. 
Such  a  coast  is  obviously  the  work  of  wave  destruction.  The 
serrations  are  due  to  differences  in  the  hardness  of  the  rocks, 
the  softer  rocks  being  cut  into  bays  and  the  more  resistant  ones 
standing  out  as  headlands.  The  bays,  however,  remain  small, 


494  SEA-COASTS 

because  in  them  the  power  of  the  waves  is  diminished,  and  soon 
a  point  is  reached  where  no  further  retreat  of  the  land  is  possible 
until  the  headlands  have  been  cut  back. 

In  brief,  the  low-lying  flat  coasts  which  border  coastal  plains 
are  areas  of  accumulation,  where  the  land  is  still,  or  has  lately 
been,  gaining  at  the  expense  of  the  sea,  while  steep,  rocky  coasts, 
bordered  by  lines  of  cliff,  are  areas  along  which  the  sea  is  eating 
away  the  land. 

Diastrophic  movements  have  a  much  greater  power  in  changing 
the  character  of  the  cliff  coasts  than  of  those  which  are  low- 
lying  and  flat.  If  the  land  is  sinking,  its  valleys  become  sub- 
merged and  converted  into  bays,  thus  forming  an  irregular  coast- 
line, while,  if  the  land  is  rising,  the  wave-cut  platform  forms  a 
plain  at  the  foot  of  the  cliffs,  which  are  now  inland  and  beyond 
the  reach  of  the  surf.  Only  in  the  very  rare  instances  of  the 
cutting  action  of  the  sea  keeping  exact  pace  with  the  movement 
of  elevation  or  depression,  will  the  steep  and  regular  character 
of  the  coast  be  maintained.  Hence,  such  coasts  are  restricted 
to  regions  which  are  stationary  or  in  extremely  slow  movement. 

Geographical  cycles  are  seldom  so  clearly  distinguishable  along 
the  sea-coast  as  in  the  interior  of  the  continents,  chiefly  because 
coast  topography  is  determined  more  by  diastrophic  movements 
than  by  marine  erosion,  which  works  very  slowly  on  account  of 
its  limited  sphere  of  action.  Subaerial  denudation  iJso,  as  will 
be  shown  in  the  sequel,  is  an  extremely  important  factor  in  con- 
trolling the  character  of  the  coast.  Nevertheless,  indications 
of  the  cycle  may  not  infrequently  be  found.  A  newly  upheaved 
coast  tends  to  be  regular  and  straight,  because  the  sea-bottom 
is  nearly  flat  and,  when  elevated,  the  sea-level  marks  a  straight 
line  upon  it.  As  is  true  of  the  subaerial  agencies,  the  first  effect 
of  wave  erosion  is  to  produce  irregularities,  cutting  out  bays  along 
the  softer  rocks  and  leaving  the  more  resistant  .ones  to  stand  out 
as  headlands.  But,  as  already  explained,  the  depth  to  which 
these  bays  can  invade  the  land  is  very  limited,  since  in  them 
the  power  of  the  waves  is  greatly  reduced,  and  hence  the  surf 


IRREGULAR  COASTS  495 

is  unable  to  produce  an  irregular  coast  in  the  full  sense  of  that 
term.  However,  such  a  coast  may  be  formed  by  the  combined 
work  of  the  surf  and  of  depression  upon  a  land  of  strong  relief. 
The  submerged  valleys  of  subaerial  origin  become  bays  and 
estuaries  that  run  far  into  the  land,  like  Delaware  and  Chesa- 
peake Bays,  and  the  coast-line  becomes  highly  irregular. 

In  either  case,  whether  the  irregularities  are  relatively  insig- 
nificant and  formed  by  surf  erosion,  or  of  great  prominence  and 
due  to  depression,  the  tendency  of  marine  action  is  to  straighten 
the  coast-line  and  remove  the  irregularities  and  thus  to  produce 
an  adjusted  coast.  This  adjustment  is  brought  about  by  the 
combined  work  of  erosion  and  deposition.  The  bays  are  places 
of  sedimentary  accumulation,  especially  if  rivers  enter  them; 
the  mouth  of  the  bay  is  first  partially  or  completely  closed  by 
a  barrier  deposited  by  the  shore-current,  behind  which  the  lagoon 
is  silted  up  and  converted  into  a  marsh  and  eventually  into  a 
plain. .  Meantime  the  headlands  are  slowly  worn  back,  until  the 
power  of  the  waves  is  insufficient  to  cut  them  further.  Provided 
the  coast  remains  stationary  for  a  long  period,  adjustment  follows, 
and  the  ancient  coast  becomes  regular,  as  the  youthful  one  was, 
but  with  a  difference  of  structure,  for  the  adjusted  coast  is  made 
up  of  bold,  truncated  headlands,  alternating  with  low-lying 
plains  and  marshes.  The  Italian  coast,  with  its  broad,  gently 
curving  gulfs,  is  an  example  of  an  adjusted  coast,  and  in  the 
province  of  Tuscany  these  changes  have  largely  taken  place 
within  historic  times:  "  The  bays  of  Piombino  and  Grosseto 
were  cut  off  from  the  sea  by  bars,  the  lagoons  thus  formed  were 
transformed  into  swamp,  the  dreaded  maremmas,  which  in  their 
turn  were  filled  in.  The  former  island  of  Monte  Argentario 
was  connected  with  the  mainland  by  two  bars."  (Penck.) 

2.  Irregular  Coasts  display  a  great  variety  of  forms  due  to  the 
manner  of  their  origin,  and  several  subdivisions  are  employed 
to  express  this  diversity  of  origin.  All  the  forms,  however,  have 
this  in  common,  that  they  .are  produced  by  the  depression  and 
submergence  of  land-surfaces,  and  it  is  the  variety  of  the  latter 


496  SEA-COASTS 

which  causes  the  manifold  differences  of  the  irregular  coasts. 
On  such  coasts  the  numerous  bays  penetrate  far  into  the  land, 
sometimes  diminishing  regularly  in  width,  but  frequently  of 
varying  width,  now  expanding  into  lake-like  form,  now  con- 
tracting to  a  strait,  often  winding  and  even  branching,  and  always 
ending  in  a  land  valley.  Islands  are  numerous  and  are  in  line 
with  the  land  between  the  bays.  The  land  may  be  high  or  low, 
gently  or  steeply  sloping,  and  the  subaerial  slope  is  continued 
beneath  the  water  without  change,  until  the  flat  sea-bottom  is 
reached.  The  type  of  an  irregular  coast  is  given  by  its  bays, 
which  vary  from  short,  funnel-shaped  indentations,  to  long, 
narrow,  winding,  and  branching  channels.  The  subdivisions 
of  the  class  are  made  in  accordance  with  these  variations. 

a.  Fjord  Coasts  are  found  in  the  high  latitudes  of  both  hemi- 
spheres and  in  regions  which  have  undergone  intense  glaciation; 
in  the  northern  hemisphere  they  are  limited  by  the  49th  parallel, 
and  in  the  southern  hemisphere  by  the  4ist.  Alaska,  British 
Columbia,  Greenland,  Scotland,  Norway,  the  southern  end  of 
South  America,  and  New  Zealand,  are  typical  examples  of  fjord 
coasts.  The  fjords  are  long,  narrow,  frequently  branched,  and 
usually  very  deep;  the  bottom  is  divided  into  several  basins  and 
the  fjords  are  generally  much  deeper  in  the  middle  of  their  course 
than  at  the  seaward  end,  though  sometimes  they  are  continued 
across  the  sea-floor  as  submarine  valleys.  The  ridges  of  land 
which  separate  adjoining  fjords  are  frequently  notched  by  low 
passes,  which  seaward  become  straits,  connecting  the  fjords 
and  cutting  up  the  ridges  into  islands,  which  are  always  very 
numerous  along  coasts  of  this  class.  The  famous  "  inside  pas- 
sage "  from  Puget  Sound  to  Sitka,  Alaska,  is  a  network  of  deep 
waterways  among  countless  islands. 

Fjords  are  not  confined  to  any  particular  type  of  land  topogra- 
phy, nor  to  any  single  kind  of  structure.  In  Norway,  western 
North  America  and  southern  Chili,  they  pierce  lofty,  mountainous 
coasts;  in  Scotland  the  coasts  are  of  low  mountains,  while  in 
southern  Sweden  and  Finland  the  fjord  coasts  are  flat.  Simi- 


IRREGULAR  COASTS 


497 


larly,  they  occur  on  coasts  where  the  lines  are  determined  by 
great  fault-scarps,  as  well  as  on  those  where  the  control  is  due 
to  folding.  The  one  indispensable  condition  is  former  or 
present  glaciation,  and  in  Norway,  Greenland,  and  Alaska 
the  landward  extensions  of  many  fjords  are  still  occupied  by 
glaciers. 

Fjords  are  clearly  glaciated  valleys;    whether  they  have   been 
merely  remodelled  by  glaciers,  or  whether  they  are  entirely  due 


FIG.  250.  —  Branching  fjord,  Lynn  Canal,  Alaska.     (U.  S.  G.  S.) 

to  glacial  excavation,  they  bear  all  the  characteristic  marks  of 
ice-action,  as  these  have  been  elsewhere  enumerated  (see  p.  162). 
Glaciers  have  the  power  of  overdeepening  their  valleys  and  of 
excavating  them  below  sea-level,  but  it  is  not  yet  definitely  known 
just  how  far  this  overdeepening  may  proceed.  At  all  events, 
the  known  fjord  coasts  show  other  evidence  of  being  much  de- 


498 


SEA-COASTS 


pressed  and  invaded  by  the  sea,  and  thus  a  fjord  coast  results 
from  the  partial  submergence  of  a  glacially  modelled  region. 
The  great  depths  of  the  fjords  and  their  freedom  from  sedimentary 


FIG.  251.  —  Fjord,  Wrangel,  Alaska.     (U.  S.  G.  S.) 


IRREGULAR  COASTS  499 

deposits  are  explained  by  the  fact  that  at  the  time  of  their  sub- 
mergence these  valleys  were  occupied  by  the  ice,  which  thus  pre- 
vented the  accumulation  of  sediments.  Had  rivers  been  flowing 
in  them  when  the  depression  occurred,  their  mouths  would  first 
have  been  drowned,  checking  the  current  and  causing  a  deposi- 
tion proportionate  to  the  load.  This  has  happened  in  the  case 
of  the  Hudson  River,  which  is  a  drowned  canon  of  great  depth 
in  which  are  accumulated  immense  thicknesses  of  river  mud,  even 
above  the  Highlands.  That  a  fjord  coast  is  due  to  depression  is 
not  contradicted  by  the  fact  that  many  such  coasts  are  now  ris- 
ing, like  that  of  Scandinavia;  the  elevation  is  still  far  from  com- 
pensating for  the  depression. 

b.  Rias  Coasts.  —  This  term  is  derived  from  northwestern 
Spain,  where  the  Ria  de  Vigo,  de  la  Coruna,  del  Ferrol,  and  several 
others,  form  long,  fjord-like  bays,  though  branching  little,  which 
extend  far  into  the  land.  Coasts  of  this  type  have  frequently 
been  regarded  as  fjord  coasts,  but  there  are  essential  differences; 
the  bays  are  shorter,  more  funnel-shaped,  broadening  and  deepen- 
ing seaward,  and  are  not  nearly  so  deep  as  fjords.  The  excava- 
tion was  not  glacial,  and  hence  the  rias  are  not  confined  to  high 
latitudes,  but  occur  abundantly  in  the  temperate  and  tropical 
regions,  as  in  Brittany,  Cornwall,  Ireland,  the  east  shore  of  the 
Adriatic,  Brazil,  southern  China,  and  eastern  Australia. 

Rias  coasts  are  most  frequent  at  the  margin  of  low  mountains 
and  lands  of  moderate  height,  but  they  may  occur  along  areas 
of  any  elevation.  Nor  are  they  associated  with  any  special  type 
of  geological  structure;  in  Spain,  Brittany,  Cornwall,  and  other 
regions  the'  rock  is  a  granite  without  recognizable  structure. 
In  other  instances,  structure  has  exerted  an  evident  control,  as 
in  the  southwest  of  Ireland,  where  the  bays  follow  the  strike  of 
the  rocks  and  thus  occupy  longitudinal  valleys,  which  are  cut  out 
along  the  soft  sandstones,  while  the  intervening  ridges  are  made 
up  of  more  resistant  limestones.  In  still  other  cases  the  rias 
are  found  in  valleys  of  folding,  where  the  coast-line  intersects 
the  line  of  strike.  An  unusual  case  is  the  Bay  of  San  Francisco,. 


5oo 


SEA-COASTS 


which  is  T-shaped  and  fills  a  longitudinal  strike-valley  parallel 
to  the  coast,  with  a  transverse  connection  with  the  Pacific  by  way 
of  the  Golden  Gate. 

The  mode  of  origin  of  rias  coasts  is  by  the  depression  and 
partial  submergence  of  short  slopes  cut  by  deep  valleys  of  sub- 
aerial  origin,  whether  excavated  by  rivers  or  formed  by  tectonic 
processes.  The  rivers  were  short  and  carried  no  great  load  of 
sediment,  hence  the  bays  were  not  filled  up  with  silt  and  mud 
during  the  slow  submergence.  In  some  examples,  as  in  those  of 
Brittany,  the  valleys  are  continued  for  some  distance  across  the 
sea-floor,  a  circumstance  which  in  itself  is  an  evidence  of  de- 
pression. The  characteristic  difference  between  fjord  and  rias 
coasts  is  that  the  former  are  due  to  glaciation  and  the  latter  are 
not. 

c.  Colas  Coasts  are  typically  displayed  in  the  Balearic  Islands 
and  are  marked  by  numerous  short,  semicircular,  and  rather 
shallow  bays,  separated  by  narrow  peninsulas.  On  the  coasts 
of  the  Red  Sea  the  bays  have  a  more  or  less  rectangular  outline 
not  narrowing  inland.  Obviously,  coasts  of  this  class  differ  but 
little  from  the  serrate  regular  coasts  into  which  they  grade;  their 
mode  of  origin,  however,  renders  it  important  to  make  the 
distinction.  Galas  coasts  owe  their  irregularities  .not  to  wave 
erosion,  but  to  the  submergence  of  land  valleys;  those  of  the 
typical  kind  are  due  to  the  depression  of  mountain  slopes,  fur- 
rowed by  numerous  short  ravines.  The  coasts  of  the  Red  Sea 
type  arise  on  the  depression  of  desert  mountains,  in  which  valleys 
are  few  and  small. 

The  irregular  coasts  are  thus  in  all  cases  due  to  the  submergence 
of  land,  and  their  characteristic  features  are  to  be  explained  by 
the  differences  of  the  land-surfaces  before  submergence,  which  in 
turn  are  determined  chiefly  by  the  subaerial  agents. 

While  the  classification  of  coastal  forms  serves  a  useful  purpose, 
it  must  not  be  supposed  that  it  is  always  easy  to  refer  a  given  coast 
to  a  definite  type.  In  travelling  along  a  coast,  one  type  is  fre- 


LOBATE  COASTS        .  $01 

quently  found  to  change  and  give  way  to  another,  as  in  eastern 
North  America,  for  example.  This  entire  coast  has  recently  been 
depressed,  and  from  the  end  of  Florida  to  45°  N.  lat.  the  sub- 
sidence is  still  in  progress,  but  the  effects  of  the  submergence  are 
very  different  in  accordance  with  the  former  land-surfaces.  The 
depressed  margin  of  .the  coastal  plain  is  a  regular  coast,  with  very 
few  islands,  while  that  of  Maine,  Nova  Scotia,  and  New  Brunswick 
is  highly  irregular  and  has  numerous  rocky  islands,  while  the 
Hudson  River,  Delaware  and  Chesapeake  Bays  are  drowned 
valleys.  Between  the  two  types  the  transition  is  gradual.  Here 
one  kind  of  coast  succeeds  another,  but  two  or  more  may  occur 
together;  calas  are  frequent  on  rias  coasts,  and  rias  are  found 
among  fjords.  In  such  cases  the  general  character  of  the  coast 
determines  its  reference. 

3.  Lobate  Coasts.  —  In  the  preceding  classes  of  irregular  coasts 
the  bays  are,  after  all,  of  comparatively  small  dimensions,  and  the 
general  trend  of  the  coast  is  not  greatly  affected  by  them,  but  there 
are  other  coasts,  where  the  land  is  invaded  by  very  broad,  deep 
gulfs,  which  are  not  mere  indentations,  and  the  interlocking  gulfs 
and  peninsulas  are  of  the  same  order  of  magnitude.  Greece  is 
a  typical  instance  of  the  lobate  coast,  and  the  islands  of  Celebes, 
Japan,  and  Haiti  are  other  examples  of  the  same  type.  The  origin 
of  the  gulfs,  which  are  usually  very  deep,  is  probably  to  be  ascribed 
to  faulting. 

Professor  Penck  has  calculated  that,  in  round  numbers,  37% 
of  the  sea-coasts  of  the  earth  belong  to  one  or  other  of  the  irregular 
types;  and  of  this  amount  nearly  one-third  is  of  the  fjord  coast  class 
and  rather  less  than  half  of  the  rias  coast  class.  Of  the  continental 
coasts  43  %  are  regular,  and  two-thirds  of  these  are  low,  flat  coasts, 
and  nearly  one-third  adjusted.  As  the  flat  coasts  are  due  to  accu- 
mulation and  many  of  them  to  upheaval,  but  a  small  amount  re- 
mains as  directly  formed  by  wave  erosion.  From  these  figures  it 
appears  that  diastrophism  is  of  more  importance  than  marine 
denudation  in  determining  the  character  of  the  coast-lines. 


502  .  SEA-COASTS 

Relations  of  Sea- coasts  to  Structure.  —  As  has  been  repeatedly 
mentioned  in  the  foregoing  paragraphs,  the  details  of  coastal 
topography  often  show  no  obvious  relations  to  the  geological 
structure  of  the  rocks.  On  a  grand  scale,  however,  the  position 
and.  trend  of  coast-lines  are  controlled  by  tectonic  features,  and 
frequently  the  details  are  similarly  determined.  Many  coasts  are 
due  to  great  systems  of  faults,  which  may,  as  in  the  Atlantic  shores 
of  Europe,  intersect  the  prevailing  lines  of  strike,  or  may  run 
approximately  parallel  with  those  lines,  as  in  eastern  Asia,  where 
a  series  of  tilted  fault-blocks  form  the  plains,  the  coast-lines,  and 
the  lines  of  fringing  islands.  Other  coasts  are  coincident  with 
long  lines  of  folding,  as  is  illustrated  by  the  entire  west  coast  of 
North  and  South  America,  the  foreland  of  the  great  mountain 
chains  being  more  or  less  completely  submerged.  The  Dalma- 
tian coast  on  the  east  side  of  the  Adriatic  is  a  half-submerged  belt 
of  folding  and  the  coast-line  obliquely  truncates  the  strike.  Fjord, 
rias  and  calas  coasts  may  occur  in  association  with  any  type  of 
structure;  they  are  determined  by  the  dominant  class  of  subaerial 
denuding  agents. 


- 


CHAPTER   XXIII 
MOUNTAIN  RANGES 

THE  term  mountain  is  somewhat  loosely  employed  for  any  lofty 
eminence,  and  the  distinction  between  mountains  and  hills,  as 
ordinarily  made,  is  principally  a  question  of  height.  Some  so- 
called  mountain  peaks  and  ridges  are  merely  the  portions  of 
dissected  plateaus  left  standing,  such  as  Lookout  Mountain  and 
Missionary  Ridge  in  Tennessee,  and  the  Alleghany  Front  in  Penn- 
sylvania. Such  mountains  usually  have  flat  tops  (table  mountains) , 
are  composed  of  strata  which  are  nearly  or  quite  horizontal,  and 
owe  their  existence  either  to  their  being  composed  of  more  re- 
sistant rocks  than  the  denuded  parts  of  the  plateau,  or  to  their 
favourable  situation  with_reference  to  the  drainage  lines.  Another 
type  of  mountain  is  the  volcanic,  which  is  usually  an  isolated  cone 
and  may  be  built  up  to  great  heights;  it  is  simply  the  accumula- 
tion of  volcanic  material  which  has  been  piled  up  around  the 
vent.  To  the  same  general  class,  as  due  to  igneous  rocks,  might  be 
referred  the  laccolithic  mountains,  in  which  an  intrusive  magma 
has  pushed  up  the  overlying  strata  into  a  dome.  Such  mountains 
may  stand  isolated  (Fig.  220),  or  several  may  be  grouped  together 
(Henry  Mountains  of  southern  Utah) ,  or  they  may  form  extensive 
parts  of  true  ranges  (Elk  Mountains,  Colorado).  Block  moun- 
tains, which  are  tilted  and  eroded  fault-blocks,  form  a  third 
class;  these  may  be  single  or  in  groups,  or  lineally  extended  as 
a  range.  Typical  mountain  ranges  and  chains  are  mountains  of 
folding,  and  differ  materially  from  any  of  these  classes,  both  in 
their  structure  and  their  mode  of  origin.  Before  proceeding  to 
discuss  the  origin  and  history  of  mountains,  it  will  be  necessary 
to  define  the  terms  to  be  used. 

503 


504  MOUNTAIN   RANGES 

A  Mountain  Range  is  made  up  of  a  series  of  more  or  less  par. 
allel  ridges,  all  of  which  were  formed  within  a  single  geosyncline 
(p.  330)  or  on  its  borders.  The  ridges  are  separated  from  one 
another  by  longitudinal  valleys,  and  may  be  formed  either  by  the 
successive  folds  or  by  denudation  within  the  limits  of  the  folds. 
In  the  latter  case  the  outcropping  harder  strata  make  the  ridges. 
A  mountain  range  is  always  very  long  in  proportion  to  its  width, 
and  its  ridges  have  a  persistent  trend.  These  features  distinguish 
a  true  range  from  the  ridges  cut  out  of  a  plateau  by  denudation. 
The  Appalachian  range,  the  Wasatch,  the  Coast  Range,  are  ex- 
amples of  typical  mountain  ranges. 

A  Mountain  System  is  made  up  of  a  number  of  parallel  or  con- 
secutive ranges,  formed  in  separate  geosynclines,  but  of  approxi- 
mately similar  dates  of  upheaval.  The  Appalachian  system  com- 
prises the  Appalachian  range,  running  from  New  York  to  Georgia, 
the  Acadian  range  in  Nova  Scotia  and  New  Brunswick,  and  the 
Ouachita  range  in  Arkansas.  Each  of  these  ranges  was  formed  in 
a  different  geosynclinal,  but  at  the  same  geological  date,  and  they 
are  consecutive,  having  a  common  direction. 

A  Mountain  Chain  comprises  two  or  more  systems  in  the  same 
general  region  of  elevation,  but  of  different  dates  of  origin.  The 
Appalachian  chain  includes  the  Appalachian  system,  the  Blue 
Ridge,  the  Highlands  of  New  Jersey  and  the  Hudson,  a  system  of 
different  date,  and  the  Taconic  system  of  western  New  England, 
which  was  not  formed  at  the  same  time  as  either  of  the  others. 

A  Cordillera  consists  of  several  mountain  chains  in  the  same 
part  of  the  continent.  Thus,  the  chains  of  the  Rocky  Mountains, 
Sierra  Nevada,  Coast  Range,  and  their  prolongations  in  Canada, 
together  make  up  the  Rocky  Mountain  or  Western  Cordillera. 

From  these  definitions  it  will  appear  that  the  mountain  range 
has  a  unity  of  structure  and  origin  which  fits  it  especially  for  study. 
If  the  history  of  the  ranges  be  understood,  the  systems  and  chains 
will  offer  little  additional  difficulty. 

A  mountain  range  (disregarding,  for  the  present,  certain  excep- 
tional cases)  consists  of  a  very  thick  mass  of  strata,  which  are 


MOUNTAIN   RANGES  505 

much  thicker  in  the  mountains  than  the  same  strata  in  the  adjoin- 
ing plains.  In  the  Appalachian  range,  for  example,  the  stratified 
rocks  are  more  than  25,000  feet  thick,  but  on  tracing  the  same 
series  of  beds  westward  into  the  Mississippi  Valley,  they  are  found 
to  become  very  much  thinner,  hardly  exceeding  one-tenth  of  the 
thickness  in  the  mountains.  This  immense  thickness  of  the 
component  strata  is  not  peculiar  to  the  Appalachians,  but  reappears 
in  the  typical  mountain  ranges  everywhere;  the  Wasatch  range  has 
31,000  feet  of  strata,  the  Coast  Range  30,000  feet,  the  Alps  50,000 
feet,  etc.  The  thick  series  of  strata  which  make  up  a  mountain 
range  are  usually  conformable  throughout,  though  this  conformity 
may  in  some  cases  be  deceptive  and  due  to  the  obliteration  of 
unconformities  by  folding.  Deposition  usually  appears  to  have 
been  without  conspicuous  breaks,  and  there  was  little  or  no  loss 
from  denudation,  though  in  some  cases  the  region  which  subse- 
quently was  upheaved  into  the  range  had  its  oscillations  of  level, 
recorded  now  in  unconformities.  This  may  be  seen,  for  example, 
in  the  Ouachita  range  of  Arkansas. 

Another  well-nigh  universal  fact  concerning  the  structure  of 
mountain  ranges  is  the  intense  folding  or  plication  of  their  strata, 
often  accompanied  by  great  thrusts.  The  degree  of  plication 
varies  much  in  different  ranges.  The  Uinta  Mountains  are  formed 
by  a  single  great  and  gently  swelling  arch  of  strata,  faulted  along 
its  northern  slope.  So  gentle  is  the  curvature  of  the  beds  that  in 
a  single  view  they  often  seem  to  be  quite  horizontal.  The  Black 
Hills,  South  Dakota,  form  a  great  dome,  with  somewhat  oval 
ground-plan.  Much  more  commonly  the  strata  are  thrown  into 
a  series  of  parallel  folds,  which  sometimes  are  open,  upright,  and 
symmetrical,  as  in  some  of  the  ridges  of  the  Jura  Mountains  of 
Switzerland,  in  which  the  folds  are  so  symmetrical  and  regu- 
lar that  a  section  across  the  parallel  ridges  looks  like  a  dia- 
gram. This  comparatively  gentle  folding  is,  however,  not  the 
rule,  but  rather  an  intense  compression  and  plication.  The 
Appalachians  are  thrown  into  closed,  asymmetrical,  and  over- 
turned folds,  with  frequent  great  thrusts.  The  Sierra  Nevada 


506  MOUNTAIN   RANGES 

is  so  intensely  plicated  that  the  thickness  of  its  strata  has  not  yet 
been  estimated.  The  Alps  have  undergone  such  enormous  com- 
pression that  many  of  the  ridges  are  in  the  form  of  fan  folds  (i.e. 
the  anticlines  are  broader  at  the  crest  than  at  the  base),  while 
others  have  been  pushed  over  to  an  inverted  position.  The  com- 
bination of  this  violent  contortion  with  faults  and  thrusts  often 
results  in  an  indescribable  confusion  and  chaos  of  forms,  which 
it  is  exceedingly  difficult  to  comprehend. 

In  folded  mountain  ranges  three  zones  may  be  distinguished: 
(i)  A  rigid,  unyielding  mass  which  is  not  folded,  (2)  the  zone  of 
folding,  (3)  the  zone  of  diminishing  action,  where  the  folding  gradu- 
ally dies  away  or  ends  in  a  fault.  Many,  perhaps  most,  ranges  are 
bounded  by  faults  on  one  or  more  sides,  as  is  true  of  the  Sierra 
Nevada,  Wasatch  and  Uinta  Mountains,  the  Alps,  etc.  The  side 
of  the  range  toward  which  the  overturned  folds  incline  is  called 
the  foreland,  and  may  be  either  the  unfolded  mass  or  the  zone  of 
diminishing  action;  the  former  arrangement  occurs  in  the  Alps, 
the  latter  in  the  Appalachians. 

The  two  main  characteristic  features  of  mountain  ranges  are, 
then,  the  immense  thickness  of  the  strata  of  which  they  are  made, 
and  the  compression  and  folding  or  thrusting  which  they  have  un- 
dergone. Certain  minor  structures  which  accompany  these  more 
striking  features  should,  however,  not  be  overlooked.  In  the  first 
place,  the  folded  strata  of  mountain  ranges  are  very  generally 
cleaved,  or  fissile,  or  both,  the  planes  of  cleavage  or  fissility  run- 
ning parallel  with  the  axes  of  the  folds.  (2)  The  major  folds  are 
themselves  composed  of  successive  series  of  minor  folds  in  de- 
scending order  of  magnitude,  the  smallest  of  them  being  visible 
only  with  the  microscope.  (3)  Dynamic  metamorphism  is  an 
almost  universal  feature  of  mountain  ranges,  the  transformation 
of  the  rocks  being  in  proportion  to  the  intensity  of  the  plication. 
The  microscope  gives  eloquent  testimony  to  the  enormous  forces 
which  have  been  at  work,  by  showing  how  the  minerals  have  been 
mashed  and  flattened,  rendered  plastic  and  flowing  like  wax 
in  a  hydraulic  press.  (4)  Masses  of  igneous  rocks  are  very  often. 


ORIGIN  OF  MOUNTAIN   RANGES  507 

though  not  always,  associated  with  mountain  ranges,  and  many 
such  ranges  have  a  core  of  igneous  rock,  often  granite,  with  strata 
flanking  it  on  both  sides. 


ORIGIN  OF  MOUNTAIN  RANGES 

The  manner  in  which  mountain  ranges  have  been  formed  must 
be  deduced  from  a  careful  study  of  their  structure,  for  no  one 
has  ever  witnessed  the  process  of  that  formation.  Mountain 
building  may  be  going  on  at  the  present  time;  indeed,  there 
is  no  reason  to  suppose  that  it  is  not,  but  so  slowly  is  the  work 
carried  on  that  it  withdraws  itself  entirely  from  observation. 
Nevertheless,  the  general  course  of  events  may  be  inferred  with 
much  confidence  from  the  structure  of  the  range. 

The  first  step  in  the  formation  of  a  mountain  range  must  evi- 
dently be  the  accumulation  of  an  immensely  thick  body  of  strata. 
This,  of  course,  must  have  taken  place  chiefly  under  water,  and  the 
only  body  of  water  large  enough  is  the  sea.  Furthermore,  our 
studies  of  modern  marine  deposits  have  taught  us  that  thick  strata 
can  be  accumulated  only  in  rather  shallow  water  and  parallel  with 
shore-lines.  This  shoal-water  origin  of  their  strata  is  confirmed  by 
the  examination  of  actual  mountain  ranges,  where  we  find  great 
masses  of  conglomerates,  ripple-marked  and  sun-cracked  sand- 
stones and  shales,  and  abundant  other  testimony  of  deposition  in 
shallow  water,  in  deltas  and  on  flood  plains  of  rivers.  To  accumu- 
late thick  strata  in  shoal  water,  the  bottom  must  subside  as  the 
sediments  are  piled  upon  it,  else  the  water  would  be  filled  up  and 
deposition  cease.  Such  a  sinking  trough  is  a  geosyncline,  and  in 
geosyclines  filled  with  sediments  is  the  cradle  of  the  mountains. 
The  area  of  the  trough  varies  from  time  to  time,  as  do  also  the 
position  of  the  line  of  maximum  subsidence  and  the  relative  rate 
of  depression  and  sedimentation,  so  that  the  depth  of  water  varies. 
We  saw  above  that  the  strata  of  .mountain  ranges  are  very  much 
thicker  than  the  same  strata  in  the  adjoining  plains,  which  means 


508  MOUNTAIN   RANGES 

that  the  ranges  have  been  formed  along  the  lines  of  maximum 
sedimentation. 

The  second  stage  in  the  building  of  a  range  is  the  upheaval  of 
the  thick  mass  of  strata  into  a  series  of  anticlinal  and  synclinal 
folds,  which  may  be  upright,  open,  and  symmetrical,  or  closed, 
asymmetrical,  inclined,  or  inverted.  This,  as  we  have  already 
learned,  can  be  produced  only  by  lateral  compression,  a  con- 
clusion which  is  sustained  not  only  by  the  mechanics  of  folding  and 
faulting,  but  also  by  the  less  obvious  structures,  such  as  cleavage 
and  fissility,  metamorphism,  the  microscopic  crumplings  and  pli- 
cations, and  the  crushing  and  flowage  of  the  mineral  particles. 
The  compressing  force  does  not  raise  anticlines  with  great  cavities 
beneath  them,  for  such  arches  could  not  well  be  self-supporting, 
but  mashes  together  the  whole  mass  of  strata,  raising  them  into 
folds  and  wrinkles,  crowding  the  beds  into  a  greatly  reduced  breadth ; 
or,  when  they  are  not  sufficiently  loaded  to  be  plastic,  breaking 
and  dislocating  them  in  great  thrusts.  It  is  not  necessary  to  suppose 
that  a  mountain  range  was  thrown  up  by  one  steady  movement. 
On  the  contrary,  there  is  good  reason  to  believe  that  repeated  move- 
ments, separated,  it  may  be,  by  long  intervals  of  time,  have  been 
engaged  in  the  work. 

The  great  forces  of  compression  which  have  upheaved  mountain 
ranges  have  manifested  themselves  recurrently  from  the  earliest 
to  the  latest  recorded  periods  of  the  earth's  history,  and  from  these 
recurrences  form  conspicuous  landmarks  in  the  chronological 
scheme. 

There  are  certain  mountain  ranges  which  have  a  different  struc- 
ture and  must  have  had  a  correspondingly  different  mode  of  origin. 
As  already  pointed  out,  in  the  Great  Basin,  which  lies  between 
the  Sierra  Nevada  and  the  Wasatch  Mountains,  are  a  number  of 
parallel  mountain  ranges  with  a  prevalent  north  and  south  trend, 
which  are  collectively  called  the  Basin  Ranges.  These  mountains 
are  not  folds  of  very  thick  strata,  but  tilted  fault-blocks,  which  have 
been  made  by  normal  faults,  each  upthrow  side  standing  as  a  great 
escarpment,  but  with  a  tilted  top  that  gradually  slopes  back  to  the 


DENUDATION   OF   MOUNTAINS  509 

foot  of  the  next  block,  to  which  it  stands  as  the  downthrow  side. 
The  processes  of  denudation  have  carved  these  tilted  blocks  into 
peaks  and  ridges  of  the  ordinary  kind.  The  boundary  ranges, 
the  Sierra  Nevada  and  the  Wasatch,  although  mountains  of  fold- 
ing, have  themselves  been  modified  by  the  same  process,  for  each 
of  these  ranges  has  a  great  fault  along  its  base,  the  Great  Basin 
being  on  the  downthrow  side  with  reference  to  each  of  them. 

The  Date  of  Mountain  Ranges  means  the  geological  period  in 
which  they  were  first  upheaved  above  the  sea.  This  date  is  sub- 
sequent to  the  newest  strata  which  are  involved  in  the  movement, 
and  earlier  than  that  of  the  oldest  strata  which  did  not  take  part 
in  it,  but  must  have  done  so,  had  they  been  present.  Strata  which 
rest  unconformably  against  the  flanks  of  a  range  must  have  been 
deposited  after  the  folding  movement  was  accomplished.  If  the 
newest  folded  strata  and  the  oldest  unmoved  strata  be  of  successive 
geological  periods,  the  date  of  the  upheaval  is  placed  between  those 
two  periods  and  said  to  close  the  older  one  for  the  particular  region 
involved.  The  subsequent  history  of  a  mountain  range  after  its 
final  upheaval  above  the  sea  must  be  read  in  its  denudation  and  in 
the  evolution  of  its  topography  and  drainage. 

DENUDATION  OF  MOUNTAINS 

Mountains  as  we  see  them  are  never  in  the  shape  which  they 
would  present  were  the  forces  of  compression  and  upheaval  alone 
concerned  in  their  formation.  Every  mountain  range  has  been 
profoundly  affected  by  the  agencies  of  denudation,  and  -their 
ridges  and  peaks,  their  cliffs  and  valleys,  have  been  carved  out  of 
swelling  folds  and  domes,  or  angular,  tilted  fault-blocks.  As 
upheaval  is  a  slow  process,  denudation  must  have  begun  its  work 
as  soon  as  the  crests  of  the  folds  made  their  appearance  above 
the  sea,  or  above  the  level  of  the  ground,  so  that  probably  no 
range  ever  had  the  full  height  which  the  strata,  if  free  from  de- 
nudation, would  have  given  to  it.  Upheaval,  though  sometimes 
slow  enough  to  allow  rivers  to  keep  their  channels  open,  is  yet  too 


510  MOUNTAIN    RANGES 

rapid  to  be  kept  in  check  by  the  processes  of  general  atmospheric 
weathering,  and  so  the  ranges  grew  into  great  uplifts.  But  as  soon 
as  the  movement  of  elevation  ceased,  denudation  began  to  get  the 
upper  hand,  for  as  we  have  learned,  mountains  are  the  scene  of 
rapid  erosion. 

Lofty,  or  alpine,  ranges  are  subject  to  peculiarly  rapid  and  effec- 
tive denudation,  quite  different  in  character  from  that  which 
operates  in  the  lowlands.  Above  the  limit  of  the  growth  of  trees 
(tree-  or  timber-line)  rock  destruction  goes  on  with  great  rapidity, 
as  is  indicated  by  the  wild  and  chaotic  confusion  of  rock-pieces. 
In  German  this  is  called  Felsenmeer  (sea  of  rock) ,  but  there  is  no 
English  term  for  it,  and  it  is  preeminently  characteristic  of  lofty 
mountain  slopes.  These  masses  of  shattered  rock  are  not  only 
evidences  of  rapid  disintegration  by  frost,  but  afford  an  immensely 
increased  surface  to  destructive  weathering.  The  wind,  which 
blows  with  great  violence,  is  an  important  agent  of  destruc- 
tion; avalanches  carry  down  great  quantities  of  rock,  and  the 
combined  agencies  of  frost  and  gravity  produce  the  vast  talus 
slopes  of  all  high  mountains. 

Another  very  effective  agent  among  alpine  summits  is  the  glacier, 
which,  by  widening  and  cutting  back  the  cirque  at  its  head,  eats 
rapidly  into  the  mountain  mass.  When  many  glaciers  rise  on  the 
different  sides  of  a  mountain,  the  recession  of  the  cirques  will  de- 
velop sharp  crests  and  knife-edges.  To  this  cause  has  been  chiefly 
attributed  the  extreme  ruggedness  of  the  high  Alps,  Sierra  Nevada, 
and  other  ranges. 

For  a  long  period  the  effect  of  denudation  is  greatly  to  increase 
the  ruggedness  of  the  mountains,  carving  folds  into  ridges  and  cliffs, 
and  ridges  into  bold  and  inaccessible  peaks,  but  sooner  or  later  the 
mountains  are  worn  down  lower  and  lower,  and  are  eventually 
levelled  with  the  plains  from  which  they  spring.  In  the  process 
of  degradation,  the  synclines  often  resist  wear  better  than  the 
anticlines,  and  standing  up  above  the  level,  form  the  synclinal 
mountains  of  many  ancient  ranges. 

From  the  geological  point  of  view  mountains  must  be  regarded 


APPALACHIAN  CYCLES  511 

as  short-lived  and  ephemeral ;  low-lying  plains  persist  for  a  longer 
time  than  do  lofty  ranges,  as  rivers  may  outlast  many  generations 
of  lakes.  Consequently,  among  the  mountain  chains  of  the  globe, 
we  everywhere  find  that  the  lofty  ranges  are  those  of  compara- 
tively recent  date,  while  ancient  mountains  have  been  worn  down 
into  mere  stumps.  The  Appalachians  have  been  reduced  nearly 
to  base-level,  and  their  present  condition  is  that  of  a  reelevated 
and  dissected  peneplain,  the  ridges  and  valleys  of  which  are  deter- 
mined by  the  position,  attitude,  and  alternation  of  harder  and 
softer  strata.  In  its  pristine  state  this  very  ancient  range  may 
have  been  as  lofty  as  the  Alps  or  Andes.  Of  course,  there  is  no 
mathematical  ratio  between  the  youth  of  a  range  and  its  height, 
for  moderately  folded  strata  of  moderate  thickness  never  could 
have  formed  very  high  mountains,  but  in  a  general  way  it  is  true, 
that  very  high  ranges  are  youthful,  and  that  very  old  ranges  are 
low.  The  process  of  degradation  may  go  so  far  as  to  sweep  away 
a  mountain  range  to  its  very  roots,  leaving  only  the  intensely 
plicated  strata  of  the  plain  as  evidence  that  mountains  ever  ex- 
isted there.  Of  such  a  nature  is  the  upland  of  southern  New 
England  and  the  great  metamorphic  area  of  Canada,  both  of  which 
probably  once  carried  ranges  of  high  mountains. 

.     APPALACHIAN  CYCLES 

We  have  seen  that  any  region,  however  lofty  and  rugged,  must 
eventually  be  worn  down  to  base-level,  provided  only  that  the 
country  remain  stationary  with  reference  to  the  sea  until  the  pro- 
cess of  degradation  is  complete.  It  is  doubtful,  however,  whether 
any  extensive  region  of  hard  rocks  has  ever  been  absolutely  re- 
duced to  base-level:  the  usual  result  is  the  formation  of  a  pene- 
plain, a  low-lying,  featureless  surface  of  gentle  slopes  and  with  only 
occasional  eminences  rising  above  the  general  level.  Reelevation 
of  such  a  peneplain  at  once  revivifies  the  streams  and  gives  all  the 
destructive  agencies  new  powers.  The  peneplain  is  attacked  and 
carved  into  valleys  and  hills,  the  valleys  being  rapidly  cut  down 


512  MOUNTAIN   RANGES 

to  base-level,  while  the  divides  and  hills  are  much  more  slowlv 
removed.  If  time  enough  be  granted,  the  rugged  country  formed 
from  a  dissected  peneplain  is  in  its  turn  worn  down  to  a  second 
peneplain  at  a  lower  base-level.  This  alternate  upheaval  and 
wearing  down  together  constitute  a  cycle  of  denudation,  from  base- 
level  back  to  base-level.  A  complete  cycle  is  one  in  which  the 
whole  region  is  reduced  to  a  peneplain  before  the  reelevation 
occurs,  and  a  partial  or  incomplete  cycle  is  one  which  is  inter- 
rupted by  upheaval  before  the  region  is  cut  down,  and  only  small 
and  local  peneplains  have  been  formed.  From  a  study  of  an  old 
region  several  cycles  of  denudation  may  frequently  be  made  out, 
represented  by  the  remnants  of  dissected  peneplains  at  different 
levels  preserved  in  the  harder  rocks.  The  successive  adjustments 
of  the  drainage  system  are  a  valuable  auxiliary  in  working  out  the 
history  of  the  cycles. 

As  an  excellent  example  of  these  cycles  of  denudation  whose 
marks  are  preserved  in  the  structure  of  the  rocks,  we  may  take  the 
Appalachian  Mountains,  which  have  been  studied  with  great 
care.  The  cycles  have  been  worked  out  elaborately,  but  only  an 
outline  of  the  more  striking  events  can  be  given  here. 

These  mountains  began  as  a  great  geosyncline  in  which  through- 
out the  vast  lengths  of  the  Palaeozoic  era  were  accumulated  enor- 
mously thick  masses  of  shoal-water  sediments.  Toward  the  close 
of  that  era  a  number  of  crustal  movements  set  in,  crushing  the  sides 
of  the  geosynclinal  trough,  and  crumpling  the  mass  of  strata  con- 
tained in  it  into  a  series  of  roughly  parallel,  closed,  inclined,  or 
overturned  folds,  forming  doubtless  a  very  lofty  range  of  moun- 
tains. During  the  long  ages  of  the  Mesozoic  era  the  mountains 
were  attacked  and  worn  down  by  the  destructive  agencies;  and 
by  the  time  the  Cretaceous  period  was  reached  the  range  had 
been  reduced  to  a  peneplain,  with  only  a  few  hills  rising  above  its 
almost  featureless  level,  —  hills  whic  n  are  now  the  peaks  of  west- 
ern North  Carolina,  the  highest  points  of  the  range  at  present. 
The  present  height  of  these  peaks  is  due  to  subsequent  reelevation. 
This  plain  is  called  ib*  Kittatinny  peneplain,  because  the  ridge 


ACCORDANCE  OF  ALPINE  SUMMITS 

of  that  name  in  Pennsylvania  and  New  Jersey  is  one  of  the  rem- 
nants of  it.  To  the  observer  who  can  overlook  the  billowy  ridges 
of  the  present  range  their  even  sky-line  is  very  striking,  and  these 
ridges  are  all  composed  of  the  hardest  rocks,  which  all  rise  to 
nearly  the  same  level.  To  reproduce  the  plain  it  would  be  nec- 
essary to  fill  the  valleys  between  the  Blue  Ridge  on  the  east  and 
the  plateau  on  the  west  up  to  the  level  attained  by  the  hard  ridges, 
and  this  would  give-  a  gently  arched  surface,  sloping  very  gradu- 
ally to  the  Mississippi  Valley  and  the  Atlantic.  On  this  peneplain 
were  already  established  the  great  streams  which  flow  to  the 
ocean,  such  as  the  Susquehanna  and  the  Potomac. 

Next  the  peneplain  was  raised  very  gradually  to  a  height  of 
1400  feet  in  Virginia,  diminishing  in  both  directions  from  this 
point,  and  the  denuding  forces  once  more  attacked  and  dissected 
the  plateau,  the  larger  streams  holding  their  transverse  courses  and 
sawing  through  the  hard  strata,  which  were  left  standing  as  ridges 
by  the  cutting  of  the  longitudinal  valleys  along  the  more  destruct- 
ible beds.  Denudation  had  cut  down  the  softer  beds  to  one  gen- 
eral level,  called  the  Shenandoah  peneplain,  the  period  of  rest 
being  long  enough  to  bring  all  the  areas  of  soft  and  soluble  beds 
to  this  level,  but  not  materially  to  lower  the  ridges  of  the  more 
resistant  strata. 

"  The  swelling  of  the  Appalachian  dome  began  again.  It  rose 
200  feet  in  New  Jersey,  600  feet  in  Pennsylvania,  1700  feet  in 
southern  Virginia,  and  thence  southward  sloped  to  the  Gulf  of 
Mexico.  ...  In  consequence  of  the  renewed  elevation,  the 
streams  were  revived.  Once  more  falling  swiftly,  they  have 
sawed,  and  are  sawing,  their  channels  down,  and  are  preparing 
for  the  development  of  a  future  base-level."  (Willis.) 

Accordance  of  Alpine  Summits.  —  It  is  perhaps  generally  true 
of  very  high  ranges  that  their  highest  peaks  and  ridges  are  ar- 
ranged so  as  to  be  in  accordance.  If  we  imagine  a  surface  which 
shall  everywhere  touch  these  summit-levels,  it  will  be  found  to 
form  a  gently  arching  dome,  with  major  axis  coinciding  with  the 
general  trend  of  the  mountains  and  highest  in  the  interior  of  the 

2L 


514  MOUNTAIN   RANGES 

range.  This  accordance  has  been  observed  in  many  alpine 
ranges,  such  as  the  Pyrenees,  Alps,  and  Caucasus,  the  Sierra 
Nevada  and  Cascades,  the  Selkirks  and  Coast  Range  of  British 
Columbia,  and  the  mountains  of  Alaska. 

In  attempting  to  explain  this  remarkable  coincidence,  the  hy- 
pothesis of  peneplanation  and  subsequent  upwarping,  which  is  so 
satisfactory  and  so  widely  accepted  as  applied  to  the  Appalachians 
and  similar  cases  in  Europe,  has  also  been  employed  for  alpine 
ranges  of  comparatively  recent  geological  date.  This  explanation 
has  not,  however,  found  so  general  an  acceptance  for  the  high 
mountains,  and  other  hypotheses  should  be  considered  as  possible 
alternatives  in  each  case.  These  rival  hypotheses  may  be  grouped 
into  two  classes:  (i)  those  which  regard  the  accordance  as  due  to 
original  structure  in  the  processes  of  upheaval,  and  (2)  those 
which  refer  the  phenomenon  to  the  spontaneous  action  of  the 
denuding  forces. 

(1)  There  is  no  reason  to  believe  that  mountains  could,  or  ever 
did,  rise  to  indefinite  heights;  on  the  contrary,  their  height  must  be 
limited  by  the  ability  of  their  foundations  to  sustain  them,  and  the 
principle  of  isostatic  adjustment  might  well  operate  to  produce 
some  rough  accordance  of  height,  while  denudation  during  the 
slow  upheaval  would  tend  to  remove  the  higher  summits  faster 
than  the    lower  ones.     "The  downcrushing  of    higher,  heavier 
blocks  with  the  simultaneous  rise  of  their  lower,  lighter  neigh- 
bors, coupled  with  the  likewise  simultaneous,  especially  rapid 
loss  of  substance  on  the  higher  summits,  form  a  compound  pro- 
cess leading  toward  a  single,  relatively  simple  result."     (Daly.) 

(2)  The  core  of  many  high  ranges  is  a  granite  batholith,  from 
which  are  carved  the  higher  peaks  and  ridges,  the  original  cover- 
ing of  strata  having  been  swept  away  by  denudation.    The  form 
of  the  uneroded  batholith  was  that  of  a  gently  arching  dome,  such 
as  might  be  expected  to  give  rise  to  accordant  summits  in  the  peaks 
carved  from  it.     In  ranges,  like  the  Alps,  which  have  no  batho- 
lithic  core,  there  is  yet  almost  always  great  metamorphism  due  to 
the  compression  of  the  rocks  and  the  weight  of  overlying  masses. 


ACCORDANCE   OF  ALPINE   SUMMITS  515 

The  surface  of  the  metamorphic  core  was  probably  quite  regular 
before  the  covering  strata  were  removed,  and  would  tend  to  result 
in  accordant  summits  among  the  peaks  which  are  sculptured  out 
of  it.  The  thoroughly  metamorphosed  rocks,  such  as  gneisses, 
schists,  quartzites,  marbles,  etc.,  are  not  very  different  from  one 
another  in  their  resistance  to  weathering,  and  hence  denudation 
would  tend,  in  a  general  way,  to  maintain  the  accordance.  Indeed, 
many  investigators  of  the  Alps  ascribe  the  accordant  summits 
to  denudation  alone.  "The  longer  a  mountainous  region  is 
exposed  to  denudation,  the  more  completely  do  the  indications 
of  original  inequalities  in  the  relative  heights  of  its  peaks  disap- 
pear, till  finally  the  summits  are  determined  entirely  by  the  char- 
acter of  the  rocks,  the  most  resistant  rocks  forming  the  highest 
peaks."  (Penck.) 

It  is  not  necessary  to  assume  that  any  of  these  alternative 
hypotheses  is  true  to  the  exclusion  of  the  others.  All  the  factors 
mentioned  may  cooperate  to  a  common  end,  and  for  every  moun- 
tain range  the  problem  should  be  regarded  as  an  individual  one, 
without  taking  for  granted  that  one  explanation  will  cover  all 
cases. 


PART  IV 

HISTORICAL  GEOLOGY 

CHAPTER    XXIV 
FOSSILS  — GEOLOGICAL   CHRONOLOGY 

A  fossil  is  the  impression  or  remains  of  an  animal  or  plant  pre- 
served in  the  rocks. 

A  knowledge  of  fossils  is  indispensable  to  the  geologist  because 
they  give  him  the  means  of  establishing  a  consecutive  chronology 
of  the  earth,  and  teach  him  much  concerning  the  changes  of 
land  and  sea,  of  climate,  and  of  the  distribution  of  living  things 
upon  the  globe. 

I.     HOW   FOSSILS    WERE   EMBEDDED    IN   THE   ROCKS 

The  conditions  of  the  preservation  of  fossils  are  much  more 
favourable  to  some  kinds  of  organisms  than  to  others.  It  is  only 
under  the  rarest  circumstances  that  soft,  gelatinous  animals,  which 
(like  jelly-fish)  have  no  hard  parts,  can  leave  traces  in  the  rocks. 
The  vast  majority  of  fossilized  animals  are  those  which  have  hard 
shells,  scales,  teeth,  or  bones;  and  of  plants,  those  which  contain 
a  sufficient  amount  of  woody  tissue. 

Again,  the  conditions  under  which  organisms  live  have  a  great 
influence  upon  the  chances  of  their  preservation  as  fossils.  Land 
animals  and  plants  are  much  less  favourably  situated  than  are 
aquatic  forms,  and  since  the  greater  number  of  sedimentary 


HOW   FOSSILS    WERE   EMBEDDED   IN  THE   ROCKS 

rocks  were  laid  down  in  the  sea,  marine  organisms  are  much  more 
common  as  fossils  than  are  those  of  fresh  water. 

On  land,  fossils  have  been  preserved,  sometimes  in  astonishing 
numbers,  under  wind-made  accumulations  of  sand,  dust,  or  vol- 
canic ash,  and  in  the  flood-plain  deposits  of  rivers.  Peat-bogs  are 
excellent  places  for  fossilization,  and  the  coal  seams  have  yielded 
great  numbers  of  fossils,  principally  of  plants.  The  remains  of 
land  animals  and  plants,  especially  of  the  latter,  are  sometimes 
swept  out  to  sea,  sink  to  the  bottom,  and  are  there  covered  up  and 
preserved  in  the  deposits;  but  such  occurrences  are  relatively  un- 
common. Small  lakes  offer  more  favourable  conditions  for  the 
preservation  of  terrestrial  organisms.  Surrounding  trees  drop 
their  leaves,  flowers,  and  fruit  upon  the  mud-flats,  insects  fall  into 
the  quiet  waters,  while  quadrupeds -are  mired  in  mud  or  quick- 
sand and  soon  buried  out  of  sight.  Flooded  streams  bring  in 
quantities  of  vegetable  debris,  together  with  the  carcasses  of  land 
animals,  drowned  by  the  sudden  rise  of  the  flood. 

The  great  series  of  fresh-water  and  volcanic-ash  deposits,  which 
for  long  ages  were  formed  in  various  parts  of  our  West,  have  proved 
to  be  a  marvellous  museum  of  the  land  and  fresh-water  life  of  that 
region.  On  the  fine-grained  shales  are  preserved  innumerable 
insects  and  fishes,  with  multitudes  of  leaves,  many  fruits,  and 
occasionally  flowers,  while  in  the  sands,  clays,  and  tuffs,  are  en- 
tombed the  bones  of  the  reptiles,  mammals,  and,  more  rarely, 
birds  of  the  land,  mingled  with  those  of  the  crocodiles,  turtles,  and 
fishes  that  lived  in  the  water.  Similar  deposits  are  known  in  other 
continents. 

It  is  on  the  sea-bed  that  the  conditions  are  most  favourable 
to  the  preservation  of  the  greatest  number  and  variety  of  fossils. 
Among  the  littoral  deposits  ground  by  the  ceaseless  action  of  the 
surf,  fossils  are  not  likely  to  be  abundant  or  well-preserved,  but 
in  quieter  and  deeper  waters  vast  numbers  of  dead  shells  and 
the  like  accumulate  and  are  buried  in  sediments.  The  fossils  are 
not,  however,  uniformly  distributed  over  the  sea-bottom;  in  some 
places  they  are  crowded  together  in  multitudes,  while  large  areas 


518  FOSSILS  — GEOLOGICAL  CHRONOLOGY 

will  be  almost  devoid  of  them.  The  differences  are  due  to  varia- 
tions in  temperature,  in  the  character  of  the  bottom,  in  food  supply, 
and  other  conditions.  Even  under  the  most  favourable  circum- 
stances, the  fossils  can  never  represent  more  than  a  fraction  of  the 
life  of  their  times.  Indeed,  the  wonder  is  that  so  much  of  the 
life  systems  of  past  ages  has  been  preserved,  rather  than  that  so 
large  a  part  has  been  irretrievably  lost. 

The  ways  in  which  fossils  are  preserved  vary  much,  but  three 
groups  include  all  the  principal  kinds. 

(i)  Preservation  of  more  or  less  of  the  original  substance. 
In  certain  rare  instances  an  organism  may  be  preserved  intact, 


FIG.  252.  —  Artificial  external  mould  of  clam  shell  (on  left)  and  original  shell  (on 

right) 

as  have  been  the  carcasses  of  the  extinct  species  of  elephant  and 
rhinoceros  in  the  frozen  gravels  of  Siberia.  Much  more  common 
is  the  decomposition  of  the  soft  structures  and  the  preservation  of 
the  hard  parts,  —  bones,  shells,  etc.  Most  of  the  shells  and  bones 
found  in  the  rocks  of  later  geological  date  are  composed  of  the 
material  originally  belonging  to  them,  though  they  have  suffered 
much  loss  of  substance. 

(2)  Entire  loss  of  substance  and  retention  of  form.  In  this 
class  of  fossils  all  the  original  material  of  the  organism  has  been 
lost,  and  no  trace  of  its  internal  structure  is  retained,  but  only  the 
external  form  has  been  reproduced  in  some  different  material. 


HOW  FOSSILS  WERE  EMBEDDED   IN  THE  ROCKS       519 

Under  this  class  we  may  distinguish  two  principal  varieties:  (a) 
Moulds  and  (b)  Casts.  A  mould  is  formed  when  the  fossil  is  em- 
bedded in  sediments,  which  accurately  reproduce  its  external 
form,  and  harden  so  as  not  to  collapse  when  the  fossil  is  removed. 
Percolating  waters  then  dissolve  away  the  organism  entirely,  leav- 
ing only  a  cavity,  which  is  the  mould.  Impressions  of  footprints, 
which  may  be  place.d  in  the  same  category  as  moulds,  have  already 
been  explained  (see  pp.  206-7). 

Casts  are  formed  when  the  mould  is  filled  by  some  solid  sub- 
stance deposited  from  percolating  waters,  thus  reproducing  the 

>v 


FlG.  253.  —  Artificial  internal  cast  of  clam  shell  (on  left)  and  inner  view  of  original 

shell  (on  right) 

form  of  the  fossil,  as  is  done  artificially  with  plaster  or  gutta-percha. 
If  the  fossil  were  hollow,  like  a  shell,  we  frequently  find  a  com- 
bination of  internal  cast  with  an  external  mould  in  the  same 
specimen.  At  the  time  the  fossil  is  embedded  its  interior  is 
filled  with  the  same  sediment  which  hardens  and  forms  an  inter- 
nal cast,  exactly  reproducing  the  form  of  the  interior.  The  shell 
itself  is  then  dissolved  away,  leaving  a  space  between  the  outer 
mould  and  the  inner  cast.  Moulds  and  casts  are  commonest  in 
rocks  which  permit  percolating  waters  to  traverse  them  freely, 
such  as  sandstones  and  coarse-grained  limestones. 


520 


FOSSILS— GEOLOGICAL  CHRONOLOGY 


(3)  Loss  of  substance  with  reproduction  of  form  and  structure. 
Fossils  of  this  class  are  also  called  petrifactions  and  pseudomorphs 
(the  latter  a  term  borrowed  from  mineralogy).  Here  the  original 
material  of  the  organism  has  been  more  or  less  completely  re- 
moved, and  other  material  substituted  for  it;  but  the  substitution 
has  been  so  gradual,  molecule  by  molecule,  that  not  only  the 
external  form  but  also  the  microscopic  structure  has  been  perfectly 
reproduced.  Several  scantily  soluble  substances  act  as  petrifying 


FlG.  254.  —  Petrified  logs,  exposed  by  weathering  of  tuffs,  Arizona.     (Photograph 

by  Sinclair) 

materials,  the  most  perfect  results  being  given  by  silica.  A  silici- 
fied  bone,  or  tooth,  or  bit,of  wood,  differs  from  the  original  only  in 
weight,  colour,  and  hardness,  and  when  a  thin  section  is  examined 
under  the  microscope,  the  minutest  details  of  structure  may  be 
made  out  as  perfectly  as  from  the  unaltered  original.  CaCO3  is  a 
very  common  petrifying  agent,  but  it  often  obliterates  structure  by 
crystallizing  after  deposition;  less  usual  are  pyrite  and  siderite. 


DETERMINING  GEOLOGICAL  CHRONOLOGY  521 


II.   WHAT  MAY  BE  LEARNED  FROM  FOSSILS 

The  principal  value  which  fossils  possess  for  the  geologist  lies  in 
the  assistance  which  they  give  him  in  reconstructing  the  history  of 
the  globe.  This  they  do  in  several  ways. 

(i)  In  determining  Geological  Chronology.  — The  most  ob- 
vious way  in  which  to  make  out  the  relative  ages  of  a  series  of 
stratified  rocks  is  to  determine  their  order  of  superposition,  for  the 
oldest  will  be  at  the  bottom  and  the  newest  at  the  top  (see  p.  321). 
But  this  method  is  of  only  local  application  and  will  not  carry  us 
far  in  an  endeavour  to  compile  a  history  of  the  whole  earth.  It 
cannot  enable  us  to  compare  even  the  rocks  of  different  parts  of 
the  same  continent,  for  any  exposed  section  is  but  a  small  frac- 
tion of  the  whole  series  of  strata.  More  embarrassing  still,  strata 
change  their  character  from  point  to  point,  limestone  being  laid 
down  in  one  place  while  sandstone  is  accumulating  in  another. 
Still  less  can  the  order  of  superposition  help  to  determine  the 
relative  ages  of  rocks  in  different  continents,  for  this  order  in 
North  America  can  be  no  guide  to  the  succession  in  Africa  or  Aus- 
tralia. This  conclusion  does  not  imply  that  order  of  superposition 
may  be  safely  neglected;  on  the  contrary,  it  is  of  fundamental  im- 
portance, but  it  must  be  studied  in  connection  with  the  fossils. 

Life,  since  its  first  introduction  on  the  globe,  has  gone  on  ad- 
vancing, diversifying,  and  continually  rising  to  higher  and  higher 
planes.  We  need  not  stop  to  inquire  how  this  progression  has 
been  effected;  for  our  present  purpose  it  is  sufficient  to  know  that 
progress  and  change  have  been  unceasing  and  gradual,  though 
not  necessarily  occurring  at  a  uniform  rate.  Accepting,  then,  the 
undoubted  fact  of  the  universal  change^in  the  character  of  the 
organic  beings  which  have  successively  lived  on  the  earth,  it  follows 
that  rocks  which  have  been  formed  in  widely  separated  periods  of 
time  will  contain  markedly  different  fossils,  while  those  which  were 
laid  down  more  or  less  contemporaneously  will  have  similar  fossils. 
This  principle  enables  us  to  compare  and  correlate  rocks  from  all 


522 


FOSSILS— GEOLOGICAL  CHRONOLOGY 


the  continents  and,  in  a  general  way,  to  arrange  the  great  events 
of  the  earth's  history  in  chronological  order. 

The  general  principle  that  similarity  of  fossils  indicates  the  ap- 
proximate contemporaneity  of  the  rocks  in  which  they  are  found 
must  not  be  taken  too  literally,  for  it  is  subject  to  certain  limita- 
tions and  exceptions. 


•  • '-  -««O'~V.-»  -^- S—  -a N_J /—  — ir^o-^  "*> <     -1"-^f=^:~-' T* 


FIG.  255.  —  Geological  map  of  Central  New  York,  showing  the  faunal  provinces  of 
the  Upper  Devonian  (Portage  stage).     (Clarke) 

(a)  Exact  contemporaneity  is  not  meant,  for  the  progress  of 
life  is  very  slow,  and  rocks  formed  thousands  of  years  apart  may 
yet  contain  precisely  similar  fossils. 

(b)  Animals  and  plants  differ  in  different  parts  of  the  world,  so 
that  contemporaneous  rocks  formed  in  widely  separated  regions 


DETERMINING  GEOLOGICAL  CHRONOLOGY  523 

will  always  show  a  certain  amount  of  difference  in  their  contained 
fossils.  In  comparing  the  rocks  of  two  continents,  it  is  often  ex- 
ceedingly difficult  to  decide  just  how  much  of  a  given  difference 
in  the  fossils  is  to  be  ascribed  to  a  difference  in  the  time  of  rock 
formation,  and  how  much  to  mere  geographical  separation. 
There  is  a  great  difference  in  the  value  of  the  various  classes  of 
organisms  for  chronological  purposes,  the  most  useful  being  those 
which  have  the  most  efficient  means  of  very  wide  simultaneous 
dispersal.  Such  are  the  pelagic  organisms,  which  live  at  the  sur- 
face of  the  open  sea  and,  either  alive  or  dead,  are  carried  for  vast 
distances  by  the  ocean  currents. 

A  geographical  distinction  which  should  be  emphasized  is  that 
of  fades,  by  which  is  meant  the  sum  total  of  environmental  con- 


«pl«a  Ithaca  CortlandCo  Chenanjo  Vallcj 


FIG.  256.  —  East-west  section  through  area  given  in  Fig.  255,  showing  changes  of 
facies.     (Clarke) 

ditions,  organic  and  inorganic.  Deep  or  shallow  water,  salt  water 
or  fresh,  muddy  or  sandy,  or  rocky  bottom,  are  all  differences  of 
facies,  and  quite  closely  adjoining,  strictly  contemporaneous  de- 
posits may  display  them.  Different  assemblages  of  contempora- 
neous fossils  also  constitute  facial  differences;  thus  we  speak  of 
the  graptolitic,  or  the  cephalopod  facies  of  a  geological  division. 

A  possible  source  of  error  in  the  inferences  drawn  from  fossils 
lies  in  the  incorporation  of  more  ancient  remains  washed  out  of 
an  older  rock  and  embedded  in  a  newer  one.  Figure  257  is  a  pho- 
tograph taken  on  the  York  River,  Virginia,  which  shows  fossil 
shells  washed  out  of  the  bluffs  and  lying  on  the  beach,  where  they 
are  mingled  with  modern  organic  remains. 

Despite  these  limitations  we  find  that,  speaking  broadly,  the 


524 


FOSSILS  — GEOLOGICAL  CHRONOLOGY 


order  of  succession  in  the  appearance  and  extinction  of  the  great 
groups  of  fossils  is  much  the  same  for  all  parts  of  the  earth,  and  we 
may  confidently  assume  that  the  grander  divisions  of  geological 
time  are  of  world- wide  significance. 

It  will  now  be  easy  to  understand  why  the  fossils  in  two  groups 
of  unconformable  strata  are  generally  so  radically  different.     It  is 


FlG.  257.  —  Fossil  shells  (Miocene)  lying  on  a  modern  beach,  York  River,  Va. 
(Photograph  by  van  Ingen) 

because  of  the  long  lapse  of  unrecorded  time  at  that  point,  during 
which  organic  progress  continued;  when  deposition  was  resumed, 
the  animals  and  plants  were  all  new,  and  so  the  change  is  abrupt. 
If  one  is  reading  a  book  from  which  a  dozen  chapters  have  been 
torn  out,  the  change  of  subject  will  appear  violently  abrupt;  to 
bridge  over  the  gap  one  must  find  another  copy  of  the  book. 


DETERMINING   GEOLOGICAL  CHRONOLOGY  $2$ 

Likewise,  to  fill  up  the  gap  of  a  great  unconformity,  we  must  go  to 
some  region  where  deposition  went  on  uninterruptedly,  and  there 
we  may  trace  the  gradual  and  steady  change  in  the  fossils. 

A  geological  chronology  is  constructed  by  carefully  determining, 
first  of  all,  the  order  of  superposition  of  the  stratified  rocks,  and 
next  by  learning  the  fossils  characteristic  of  each  group  of  strata. 
To  many  it  has  seemed  that  this  is  reasoning  in  a  circle,  but 
that  is  because  the  argument  is  not  fully  stated,  some  of  its  steps 
being  omitted.  The  order  of  succession  among  the  fossils  is  deter- 
mined from  the  order  of  superposition  of  the  strata  in  which  they 
occur.  When  that  succession  has  been  thus  established,  it  may  be 
employed  as  a  general  standard. 

Great  physical  events,  such  as  the  upheaval  of  mountain  ranges, 
widespread  transgressions  of  the  sea  and  changes  of  climate,  often 
give  us  a  means  of  correlating  the  strata  of  different  continents 
with  greater  precision  than  can  be  done  with  the  aid  of  fossils  only. 
The  latter  are,  however,  indispensable  means  of  first  determining 
which  of  these  events  are  comparable  in  different  regions.  The 
history  is  recorded  partly  in  the  nature  and  structure  of  the  rocks, 
partly  in  the  fossils,  and  partly  in  the  topographical  forms  of  the 
land  and  the  courses  of  the  streams.  By  combining  these  differ- 
ent lines  of  evidence,  local  histories  are  constructed  for  each  region, 
until  from  these  the  story  of  the  whole  continent  may  be  compiled. 
The  comparative  study  of  the  fossils  then  gives  the  clew  for  uniting 
the  history  of  the  different  continents  into  the  history  of  the  earth. 
Much  remains  to  be  done  before  this  great  task  can  be  accom- 
plished, but  already  we  have  an  outline  of  the  scheme  which  future 
investigations  may  fill  up. 

It  necessarily  follows  from  the  way  in  which  sedimentary  rocks 
are  formed,  and  the  local  nature  of  upheavals  and  depressions  of 
land,  that  in  no  single  locality  can  the  entire  series  of  strata  be 
observed,  and  that  each  region  can  display  but  a  certain  propor- 
tion of  the  whole  record.  The  different  parts  of  our  continent 
are  of  vastly  different  geological  dates,  and  even  the  same  area 
may  have  been  many  times,  a,  land-surface,  and  as  often  a  sea- 


526  FOSSILS— GEOLOGICAL  CHRONOLOGY 

bottom.  Unconformities,  more  or  less  widespread,  offer  a  natural 
and  convenient  mode  of  dividing  the  strata  into  groups,  but  the 
difficulty  with  this  method  is  that  the  dates  of  elevation  and  de- 
pression so  seldom  correspond  in  different  regions,  that  divisions 
thus  made  are  apt  to  be  of  more  or  less  local  validity.  The  only 
standard  yet  devised  which  is  applicable  to  all  the  world  is  that 
founded  upon  the  progress  of  life. 

The  comparison  between  human  history  and  geological  history 
is  one  that  has  very  often  been  made,  but  trite  and  hackneyed  as 
it  is,  it  is  none  the  less  instructive.  The  history  of  civilized  na- 
tions is  the  record  of  continuous  development,  not  without  retro- 
gressions and  periods  of  comparative  stagnation,  but  having  no 
actual  gaps  in  it.  For  the  sake  of  convenience,  history  is  divided 
into  certain  periods  in  accordance  with  the  predominance  of  cer- 
tain great  ideas  and  principles,  and  these  periods  are  real,  repre- 
senting the  salient  facts  in  the  progress  of  development.  Each 
period  is,  however,  but  the  outcome  of  the  antecedent  periods, 
and  the  ideas  and  principles  which  characterize  it  were  slowly 
maturing,  it  may  be  through  centuries,  and  even  after  other  ideas 
have  risen  to  predominance,  older  ones  continue  to  live  and  influ- 
ence the  world.  For  example,  when  we  speak  of  the  age  of  the 
French  Revolution,  we  refer  to  a  time  when  a  certain  set  of  politi- 
cal ideas  and  principles  were  the  most  striking  and  influential 
factors  in  the  development  of  the  civilized  world,  beginning  with 
the  visible  changes  of  1789  and  ending  with  the  restoration  of  the 
Bourbons  in  1815.  But  the  tremendous  outbreak  was  slowly  pre- 
paring throughout  the  eighteenth  century;  the  conflagration  was 
proportionate  to  the  materials  that  had  been  gathered  for  it.  Nor, 
on  the  other  hand,  could  the  effects  of  the  great  movement  be 
undone  by  the  return  of  the  exiled  king.  To  this  day  the  whole 
civilized  world  feels  the  effects  of  the  convulsion,  and  the  entire 
course  of  the  nineteenth  century  would  have  been  different  but 
for  the  French  Revolution. 

Historians  are  careful  to  distinguish  between  events  and  the 
record  of  them.  Events  are  continuous  and  bound  up  into  a  chain 


DETERMINING  GEOLOGICAL  CHRONOLOGY  527 

of  consequences,  every  one  of  which  is  dependent  upon  others, 
while  the  records  may  be  scanty,  interrupted,  confused,  unin- 
telligible, even  misleading  and  falsified,  so  that  it  is  no  easy  task 
to  write  history  accurately  and  without  attributing  undue  im- 
portance to  this  or  that  principle  or  policy. 

These  considerations  fully  apply  to  geological  history;  its  divi- 
sions are  founded  upon  the  rise  and  culmination  of  great  groups 
of  animals  and  plants,  which  one  after  another  have  risen  to  pre- 
dominance and  then  declined,  their  place  being  taken  by  others 
better  fitted  for  the  new  conditions.  These  successive  culminations 
are  not  sudden,  but  gradual  and  continuous,  and  the  beginnings 
of  each  group  are  to  be  found  in  times  long  before  the  period  of 
its  predominance.  Nor  is  decline  immediately  followed  by  extinc-* 
tion;  one  group  slowly  gives  way  to  another,  but  long  after  the 
first  has  ceased  to  be  the  principal  fact  in  the  world's  life,  it  may 
linger  on  in  diminished  importance  until,  perhaps,  it  finally  disap- 
pears. The  geological  periods,  therefore,  like  historical  periods, 
had  not  definite  beginnings  and  endings,  for  one  slowly  fades  into 
another,  but  they  are  none  the  less  actual  because  the  lines  of 
separation  between  them  must  often  be  somewhat  arbitrarily 
drawn,  and  they  cannot  always  be  made  to  correspond  in  differ- 
ent regions. 

In  geology,  as  in  history,  we  must  distinguish  between  the  events 
and  the  records  of  them.  The  more  complete  the  records,  the 
more  obviously  continuous  and  gradual  was  the  course  of  events; 
only  imperfect  records  can  make  the  history  seem  broken  and  dis- 
jointed. As  our  science  was  first  developed  in  western  Europe, 
where  the  great  groups  of  strata  are  mostly  separated  by  uncon- 
formities, with  abrupt  changes  in  the  f9ssils,  the  older  geologists 
very  naturally  concluded  that  the  great  divisions  of  geological  time 
were  marked  by  frightful  catastrophes  which  devastated  the  earth, 
destroying  every  living  thing  upon  it.  Each  group  of  rocks  was 
looked  upon  as  the  product  of  a  long  and  tranquil  period,  and  its 
fossils  were  believed  to  represent  an  entirely  new  creation.  Though 
opposed  by  some  far-seeing  minds,  the  doctrine  of  Catastrophism, 


528  FOSSILS  —  GEOLOGICAL  CHRONOLOGY 

as  it  was  called,  long  held  sway,  but  was  shown  to  be  erroneous, 
when  the  study  of  geology  was  carried  to  other  parts  of  the  world. 
Then  it  appeared  that  the  supposed  catastrophes,  if  they  occurred 
at  all,  were  not  general,  but  local,  and  that  records  missing  in 
Europe  had  been  preserved,  partially  at  least,  in  other  continents. 
Enough  of  these  missing  records  has  been  recovered  to  show  that 
the  earth's  progress  was  not  by  a  series  of  abrupt  and  sudden 
changes,  but  by  a  continuous,  orderly  development. 

Major  divisions  of  geological  time  are  founded  upon  the  more 
striking  changes  in  the  animals  and  plants,  while  for  minor  divi- 
sions the  more  detailed  differences  in  the  organisms  are  employed. 
Parallel  with  the  divisions  of  time  run  the  groups  or  systems  of 
the  strata,  for  characterizing  which  both  the  physical  nature  and 
structure  of  the  rocks  and  the  fossils  are  employed.  In  the  very 
difficult  and  complicated  task  of  compiling  the  earth's  history,  no 
kind  of  evidence  can  be  ignored,  and  wide  knowledge  and  sound 
judgment  are  needed  in  the  work,  so  that  no  particular  class  of 
records  shall  be  either  over-  or  undervalued. 

Though  the  goal  of  geological  inquiries  is  to  construct  the  his- 
tory of  the  earth  as  a  unit,  this  goal  can  be  reached  only  by  the 
minute  and  exhaustive  study  of  the  local  histories.  Each  of  the 
latter  has  certain  peculiarities  of  its  own  which  must  be  deter- 
mined, and  hence  arises  the  multiplicity  of  local  names  for  groups 
of  strata,  so  confusing  to  the  student.  Local  names  are  useful, 
because  they  avoid  the  necessity  of  premature  correlations,  which 
may  lead  to  the  direst  mistakes. 

(2)  As  Evidence  of  Geographical  Changes.  —  We  have  seen 
that  from  the  composition  and  structure  of  the  stratified  rocks 
themselves  much  may  be  learned  concerning  the  geographical 
conditions  under  which  they  were  formed,  and  of  the  subsequent 
geographical  changes  of  the  region  in  which  they  occur.  Fossils 
supplement  this  information  regarding  the  body  of  water  in  which 
the  rocks  were  laid  down,  whether  fresh  or  salt,  deep  or  shallow, 
near  or  far  from  land,  in  an  open  sea  or  a  closed  basin,  and  whether 
such  a  closed  basin  had  occasional  or  constant  communication 


EVIDENCE  OF  CLIMATIC  CHANGES  529 

with  the  ocean.  Most  of  the  stratified  rocks  which  now  form  part 
of  the  land-surface  give  us  information  only  concerning  the  former 
extension  of  bodies  of  water  over  what  is  now  the  land,  but  they  can 
tell  us  nothing  of  the  land  areas  which  have  disappeared  beneath 
the  sea.  In  this  connection  fossils  are  of  great  assistance,  for,  in 
certain  instances,  the  distribution  of  marine  fossils  points  to  the 
presence  of  land  barriers  to  migration  which  no  longer  exist,  or 
to  a  continuity  of  coast-lines  which  are  now  broken  up,  while 
the  fossils  of  land  animals  may  demonstrate  the  former  existence 
of  land  bridges  between  regions  which  have  long  been  separated 
by  water.  Thus  it  may  be  shown  that  North  America  was  fre- 
quently and  for  long  periods  of  time  connected  with  Asia  across 
Bering  Sea,  and  that  its  union  with  South  America  is  of  geologi- 
cally late  date. 

(3)  As  Evidence  of  Climatic  Changes.  —  The  remarkable  cli- 
matic changes  through  which  various  parts  of  the  earth  have 
passed  are  indicated  by  fossils.  Indeed,  with  the  exception  of 
glacial  marks  and  ice-formed  deposits,  fossils  offer  almost  the 
only  trustworthy  evidence  available  as  to  changes  of  temperature. 
Thus,  when  we  find  in  the  rocks  of  Greenland  the  remains  of  ex- 
tensive forests  of  such  trees  as  now  grow  in  temperate  latitudes, 
the  only  possible  inference  is  that  Greenland  now  has  a  far  colder 
climate  than  when  those  forests  existed.  The  same  conclusion 
follows  from  the  presence  in  the  rocks  of  Wyoming  and  Idaho  of 
great  palm  leaves  and  other  subtropical  plants  associated  with  the 
bones  of  crocodiles  and  other  reptiles,  such  as  live  only  in  warm 
regions.  In  deposits  of  a  far  later  date  occur  bones  of  the  rein- 
deer in  southern  New  England  and  in  the  south  of  France,  walrus 
bones  in  the  sands  of  New  Jersey,  and  those  of  the  musk-ox  in 
Arkansas;  all  of  which  shows  that  at  one  time  these  regions  had  a 
much  colder  climate  than  at  present. 

The  evidence  as  to  climatic  changes  which  is  presented  by  fos- 
sils must,  however,  be  treated  with  great  caution,  because  even 
nearly  allied  species  often  have  entirely  different  habits,  and 
flourish  in  quite  different  climates.  Most  fossils  belong  to  extinct 

2M 


53O  FOSSILS  — GEOLOGICAL  CHRONOLOGY 

species,  as  to  whose  climatic  relations  we  have  no  knowledge. 
Before  any  conclusion  concerning  changes  of  climate  can  be  re- 
garded as  established,  we  should  have  the  testimony  of  species 
still  living,  or,  if  that  is  not  possible,  the  evidence  must  be  drawn 
from  large  assemblages  of  different  kinds  of  animals  and  plants. 
Such  an  extreme  case  as  the  fossil  plants  of  Greenland  is  sufficient 
evidence  without  further  corroboration. 


III.    CLASSIFICATION  OF  GEOLOGICAL  TIME 

The  method  of  making  the  divisions  and  subdivisions  of  geo- 
logical time  is  not  yet  a  fixed  one,  and  there  is  much  difference 
in  the  usage  of  various  writers.  The  names  of  the  divisions  also 
have  been  given  at  various  times  and  in  many  lands,  according  to 
no  particular  system.  Most  of  these  names  have  been  taken  from 
the  locality  or  district  where  the  rocks  in  question  were  first 
studied  or  are  most  typically  displayed ;  as  Devonian  from  Devon- 
shire, Jurassic  from  the  Jura  Mountains.  Some  are  named  from 
a  characteristic  or  prevalent  kind  .of  rock,  such  as  Cretaceous 
(Latin  creta,  chalk)  and  Carboniferous.  Of  late  there  has  been  a 
tendency  toward  a  more  uniform  method  of  nomenclature,  and 
to  the  use  of  one  set  of  terms  for  the  divisions  of .  time,  and  an- 
other and  corresponding  set  for  the  divisions  of  the  strata.  The 
grander  divisions  of  time  are  called  eras,  and  in  descending  order 
we  have  periods,  epochs,  and  ages.  The  following  table  represents 
the  divisions  in  the  scale  of  time  and  the  scale  of  rocks  which  have 
been  adopted  by  the  International  Geological  Congress. 

TIME  SCALE  ROCK  SCALE 

Era  Group 

Period  System 

Epoch  Series 

Age  Stage 

Substage 

Zone 


CLASSIFICATION  OF  GEOLOGICAL  TIME 


531 


It  will  be  observed  that  the  subdivision  is  carried  farther  in  the 
scale  of  rocks  than  in  that  of  time,  because  of  the  generally  local 
character  of  these  minor  subdivisions.  The  names  employed  are, 
as  yet,  the  same  for  both  scales,  and  we  speak  of  the  Palaeozoic 
Era  or  Group,  and  of  the  Silurian  Period  or  System.  It  has  been 
proposed  to  give  separate  names  to  the  divisions  of  the  two  scales, 
and  this  would  be  an  improvement  in  some  respects. 


TABLE  OF  MAJOR  GEOLOGICAL  DIVISIONS 


Cenozoic  Era 


Mesozoic  Era 


Palaeozoic  Era 


Pre-Cambrian  Eras 


|  Quaternary  Period 

( Tertiary  Period 

[Cretaceous  Period 

•j  Jurassic  Period 

[Triassic  Period 
Permian  Period 
Carboniferous  Period 
Devonian  Period 
Silurian  Period 
Ordovician  Period 
Cambrian  Period 

J  Algonkian  Period 

1  Archaean  Period 


CHAPTER    XXV 

ORIGINAL  CONDITION  OF  THE  EARTH  —  PRE-CAMBRIAN 

PERIODS 

As  we  trace  the  history  of  mankind  back  to  very  ancient  times, 
we  find  that  the  records  become  more  and  more  scanty  and  less 
intelligible,  until  history  fades  into  myth  and  tradition.  Of  a 
still  earlier  age  we  have  not  even  a  tradition;  it  is  prehistoric. 
Similarly,  among  the  geological  records  the  earliest  are  in  a 
state  of  such  excessive  confusion  that  they  are  exceedingly  diffi- 
cult to  understand,  and  between  different  observers  there  are 
radical  differences  of  opinion  both  as  to  the  facts  and  as  to  their 
interpretation.  Furthermore,  there  must  have  been  an  inconceiv- 
ably long  time  earlier  than  the  most  ancient  recorded  periods,  as 
to  which  conjecture  and  inference  are  the  only  resource.  In  these 
difficult  straits  astronomy  offers  valuable  assistance  to  the  baffled 
geologist.  The  Nebular  Hypothesis  is  a  scheme  of  *he  develop- 
ment of '  the  solar  system  which  is  very  generally  accepted  by 
astronomers,  in  some  form,  as  essentially  true. 

The  term  nebular  hypothesis  is  usually,  though  not  with  exact- 
ness, limited  to  one  particular  form,  according  to  which  the  place 
of  the  present  solar  system  was  originally  occupied  by  a  vast 
rotating  nebula,  a  mass  of  intensely  heated  vapour,  or  possibly 
clouds  of  meteorites,  extending  beyond  the  orbit  of  the  outermost 
planet.  As  the  nebula  cooled  by  radiation,  it  contracted,  leaving 
behind  it  successive  rings,  like  those  of  the  planet  Saturn,  but  on 
a  vastly  larger  scale.  The  rings  kept  up  the  rotation  imparted  by 
the  nebula,  and  all  of  them  lay  in  nearly  the  same  plane.  Unequal 

532 


ORIGINAL  CONDITION  OF  THE  EARTH  533 

contraction  in  various  parts  of  each  revolving  ring  caused  it  to 
break  up  and  gather  by  mutual  attraction  into  masses.  If  these 
rings  were  composed  of  relatively  small  solid  masses,  like  meteorites, 
or  if  they  had  solidified  by  condensation  of  the  vapours,  the  heat 
generated  by  the  collisions,  as  the  broken  ring  was  gathered  into 
a  mass,  would  suffice  to  raise  the  temperature  and  liquefy  or 
vapourize  the  mass.  By  revolution  the  nebulous  masses  would 
assume  a  spheroidal  shape  and  become  planets.  The  central  mass 
of  the  original  nebula  forms  the  sun,  which  is  still  in  an  intensely 
heated,  incandescent  state. 

Another  form  of  the  nebular  hypothesis,  called  for  the  sake  of 
distinction  the  Planetesimal  Hypothesis,  has  recently  been  pro- 
posed by  Professor  Chamberlin.  This  postulates,  as  the  begin- 
ning of  the  solar  system,  a  spiral  nebula,  "  and  that  the  matter 
of  this  parent  nebula  was  in  a  finely  divided  solid  or  liquid  state 
before  aggregation.  ...  It  regards  the  knots  of  the  nebula  as  the 
nuclei  of  the  future  planets,  and  the  nebulous  haze  as  matter  to 
be  added  to  these  nuclei  to  form  the  planets.  It  assumes  that 
both  the  knots  and  the  particles  of  the  nebulous  haze  moved 
about  the  central  mass  in  elliptical  orbits  of  considerable,  but  not 
excessive,  eccentricity.  ...  It  deduces  a  relatively  slow  growth 
of  the  earth,  with  a  rising  internal  temperature  developed  in  the 
central  parts  and  creeping  outward."  (Chamberlin  and  Salis- 
bury.) 

This  is  not  the  place  to  discuss  the  evidence  for  an  astronomical 
speculation,  but  it  is  clear  that  the  hypothesis  regarding  the 
development  of  the  solar  system  which  we  adopt  must  condition 
our  views  as  to  the  early  unrecorded  stages  of  the  earth's  history. 
From  the  strictly  geological  standpoint  the  most  important  differ- 
ence between  the  Nebular  and  the  Planetesimal  Hypotheses  is 
that,  according  to  the  former,  the  earth  has  passed  through  a 
gaseous  and  a  molten  stage,  and  therefore  must  have  formed  a 
crust  by  solidification,  while  the  latter  leads  to  the  conclusion  that 
the  earth  has  been  solid  from  the  beginning,  and  consequently 
never  formed  a  crust  of  solidification. 


534  THE  PRE-CAMBRIAN   PERIODS 

THE   PRE-CAMBRIAN   PERIODS  — I.   ARCHAEAN 

It  is  unfortunate  that  an  account  of  historical  geology  shoyld 
necessarily  begin  with  the  most  difficult  and  obscure  part  of  the 
whole  subject,  but  the  treatment  must  be  in  accordance  with  the 
chronological  order,  and  the  oldest  rocks  are  the  least  intelligible. 
The  ordinary  criteria  of  the  historical  method,  namely,  the  strati- 
graphical  succession  and  the  comparison  of  fossils,  fail  us  here 
almost  entirely,  and  the  only  way  of  correlating  the  rocks  of  dif- 
ferent regions  and  continents  is  by  means  of  the  characters  of  the 
rocks  themselves.  In  the  present  state  of  knowledge,  "  lithologi- 
cal  similarity  "  is  not  a  safe  guide.  So  many  metamorphic  rocks, 
once  referred  to  the  Archaean,  have  proved  to  be  of  much  later 
date,  that  some  cautious  geologists,  who  have  no  confidence  in 
"  lithological  similarities,"  prefer  not  to  use  the  term  Archcean 
at  all,  but  to  employ  local  terms  for  the  oldest  crystalline  rocks 
exposed  in  a  given  district. 

The  Archaean  includes  the  most  ancient  rocks,  often  spoken  of 
as  the  "  basement,  or  basal  complex."  Its  antiquity  is  best  as- 
sured in  regions  where  it  is  separated  by  thick  series  of  sediment- 
ary or  metamorphic  rocks  from  the  Lower  Cambrian,  which  can 
be  certainly  identified  by  its  fossils.  The  character  and  relations 
of  the  pre-Cambrian  rocks  differ  so  much  in  different  areas  that 
it  will  be  best  to  describe  them  in  two  or  three  typical  regions.  In 
the  Canadian  provinces  of  Quebec  and  eastern  Ontario  and  in  the 
Adirondack  Mountains,  the  oldest  rocks  are  a  series  of  intensely 
metamorphosed  sediments,  including  great  bodies  of  limestones, 
quartzites,  schists,  etc.  In  the  Adirondacks,  especially,  this  series 
is  invaded  by  enormous  bodies  of  intrusives,  which  preceded  and 
were  involved  in  a  great  period  of  metamorphism.  In  eastern 
Ontario  the  thickness  of  the  metamorphosed  sedimentaries  is 
exceedingly  great.  "  Along  its  whole  northern  border  this  sedi- 
mentary series  is  torn  to  pieces  by  an  enormous  volume  of  gneissic 
granite  of  igneous  origin  which  rises  from  beneath  it,  and  which 
along  its  margin  also  wells  up  through  it  in  the  form  of  great 
intrusive  bathyliths."  Though  underlying  the  metamorphosed 


THE  PRE-CAMBRIAN   PERIODS  535 

sediments  of  the  Grenville  series,  the  gneissic  granite,  or  Laurentiant 
is  the  younger,  as  the  contact   is  an  intrusive  one. 

In  the  region  around  Lake  Superior  the  pre-Cambrian  rocks 
are  displayed  in  enormous  thickness  and  are  divided  into  groups 
by  four  great  unconformities.  Of  these  the  most  ancient  is  the  Kee- 
watin,  intensely  metamorphosed  rocks  derived  from  the  transfor- 
mation of  lava  flows,  tuffs,  and  other  volcanic  rocks,  with  some  of 
sedimentary  origin,  and  forming  a  great  series  of  schists,  which 
are  underlaid  and  penetrated  by  the  Laurentian  gneissic  granites. 
It  immediately  suggests  itself  that  the  Grenville  series  of  the  east 
is  the  equivalent  of  the  Lake  Superior  Keewatin,  but  the  com- 
mittee of  the  Canadian  and  United  States  Geological  Surveys, 
which  has  investigated  these  problems,  report  that  they  consider 
it  "  inadvisable  in  the  present  state  of  our  knowledge  to  attempt 
any  correlation  of  the  Grenville  series  with  the  Huronian  or 
Keewatin."  The  classification  proposed  by  the  committee  is  con- 
tained in  the  subjoined  table,  though  emphasis  must  again  be  laid 
upon  the  fact  that  no  equivalence  between  the  Grenville  and 
the  Keewatin  is  asserted.  They  may  be  separated  by  vast  periods 
of  time,  and  yet  both  must  be  older  than  the  intrusion  of  the 
Laurentian  granites. 

PRE-CAMBRIAN  ROCKS 

LAKE  SUPERIOR  REGION  EASTERN  REGION 

Keweenawan 
Unconformity 

Upper 

Unconformity 


Huronian 


Middle 


Unconformity 
Lower 
Unconformity 
Keewatin  Grenville  series 

Intrusive  contact  Intrusive  contact 

Laurentian  Laurentian 

N.B.  —  In  the  classification  adopted  in  this  book  the  Archaean  comprises 
the  Keewatin  and  Laurentian  and  probably  also  the  Grenville  series. 


536  ORIGINAL  CONDITION  OF  THE  EARTH 

The  Archaean,  then,  is  composed  of  completely  crystalline  rocks 
of  various  types.  Massive  rocks,  such  as  granite  and  basic  erup- 
tives,  and  foliated  rocks,  like  gneissoid  granite,  gneiss,  many 
varieties  of  schists,  are  intermingled  in  the  most  intricate  way, 
a  characteristic  well  expressed  in  the  oft-used  phrase  of  the  basal 
or  fundamental  complex  for  the  Archaean.  The  component 
mineral  particles  show  plainly  the  intense  dynamic  metamor- 
phism  to  which  they  have  been  subjected,  in  their  extremely  com- 
plex arrangement  and  in  their  laminated  and  crushed  condition. 
The  rocks  thus  referred  to  the  Archaean  are  not  necessarily  all  of 
the  same  age,  but  they  are  all  of  vast  antiquity  and  older  than  any 
other  known  series.  They  are  of  very  great  but  unknown  thick- 
ness, for  the  bottom  of  them  is  nowhere  to  be  seen,  and  even 
when  thrown  up  into  mountain  ranges,  erosion  has  in  no  case  cut 
so  deeply  into  these  rocks  as  to  expose  anything  different  below 
them. 

The  reason  for  uniting  these  rocks  into  one  group  is  not 
merely  their  likeness  in  composition,  which  is  not  a  sufficient 
criterion,  but  because  of  their  unique  and  uniformly  complex 
structure,  their  resemblance  to  one  another  and  difference 
from  any  other  group  of  rocks,  and  their  invariably  funda- 
mental position. 

The  Distribution  of  the  Archaean  Rocks.  —  At  the  outset  of  our 
historical  studies  it  is  essential  to  understand  clearly  just  what  is 
meant  by  the  term  distribution  of  a  given  formation.  It  means: 
(i)  that  the  given  rock  is  at  the  surface  over  a  certain  area,  dis- 
regarding the  covering  of  soil,  drift,  or  other  loose  materials;  (2) 
that  the  concealed  extension  of  the  formation  beneath  newer  rocks 
may  be  confidently  inferred  from  surface  observations.  So  far  as 
the  Archaean  rocks  are  concerned,  their  surface  distribution  can 
at  present  be  stated  only  with  much  reserve,  for  they  often  grade 
into  crystalline  schists  of  demonstrably  later  date,  and  much  that 
once  was  referred  to  the  Archaean  is  now  known  to  be  far  more 
recent.  Accurately  to  determine  the  distribution  of  the  basal 
complex  will  require  the  most  extensive,  minute,  and  laborious 


THE  DISTRIBUTION  OF  THE  ARCH/EAN   ROGKS         537 


FlG.  258.  —  Map  of  known  pre-Cambrian  surface  exposures  in  North  America. 
The  black  areas  are  outcrops 


538  .  ORIGINAL  CONDITION  OF  THE  EARTH 

t 
investigation.    The  northern  part  of  North  America,  from  the 

Arctic  Ocean  to  the  Great  Lakes,  probably  including  Greenland, 
is  made  up  of  an  immense  area  of  schistose  rocks,  estimated  at 
more  than  2,000,000  square  miles  in  extent.  Over  this  vast  region 
occur  numerous  areas  of  Archaean  rocks,  but  it  is  not  yet  possible 
to  say  how  much  of  it  belongs  in  that  group  and  how  much  is 
newer. 

Beside  this  principal  region  are  several  other  minor  ones.  A 
narrow  band  of  schistose  rocks  extends,  with  some  interruptions, 
/rom  Newfoundland  to  Alabama,  with  shorter  parallel  belts  in 
eastern  Canada  and  New  England.  Another  great  axis  is  on  the 
site  of  the  Rocky  Mountain  chain,  with  several  shorter  and  gen- 
erally parallel  belts  from  Mexico  to  Alaska.  Isolated  areas  occur 
in  Missouri,  central  Texas,  New  Mexico,  and  Arizona.  In  all  of 
these  regions  are  found  rocks  like  the  typical  Archaean,  which  stand 
in  the  same  relation  to  the  newer  groups,  but  how  much  should  be 
referred  to  the  oldest  series  is  still  a  question. 

In  the  other  continents  occur  great  areas  of  very  ancient  gneisses 
and  crystalline  schists,  but  even  less,  than  in  North  America 
has  the  distinction  been  made  between  the  fundamental  com- 
plex and  newer  groups.  In  the  following  statements  no  attempt 
is  made  to  determine  how  much  of  the  areas  mentioned  is  properly 
Archaean. 

In  Europe  the  principal  area  lies  to  the  north,  covering  parts  of 
Ireland  and  the  Highlands  of  Scotland,  with  which  was  probably 
once  connected  the  great  continuous  mass  of  Scandinavia,  Finland, 
and  Lapland.  Considerable  areas  also  occur  in  central  and  south- 
ern Europe,  as  the  central  plateau  of  France,  parts  of  Germany 
and  Bohemia,  and  long,  narrow  belts  in  the  Pyrenees,  Alps,  and 
Balkans.  In  Asia  these  ancient  crystalline  rocks  are  found  in  the 
great  mountain  ranges,  such  as  the  Himalayas,  Altai,  etc.  They 
make  up  a  large  part  of  the  Indian  peninsula,  and  are  extensively 
displayed  in  China,  Japan,  and  the  islands  of  the  Malay  Archi- 
pelago. The  vast  central  plateau  which  occupies  so  much  of 
Africa  is  principally  composed  of  these  rocks,  which  are  also 


ORIGIN  OF  THE  ARCH^AN   ROCKS  539 

largely  exposed  in  Australia.  In  South  America  similar  rocks 
appear  in  the  highlands  of  Brazil  and  in  the  Andes. 

It  is  estimated  that  the  Archaean  rocks  form  somewhat  more 
than  one-fifth  of  the  land-surface  of  the  earth,  and  there  is  reason 
to  believe  that  they  are  actually  universal,  and  that  a  boring  made 
at  any  point,  if  sufficiently  deep,  would  encounter  them.  They 
are  found  at  the  bottom  of  many  deep  canons,  and  borings  fre- 
quently penetrate  them  at  points  where  there  are  no  surface  indi- 
cations of  their  presence.  If  these  rocks  are  really  distributed 
over  the  entire  globe,  they  are  the  only  formation  of  which  this  is 
true. 

Origin  of  the  Archaean  Rocks.  — This  is  a  problem  which  has 
given  rise  to  a  great  deal  of  discussion,  but  a  solution  appears  to  be 
near.  Independently,  in  many  countries,  observers  have  reached 
the  conclusion  that  these  rocks  are  divisible  into  two  great  series, 
a  schist  series  composed  chiefly  of  highly  metamorphosed  sedi- 
mentary and  volcanic  rocks,  and  a  gneissoid  granite  series,  which 
is  intrusive  and  later  than  the  former. 

Assuming  that  this  conclusion  is  true,  at  least  as  a  working 
hypothesis,  it  involves  certain  curious  consequences.  Surface 
lava  flows  and  volcanic  tuffs,  and  still  more,  sedimentary  rocks, 
necessarily  imply  a  solid  floor  upon  which  they  were  laid  down, 
but  of  this  floor  not  a  trace  has  anywhere  been  found.  The 
question  immediately  arises,  what  has  become  of  it  ?  No  answer 
to  this  question  can  yet  be  given,  but  apparently  the  most  likely 
suggestion  is  thatthe  ascending  floods  of  molten  magma,  which 
gave  rise  to  the  gneissoid  granites,  must  have  melted  and  assimi- 
lated it.  If  this  were  only  a  local  phenomenon,  there  would  be 
nothing  very  surprising  about  it,  but  it  would  seem  to  be  true  of  the 
entire  globe,  and  this  is  a  startling  conclusion.  We  are  then  to 
suppose  that  a  solid  crust,  however  formed,  was  for  a  very  long  time 
sufficiently  rigid  and  stable  to  allow  a  great  thickness  of  sedimen- 
tary and  volcanic  rocks  to  be  accumulated  upon  it  and  then  was 
ingulfed  and  destroyed  by  a  universally  ascending  magma,  though 
it  is  not  necessary  to  suppose  that  this  took  place  simultaneously 


540  ORIGINAL  CONDITION  OF  THE  EARTH 

over  the  whole  earth,  or  even  within  a  relatively  short  period  of 
time;  it  may  have  required  ages  in  the  accomplishment.  Further- 
more, it  must  not  be  forgotten  that  remnants  of  the  floor  may  yet 
be  discovered  in  little -known  regions.  If  this  complete  and  univer- 
sal assimilation  actually  took  place,  it  is  an  absolutely  unique  phe- 
nomenon in  the  recorded  history  of  the  earth,  though  something 
more  or  less  similar  may  have  happened  many  times  before  that 
record  began. 

Many  other  hypotheses  have  been  propounded  to  account  for  the 
origin  of  the  Archaean  rocks,  but  as  they  are  not  supported  by  any 
strong  evidence,  it  is  not  worth  while  to  consider  them  here;  several 
of  them  have  been  formally  abandoned  by  their  authors. 

That  the  oldest  known  rocks  were  not  the  first  to  be  formed  is 
manifest  from  the  derivative  nature  of  many  of  them,  for  sedi- 
ments necessarily  imply  some  preexisting  rock  to  furnish  the 
materials,  and  volcanic  outbursts  involve  a  solid  surface  through 
which  they  break. 

From  the  extreme  degree  of  dynamic  and  thermal  metamor- 
phism  which  the  Archaean  rocks  have  undergone,  we  should  not 
expect  to  find  any  recognizable  fossils  in  them.  On  the  other 
hand,  there  are  indirect  evidences  that  life  was  already  present  on 
the  earth  at  that  period.  The  limestones,  iron  ores,  and  graphite 
found  in  these  rocks  appear  to  have  been  organically  accumulated, 
but  it  is  possible  that  they  were  chemically  formed,  and  so  the 
evidence,  while  probable,  is  not  altogether  conclusive. 

II.    ALGONKIAN 

This  is  the  name  proposed  by  the  United  States  Geological 
Survey  for  the  great  series  of  sedimentary  and  metamorphic 
rocks  which  lie  between  the  basal  Archaean  complex  and  the  oldest 
Palaeozoic  strata;  it  is  but  little  used  outside  of  this  country  and 
is  not  universally  employed  even  here,  but  it  is  beginning  to  make 
its  way  in  Europe,  and  serves  a  useful,  though  possibly  a  temporary, 
purpose.  While  it  is  possible,  though  not  very  likely,  that  more 


ALGONKIAN  541 

advanced  knowledge  may  lead  us  to  distribute  these  rocks  partly 
into  the  Archaean  and  partly  into  the  Palaeozoic,  yet  for  the  present, 
at  least,  it  is  better  to  form  a  separate  grand  division  for  them. 

The  Algonkian  rocks,  which  are  widely  distributed  in  North  Amer- 
ica, form  an  immensely  thick  mass  of  strata  and  of  metamorphic 
rocks  which  are  believed  to  represent  those  strata  in  other  regions. 
These  metamorphic  rocks  were  long  generally  referred  to  as  the 
Huronian,  which  was  regarded  as  the  upper  portion  of  the  Archaean, 
but,  so  far  as  can  be  learned,  they  occupy  the  same  stratigraphical 
position  as  certain  little  changed  sediments,  between  the  funda- 
mental complex  below  and  the  Cambrian  above.  At  the  base  of 
the  magnificent  section  exposed  in  the  Grand  Canon  of  the  Colorado 
is  a  very  thick  mass  of  strata,  separated  by  great  unconformities 
from  the  Archaean  gneiss  below  and  from  the  overlying  Cambrian. 
This  mass  is  again  subdivided  by  minor  unconformities  into  three 
series.  The  lower  series,  at  least  1000  feet  thick,  and  perhaps  more, 
is  made  up  of  stratified  quartzites  and  semi-crystalline  schists,  cut 
by  intrusive  granite.  Above  this  come  nearly  7000  feet  of  sand- 
stones, with  included  lava  sheets,  and  at  the  top  more  than  5000 
feet  of  shales  and  limestones,  in  which  a  few  fossils  have  been  found. 
The  two  upper  series  are  not  at  all  metamorphic.  All  these  strata 
are  steeply  inclined,  and  upon  their  truncated  edges  rests  the 
sandstone  referred  by  Mr.  Walcott  to  the  Middle  Cambrian. 

In  central  Montana  is  a  very  extensive  exposure  of  Algonkian 
rocks,  12,000  feet  thick,  composed  of  immense  bodies  of  sandstones, 
quartzites,  limestones,  and  hard  arenaceous  shales.  These  beds, 
called  the  Belt  series,  are  upturned  in  the  Belt  Mountains,  and  un- 
conformably  overlaid  by  a  Middle  Cambrian  sandstone.  "  In 
late  Algonkian  times  an  orographic  movement  raised  the  in- 
durated sediments  of  the  Belt  terrane  above  sea-level,  .  .  .  [and] 
folding  of  the  Belt  rocks  formed  ridges  of  considerable  elevation, 
and  areal  [aerial]  erosion  and  the  Cambrian  sea  cut  away  in 
places  from  3000  to  4000  feet  of  the  upper  formations  of  the  Belt 
terrane  before  the  sands  that  now  form  the  Middle  Cambrian 
sandstones  were  deposited."  (Walcott.) 


542  ORIGINAL  CONDITION  OF  THE  EARTH 

A  very  similar  succession  of  rocks  to  that  of  the  Grand  Canon 
is  found  in  the  Lake  Superior  region,  intervening  between  the 
Archaean  complex  and  the  Upper  Cambrian,  from  both  of  which 
they  are  separated  by  great  unconformities.  As  in  the  Grand 
Canon  section,  these  rocks  are  divisible  into  four  series  by  un- 
conformities. The  lower  two  series,  with  a  maximum  thickness 
probably  exceeding  5000  feet,  are  much  crumpled,  metamorphosed, 
and  semi-crystalline.  They  comprise  limestones,  quartzites,  mica 
schists,  etc.,  cut  by  igneous  dykes,  also  much  volcanic  tuff  and 
agglomerate.  Next  follows  a  series  of  12,000  feet  of  less  intensely 
folded  but  still  metamorphic  rocks,  quartzites,  shales,  slates,  mica 
schists,  with  dykes  and  interbedded  sheets  of  diorite.  A  few 
fossils  have  been  found  in  the  quartzites  of  this  series.  The 
fourth  series  has  a  maximum  thickness  of  50,000  feet,  though 
usually  much  less.  The  lower  part  of  this  series  is  formed  by  thick 
lava  sheets,  interbedded  with  sandstone  and  conglomerate,  and 
above  is  a  mass  of  sedimentary  rocks  largely  derived  from  the 
volcanic  materials.  This  uppermost  series  is  by  some  authorities 
referred  to  the  Cambrian,  but,  in  the  absence  of  fossils,  there  seems 
to  be  no  way  of  definitely  deciding  the  question. 

Over  the  great  Archaean  area  of  Canada  occur  many  districts 
of  metamorphic  rocks  which  are  plainly  of  sedimentary  origin, 
such  as  crystalline  limestones,  schistose  conglomerates,  as  well 
as  volcanic  tuffs  and  agglomerates.  In  the  Archaean  region  of 
Canada  and  in  New  England  the  Algonkian  metamorphics  seem 
to  grade  into  the  Archaean  complex  without  unconformity.  This 
apparent  conformity  may,  however,  very  well  be  due  to  subse- 
quent dynamic  metamorphism,  which,  as  has  been  proved,  may 
obliterate  nearly  all  traces  of  a  great  unconformity.  Through  the 
Rocky  Mountain  region  and  the  Pacific  coast  mountains,  the 
Archaean  is  in  very  many  places  overlaid  by  great  thicknesses  of 
metamorphic  Algonkian  rocks,  such  as  quartzites,  sandstones,  and 
schists,  which  are  sometimes  as  much  as  12,000  feet  thick,  as  in  the 
Wasatch  and  Uinta  mountains.  Other  isolated  areas  are  found, 
as  in  the  Black  Hills,  where  a  great  mass  of  schists,  slates,  and 


LIFE  IN  THE  ALGONKIAN  543 

quartzites  is  separated  by  a  very  marked  unconformity  from  the 
overlying  Cambrian ;  also  in  Missouri  and  Texas.  The  Algonkian 
rocks  of  the  West  have  not  been  subjected  to  such  extreme  folding 
as  have  most  of  those  of  the  East,  and  hence  their  distinctness  from 
the  Archaean  is  more  clearly  marked.  In  the  southern  Appa- 
lachians are  some  little  changed  strata  which  are  referred  to  the 
Algonkian. 

In  other  continents  the  distinction  between  the  Archaean  and  the 
Algonkian  is  beginning  to  demand  recognition.  In  Great  Britain, 
for  instance,  are  found  very  interesting  parallels  with  the  Algonkian 
of  this  country.  In  Scotland  the  Torridon  sandstones,  8000  to 
10,000  feet  thick,  which  are  nearly  horizontal  and  almost  un- 
changed, lie  unconformably  between  the  oldest  Cambrian  and  the 
basal  Archaean;  and  in  other  areas,  metamorphic  rocks  of  sedi- 
mentary origin  occupy  a  similar  position.  In  Finland  and  Sweden 
10,000  feet  of  sedimentary  and  igneous  rocks  and  schists  occur 
between  the  Archaean  and  the  Cambrian.  Many  of  the  crystalline 
schists  of  the  European  pre-Cambrian  areas  appear  to  correspond 
in  character  and  position  to  the  metamorphic  Algonkian. 

Lately  the  surprising  announcement  has  been  made  of  exten- 
sive glaciation  in  the  early  Algonkian  of  North  America  The 
conglomerate  at  the  base  of  the  Lower  Huronian  of  Canada  is 
regarded  as  of  glacial  origin,  "  since  it  contains  angular  and  sub- 
angular  boulders  of  all  sizes  up  to  cubic  yards,  enclosed  in  -an 
unstratified  matrix.  These  boulders  are  often  miles  from  any 
possible  source.  Recently,  striated  stones  have  been  broken  out 
of  their  matrix  in  the  Lower  Huronian  of  the  Cobalt  Silver  region, 
giving  still  stronger  proofs  that  the  formation  is  ancient  boulder 
clay."  (Coleman.) 

In  South  Africa  we  find  clear  evidence  of  three  distinct  ice  ages, 
before  the  close  of  the  Palaeozoic  Era,  and  the  most  ancient  of  these, 
which  cannot  be  definitely  dated,  may  correspond  in  time  to  that 
above-mentioned  in  Canada. 

Life  in  the  Algonkian.  —  In  the  Grand  Canon  and  Montana 
determinable  fossils  have  been  found  in  the  less  changed  sedi- 


544  ORIGINAL  CONDITION  OF  THE  EARTH 

merits,  but  they  are  too  few  and  scanty  to  tell  us  much  of  the  life 
of  the  times.  Evidences  of  life  are  not  wanting  in  the  metamor- 
phic  rocks  of  the  eastern  and  northern  regions,  but  they  are  in- 
direct. The  strata  of  crystallized  limestone  are  indications  of  the 
presence  of  animal  life  in  the  Algonkian  seas.  The  great  quantities 
of  graphite  diffused  through  many  of  the  schists,  and  the  beds  of 
iron  ore  likewise  tend  to  show  the  existence  of  plants  at  the  same 
time.  More  conclusive  are  the  determinable  fossils  obtained  in 
the  Belt  series  of  Montana  and  in  the  Grand  Canon  series,  which 
include  the  tracks  of  worms,  brachiopods,  and  fragments  of  large 
Crustacea  referable  to  the  Eurypterida.  Such  remains  imply  a 
long  antecedent  history  of  life,  the  records  of  which  remain  to  be 
discovered. 

The  pre-Cambrian  rocks  are  remarkable  for  their  wealth  of 
valuable  minerals.  Immense  accumulations  of  iron  ore  occur  in 
Canada,  New  York,  New  Jersey,  along  the  Appalachians  from 
Virginia  to  Georgia,  in  Michigan,  the  Lake  Superior  region,  Mis- 
souri, and  the  Southwest.  The  great  copper  mines  of  Lake 
Superior  are  associated  with  igneous  rocks  which  intersect  sand- 
stones referred  to  the  Algonkian. 

It  will  be  obvious  to  the  student  how  very  little  is  really  known 
regarding  the  most  ancient  rocks  of  the  earth's  crust,  the  Archaean. 
They  are  enormously  thick  metamorphic  masses  of  vast  geograph- 
ical extent.  In  all  the  continents  they  form  the  foundation  upon 
which  the  oldest  fossiliferous  sediments  were  laid  down,  and,  in 
brief,  they  are  the  oldest,  the  thickest,  the  most  widely  distributed 
and  the  most  important  of  all  the  accessible  constituents  of  the 
earth's  crust.  Their  uniform  character,  wherever  found,  the 
extreme  plication  and  metamorphism  which  they  have  undergone, 
and  their  world-wide  distribution,  are  all  extremely  remarkable 
features,  such  as  recur  in  rocks  of  no  other  age.  The  Algonkian 
sedimentary  rocks  present  the  earliest  chapters  in  the  recorded 
history  of  life.  The  pre-Cambrian  rocks  indicate  that  vast  periods 
of  time  had  elapsed  before  the  clearly  recorded  part  of  the  earth's 
history  began,  a  time  probably  longer  than  all  subsequent  periods. 


CHAPTER   XXVI 
PALAEOZOIC  ERA  — CAMBRIAN  PERIOD 

THE  Palaeozoic  is  the  oldest  of  the  three  main  groups  into  which 
the  normal  fossiliferous  strata  are  divided;  it  forms  the  first 
legible  volume  of  the  earth's  history,  and  in  interpreting  it  specu- 
lation and  hypothesis  play  a  much  less  prominent  part  than  in  the 
pre-Cambrian  volume.  The  Palaeozoic  rocks  are  conglomerates, 
sandstones,  shales,  and  limestones,  with  quite  extensive  areas  of 
metamorphic  rocks,  and  associated  igneous  masses,  both  volcanic 
and  plutonic.  The  thickness  of  these  rocks  is  very  great,  estimated 
in  Europe  at  a  maximum  of  100,000  feet.  This  does  not  imply 
that  such  a  thickness  is  found  in  any  one  place,  but  that  if  the  maxi- 
mum thicknesses  of  each  of  the  subordinate  divisions  be  added 
together,  they  will  amount  to  that  sum.  In  this  country  more 
than  25,000  feet  of  Palaeozoic  strata  are  exposed  in  the  much 
folded  and  profoundly  denuded  Appalachian  Mountains,  but  in 
the  Mississippi  valley  they  attain  only  a  fraction  of  that  thickness. 
These  rocks  are,  in  the  vast  majority  of  cases,  of  marine  origin, 
but  some  fresh-water  beds  are  known,  and  very  extensive  swamp 
and  river  deposits  have  preserved  a  record  of  much  of  the  land 
life  of  the  era,  especially  of  its  later  portions.  That  there  must 
have  been  land-surfaces  is  abundantly  shown  by  the  immense 
thickness  and  extent  of  the  strata,  all  of  which  were  derived  from 
the  waste  of  the  land.  Both  in  Europe  and  in  North  America,  the 
land  areas  were  prevailingly  toward  the  north,  and  are  doubtless 
indicated,  in  part,  by  the  great  regions  of  the  pre-Cambrian  meta- 
morphic rocks.  The  general  character  of  the  Palaeozoic  beds 
shows  that  they  were,  in  large  measure,  laid  down  in  shallow  water 
in  the  neighbourhood  of  land.  Their  great  thickness  indicates, 
2N  545 


546  PALEOZOIC   ERA 

further,  the  enormous  denudation  which  the  land  areas  under- 
went. The  calculation  has  not  been  made  for  this  country,  but 
for  Great  Britain  Geikie  states  that  the  lower  half  of  the  Palaeo- 
zoic group  represents  the  waste  of  a  plateau  larger  than  Spain  and 
5000  feet  high,  cut  down  to  base-level. 

Very  widespread  disturbances  of  the  earth's  crust  before  the 
beginning  of  the  Palaeozoic  era  and  at  its  close  have  produced  well- 
nigh  universal  unconformities  with  both  the  underlying  pre- 
Cambrian  and  the  overlying Mesozoic  rocks;  at  only  a  few  points 
are  transitional  series  found. 

Early  in  Palaeozoic  time  were  established  the  main  geographical 
outlines  which  dominated  the  growth  of  the  North  American 
continent,  —  a  growth  which  was,  for  the  most  part,  steady  and 
tranquil.  These  conditions  may  be  briefly  stated  as  the  forma- 
tion of  a  great  interior  continental  sea,  divided  from  the  Atlantic 
and  the  Pacific  by  more  or  less  extensive  and  variable  land  areas. 
There  are  thus  three  principal  regions  of  continental  develop- 
ment: those  of  the  Atlantic  and  Pacific  borders  and  the  interior. 
In  addition,  the  eastern  border  is  subdivided  by  pre-Cambrian 
ridges  into  subordinate  areas  of  deposition.  At  the  present  time 
the  surface  rocks  over  the  eastern  half  of  the  continent  are  pre- 
vailingly Palaeozoic,  extending  chiefly  southward  and  south- 
eastward from  the  great  pre-Cambrian  mass  of  the  north. 

Palaeozoic  time  was  of  vast  length,  perhaps  exceeding  that  of 
the  combined  Mesozoic  and  Cenozoic  eras. 

The  subdivisions  of  the  Palaeozoic  are  very  clearly  marked, 
locally  often  by  unconformities,  but  on  a  wide  scale  by  the  changes 
in  the  character  of  the  fossils.  There  is  some  difference  in  the 
practice  concerning  these  divisions,  not  as  to  their  limits  or  order 
of  succession,  but  merely  as  to  their  rank,  whether  certain  ones 
should  be  called  systems  (periods)  or  series  (epochs).  This  is 
a  difference  more  about  names  than  facts.  The  successive  steps 
of  organic  and  geographical  development  are  best  displayed  by 
dividing  the  group  into  six  systems,  or  periods,  which  are  as  fol- 
lows, beginning  with  the  oldest:  i.  Cambrian;  2.  Ordovician; 


PALAEOZOIC  LIFE  547 

3.  Silurian;  4.  Devonian;  5.  Carboniferous;  6.  Permian.  By 
many  geologists  the  Ordovician  and  Silurian  are  comprised  in 
one  system,  and  the  Carboniferous  and  Permian  in  another; 
but  the  present  tendency  is  in  favour  of  maintaining  all  six  as 
equal  in  rank.  It  must  not  be  supposed  that  these  systems 
represent  equal  spaces  of  time  as  measured  by  the  thickness  of 
rocks,  or  equal  geographical  extent;  on  the  contrary,  they  are 
very  unequal  in  both  these  respects.  The  classification  means 
that  the  six  systems,  or  periods,  stand  for  approximately  equiva- 
lent changes  in  the  character  of  the  animals  and  plants. 

Palaeozoic  Life  possesses  an  individuality  not  less  distinctly 
marked  than  that  of  the  group  of  strata,  which  demarcates  it 
very  sharply  from  the  life  of  succeeding  periods,  and  gives  a  cer- 
tain unity  of  character  to  the  successive  assemblages  of  plants 
(floras}  and  of  animals  (faunas).  The  era  is  remarkable  both 
for  what  it  possesses  and  what  it  lacks.  Among  plants,  the  vege- 
tation is  made  up  principally  of  Cryptogams,  seaweeds,  ferns, 
club-mosses,  and  horsetails.  Especially  characteristic  are  the 
gigantic,  tree-like  club-mosses  'and  horsetails,  which  are  now 
represented  only  by  very  small,  herbaceous  forms.  The  only 
flowering  plants  known  are  the  Gymnosperms,  the  Cycads,  and 
their  allies;  no  Angiosperms  have  been  discovered.  Palaeozoic 
forests  must  have  been  singularly  gloomy  and  monotonous,  lack- 
ing entirely  the  bright  flowers  and  changing  foliage  of  later  periods. 

The  Palaeozoic  fauna  is  largely  made  up  of  marine  inverte- 
brates, in  the  earlier  periods  entirely  so;  i.e.  so  far  as  we  have  yet 
learned,  though  land  life  surely  began  before  the  oldest  records 
of  it  yet  discovered.  Graptolites  and  Hydroid  Corals,  true  Corals, 
Echinoderms  (especially  Crinoids,  Cystideans,  and  Blastoids), 
long-hinged  and  hingeless  Brachiopods,  Mollusca  (particularly 
the  Nautiloid  Cephalopods),  and  the  crustacean  groups  of  Trilo- 
bites  and  Eurypterida  are  the  most  abundant  and  character- 
istic types  of  animal  life.  Insects,  centipedes,  and  spiders  were 
common  toward  the  end  of  the  era.  Cambrian  rocks  con- 
tain no  fossil  vertebrates,  but  they  make  their  appearance  in 


548  PALAEOZOIC   ERA 

the  Ordovician.  For  long  ages  the  only  vertebrates  were 
fishes  and  certain  low  types  allied  to  the  fishes,  but  at  the  end 
of  the  Devonian  and  in  the  Carboniferous  appeared  the  Am- 
phibia, followed  in  the  Permian  by  true  Reptiles.  Teleosts, 
such  as  make  up  by  far  the  largest  part  of  the  modern  fish-fauna, 
both  marine  and  fresh-water,  as  well  as  birds  and  mammals,  are 
entirely  absent  from  the  Palaeozoic. 

The  overwhelming  majority  of  Palaeozoic  species,  and  even 
genera,  fail  to  pass  over  into  the  Mesozoic,  and  even  in  the  larger 
groups  which  continued  to  flourish  almost  always  a  more  or  less 
complete  change  of  structure  occurs,  so  that  Palaeozoic  corals, 
Echinoderms,  and  fishes,  for  example,  are  very  markedly  distinct 
from  those  which  succeeded  them.  The  difference  is  generally 
in  the  direction  of  greater  primitiveness  of  structure  in  the  older 
forms,  Palaeozoic  types  standing  in  somewhat  the  same  relation 
to  subsequent  types  as  the  embryo  does  to  the  adult. 

In  the  vast  periods  of  time  included  in  the  Palaeozoic  era 
occurred  some  remarkable  climatic  vicissitudes,  which  will  be 
more  fully  described  in  the  succeeding  sections.  Times  of  wide- 
spread glaciation  occurred  in  the  Lower  Cambrian  of  Norway 
and  China,  probably  of  Australia,  and  perhaps  also  of  South 
Africa;  in  the  Devonian  of  South  Africa,  and  in  the  Permian  of 
the  latter  region,  India,  Australia,  and  South  America,  perhaps 
also  in  Europe  and  North  America. 

For  most  of  the  era,  however,  the  climate  appears  to  have 
been  mild  and  equable  on  the  whole,  very  much  the  same  kinds 
of  animals  and  plants  occurring  in  high  as  in  low  latitudes.  In 
short,  we  can  detect  no  evidence  of  climatic  zones  as  being  dis- 
tinctly marked  in  those  periods. 

THE  CAMBRIAN  PERIOD 

The  rocks  older  than  the  coal  measures  were  for  a  long  time 
heaped  indiscriminately  together,  under  the  name  of  Greywacke, 
or  Transition  Rocks,  and  were  little  regarded  by  geologists. 


THE  CAMBRIAN  PERIOD  549 

About  1831,  the  problem  of  these  ancients  rocks  was  attacked 
by  two  eminent  English  geologists,  Sedgwick  and  Murchison, 
who  soon  brought  order  out  of  the  chaos.  There  was  much  dis- 
cussion and  dispute  as  to  the  limits  of  the  systems  into  which 
the  Greywacke  should  be  divided,  and  as  to  the  names  which 
should  be  given  to  them.  The  oldest  fossiliferous  strata  were 
by  Sedgwick  called  Cambrian  (from  the  Latin  name  for  Wales), 
but  were  included  by  Murchison  in  his  Lower  Silurian.  The 
latter  example  was  long  followed  by  most  geologists,  but  the 
advance  of  knowledge  has  fully  vindicated  the  claim  of  the  Cam- 
brian to  rank  as  a  distinct  system.  The  divisions  of  the  American 
Cambrian  are  as  follows:  — 

3.  Upper  Cambrian,  Saratogan  Epoch,  Dikellocephalus 
Fauna. 

2.  Middle  Cambrian,  Acadian  Epoch,  Paradoxides  Fauna. 

i.  Lower  Cambrian,  Georgian  Epoch,  Olenellus  Fauna. 

American.  —  In  North  America,  Cambrian  rocks  are  not  ex- 
posed at  the  surface  over  large  areas,  being,  for  the  most  part, 
deeply  buried  under  later  sediments;  their  maximum  thickness, 
so  far  as  known,  does  not  exceed  12,000  feet.  While  not  forming 
extensive  areas  of  the  present  surface,  Cambrian  strata  are  very 
widely  distributed  over  the  continent,  usually  resting  uncom- 
formably  upon  the  plicated  and  metamorphosed  rocks  of  the 
Archaean  and  Algonkian.  These  strata  are  found  in  the  pre- 
Cambrian  depressions,  from  the  Adirondacks  to  Newfoundland, 
and  along  the  flank  of  the  Appalachian  uplift,  from  the  St.  Law- 
rence to  Alabama.  They  also  fringe  Archaean  or  Algonkian 
areas  in  other  regions,  as  in  Wisconsin,  Missouri,  Texas,  in  the 
Rocky  Mountain  chain,  from  Colorado  to  British  Columbia,  and 
in  the  mountains  of  Nevada.  Cambrian  beds  are  exposed  in  the 
Colorado  Canon,  and  doubtless  would  be  found  throughout  the 
larger  part  of  the  continent  were  the  overlying  beds  stripped 
away. 

So  far  as  they  are  accessible  to  observation,  the  Cambrian  rocks 
are  chiefly  such  as  are  laid  down  in  shallow  water  near  shore, 


550 


PALEOZOIC   ERA 


conglomerates,  sandstones,  shales,  which  are  ripple-marked  in 
a  way  that  betrays  their  shoal-water  origin.  There  are  also  some 
areas  of  deeper  water  accumulations,  found  in  the  thick  lime- 
stones of  western  Vermont,  the  Appalachian  Mountains,  Nevada, 
and  British  Columbia.  Very  little  igneous  rock  is  found  in  the 
Cambrian  of  North  America.  Small  intrusions  occur  in  New- 
foundland and  New  England,  and  quite  considerable  ones  in 
British  Columbia,  but  some  of  these  may  be  long  post-Cambrian 
in  date. 


FlG.  259.  —  Map  of  known  Cambrian  outcrops  in  the  United  States  and  Canada 

As  is  indicated  by  the  geographical  distribution  of  the  fossils, 
North  America  was,  in  the  Cambrian  period,  divided  into  two 
provinces  of  very  unequal  size  and  faunally  very  different.  The 
Atlantic  province,  comprising  Newfoundland,  New  Brunswick, 
Nova  Scotia,  and  New  England,  shows  so  close  a  connection 
with  Europe  as  to  justify  the  inference  that  in  high  latitudes  a 
land  bridge  spanned  the  Atlantic,  or  at  least  that  a  chain  of 
islands  and  shoals  permitted  the  migration  of  shore-loving  marine 
animals  from  one  continent  to  the  other.  All  the  rest  of  North 


THE  CAMBRIAN   PERIOD  551 

America  belongs  to  the  Pacific  province,  though  there  are  many 
local  faunas  within  that  vast  area. 

During  Cambrian  times  the  sea  was  slowly  advancing  over  the 
land  in  North  America,  and  the  geography  of  the  continent  was 
very  different  at  the  close  of  the  period  from  what  it  had  been  at 
the  beginning.  In  the  Lower  Cambrian  the  land  areas  are  in- 
ferred to  have  been  somewhat  as  follows:  First,  there  was  the 
great  northern  mass  of  crystalline  Archaean  and  Algonkian  rocks, 
but  this  was  probably  much  more  extensive  than  the  present 
exposures  of  pre-Cambrian  rocks  would  indicate.  It  probably 
covered  the  whole  Mississippi  valley  down  to  30°  N.  lat.  and 
extended  westward  beyond  the  Rocky  Mountains.  Long,  narrow 
strips  of  land,  alternating  with  narrow  sounds,  occupied  part 
of  New  England  and  the  maritime  provinces  of  Canada,  while  an 
Appalachian  land,  whose  western  line  is  marked  by  the  present 
Blue  Ridge,  extended  eastward  an  unknown  distance  into  the 
Atlantic.  On  the  western  shore  of  the  Appalachian  land  was  a 
narrow  arm  of  the  sea,  which  opened  south  and  nearly  separated 
this  land  area  from  the  great  mass  of  the  continent.  During  the 
Lower  and  Middle  Cambrian,  this  long  and  narrow  bay  or  sound 
must  have  been  closed,  or  only  occasionally  and  partially  opened, 
at  the  northern  end.  In  later  Cambrian  times  it  was  perhaps 
open.  The  site  of  the  Sierra  Nevada  was  occupied  by  a  long, 
narrow  land,  running  from  Puget  Sound  to  Mexico,  and  another 
such  area  was  found  in  eastern  British  Columbia.  The  Great 
Basin  region  was  under  water.  Around  these  shores  were  laid 
down  the  coarser  deposits  of  the  Lower  Cambrian,  with  great 
masses  of  shales  and  thick  limestones  in  deeper  water. 

Middle  Cambrian  sediments  have  quite  a  similar  distribution 
to  those  of  the  Lower,  but  the  sea  was  slowly  advancing  over  the 
continent  from  the  south.  Nothing  is  known  of  the  condition 
of  Central  America  and  Mexico  at  this  time,  but  from  Arizona 
eastward  to  Alabama  the  land  was  submerged.  This  trans- 
gression of  the  sea  continued,  and  reached  its  maximum  in  the 
Upper  Cambrian.  Toward  the  close  of  the  period  a  large  part 


552  PALEOZOIC   ERA 

of  the  continent  had  been  submerged  and,  in  particular,  a  vast 
interior  sea  had  been  established  over  the  Mississippi  Valley. 

The  Cambrian  of  Other  Continents.  —  In  Europe  the  Cambrian 
is  very  extensively  developed,  with  remarkable  differences  of 
thickness  in  different  regions.  The  maximum  thickness  occurs 
on  the  western  side  of  the  continent  in  Wales  and  Spain.  In 
Wales  are  20,000  feet  of  conglomerates,  sandstones,  shales,  slates, 
and  volcanic  rocks,  while  in  southern  Sweden  and  northwestern 
Russia  the  entire  period  is  represented  by  only  400  feet  of  beds. 
Much  of  the  Welsh  Cambrian  is  regarded  by  Professor  Penck  as 
being  of  continental  origin,  with  occasional  incursions  of  the  sea. 
The  Lower  Cambrian  appears  to  be  limited  to  the  north  of  Europe, 
while  the  Middle  Cambrian  witnessed  the  widest  transgression  of 
the  sea,  beds  of  this  date  occurring  in  France,  Germany,  Bohemia, 
Spain,  and  Sardinia.  The  Middle  Cambrian  is  characterized  by 
the  Trilobite  genus  Paradoxides  (Plate  III,  Fig.  6)  which  is  very 
common  in  Europe  and  in  the  Atlantic  province  of  North  America, 
but  is  not  found  in  other  parts  of  the  latter  continent.  The  Upper 
division,  like  the  Lower,  is  restricted  to  northern  Europe,  so  that 
there  was  extensive  submergence  in  the  Middle  Cambrian,  but 
a  withdrawal  of  the  sea  before  the  beginning  of  the  later  portion 
of  the  period.  This  is  in  decided  contrast  to  the  geographical 
changes  of  North  America,  where  the  most  widespread  extension 
of  the  sea  took  place  in  the  Upper  Cambrian.  In  Russia  the 
Cambrian  sediments  are  remarkable  for  their  unconsolidated 
condition;  some  of  them  look  as  though  just  abandoned  by  the 
sea. 

In  Norway,  70°  N.  lat.,  have  been  found  glacial  deposits 
which  are  either  basal  Cambrian  or  late  Algonkian.  "  The  con- 
glomerate in  some  places  is  seen  to  be  formed  of  old  moraines.  .  .  . 
The  stones  have  not  the  habitus  of  water-worn  rolling  stones,  but  of 
ice-worn  stones.  On  some  of  the  dolomite  ones  clear  glacial  striae 
were  observed.  ...  At  one  place  plain  glacial  striae  have  been 
found  upon  the  surface  of  the  hard  sandstone  under  a  mass  of 
conglomerate."  (Reusch.) 


CAMBRIAN  LIFE  553 

Cambrian  rocks  cover  great  areas  in  eastern  Asia,  northern 
Siberia,  Korea,  and  China.  In  China  these  strata,  which  are 
but  little  disturbed,  attain  the  great  thickness  of  20,000  feet,  and 
consist  predominantly  of  sandstones  and  limestones.  In  the 
lower  part  are  170  feet  of  boulder  clay  of  evidently  glacial  origin. 
"On  the  Yangtse  River,  inv3i0  lat.,  i.e.  as  far  south  as  New 
Orleans,  not  high  above  sea-level,  a  large  body  of  glacial 
material  was  discovered.  ...  It  demonstrates  the  existence  of 
glacial  conditions  in  a  very  low  latitude  in  the  early  Palaeozoic. " 
(Willis.) 

Cambrian  also  occurs  in  northern  India,  but  none  has  yet  been 
identified  in  Africa.  It  is  found  in  the  south  of  Australia  and 
in  Tasmania,  apparently  belonging  to  all  three  of  the  divisions. 
Evidences  of  glacial  action  have  been  observed  in  the  Australian 
Cambrian,  showing  that  this  climatic  change  was  not  local  but 
very  widespread,  especially  as  the  earliest  of  the  South  African 
ice  ages,  mentioned  under  the  Algonkian,  may  have  been  early 
Cambrian  in  date. 

In  South  America,  Cambrian  has  as  yet  been  found  only  in 
the  northern  part  of  Argentina;  it  is  apparently  referable  to  the 
Middle  division. 

CAMBRIAN  LIFE 

The  Cambrian  fauna  is  of  extraordinary  interest,  because  it  is 
the  most  ancient  that  we  know  with  any  fulness,  though,  of  course, 
it  does  not  represent  the  beginnings  of  life.  Almost  all  the  great 
types  of  invertebrates  are  already  present  and  very  definitely 
characterized,  indicating  that  life  had  been  differentiating  for 
a  vast  period  before  the  lowest  Cambrian  rocks  had  been  laid 
down.  As  compared  with  the  faunas  of  other  Palaeozoic  periods, 
that  of  the  Cambrian  is  very  scanty,  but  our  knowledge  of  it  has 
been  greatly  increased  of  late  and  may  be  expected  to  increase 
in  the  future. 

Though  the  successive  Cambrian  faunas  have  a  very  uniform 
distribution  over  wide  areas,  there  are  already  indications  of 


554 


PALEOZOIC   ERA 


local  differences  which  mark  out  faunal  provinces;  thus,  the 
Lower  and  Middle  Cambrian  fossils  of  Newfoundland  are  more 
similar  to  those  of  Europe  than  to  those  of  the  Appalachian  and 


PLATE  I.  — CAMBRIAN  FOSSILS 


Fig.  i,  Orbulina  universa  Lam.,  X  12,  L.  C.  2,  Globigetina  cambrica  Matthew,  X  5, 
,.  C.  3,  Leptontitus  zitteli  Walcott,  x  %,  L.  C.  4,  Spicule  of  Protospongia,  X  %,  L.  C. 
,  Climacograptus  emmonsi  Wale.,  X  %,  L.  C.  6,  Dictyone ma  flabelliforme  Eichwald, 


X  1/2,  U.  C.  7,  Archeeocyathellus  rensselaricus  Ford,  x  %,  L.  C.  8,  Archceocyathns 
prqfundus  Billings,  X  %,  L.  C.  9,  Eocystites  longidactylus  Wale.,  M.  C.  10,  Lingulepis 
pinnifortnis  Owen,x  i,  U.  C.  n,  Linarssouia  taconica  Walc.,x  4,  L. C.  12,  Kutorgina 
cingulata  Billings,  X  %,  L.  C.  13,  Protorthis  billingsi  Hartt,  X  i,  M.  C.  14,  Camer- 
ella  antiquata  Bill.,  X  i,  L.  C.  15,  Fordilla  troyensis  Barrande,  X  B/2,  L  C.  16,  Rhaphi- 
stoma  attleborensis  S.  and  F.  X  5/2-  *7»  Stenotheca  rugosa  Hall,  X  2,  L.  C.  18,  Hyoli- 
thesprinceps  Bill.,  X  %,  L.  C. 

interior  regions  of  America.  A  fauna  of  similar  date  but  different 
facies  occurs  in  Alabama,  and  farther  north  in  the  Appalachians. 


CAMBRIAN   LIFE  555 

Of  Plants  nothing  is  surely  known;  certain  marks  on  the  bed- 
ding-planes of  strata  have  been  regarded  as  seaweeds,  but  they  are 
too  obscure  for  determination,  and  many  are  worm  tracks. 

The  fauna  is  principally  made  up  of  Brachiopods  and  Trilo- 
bites,  but  many  other  types  are  represented  also. 

Foraminifera  very  like  those  of  the  modern  seas  (Plate  I, 
Figs,  i,  2)  are  found  even  in  the  Lower  Cambrian. 

Spongida.  —  Siliceous  Sponges  are  not  uncommon. 

Ccelenterata.  —  The  Hydrozoa  are  represented  by  the  Grapto- 
lites,  a  series  of  forms  which  are  confined  to  the  older  Pa- 
laeozoic rocks.  Dictyonema  (I,  6)  is  a  complex  Graptolite, 
found  abundantly  in  a  thin  band  of  shale  near  the  top  of  the 
Upper  Cambrian,  which  is  of  nearly  world-wide  distribution. 
It  shows  the  great  value  of  the  organisms  which  live  at  the  surface 
of  the  open  sea  (pelagic  fauna)  in  fixing  contemporaneous  deposits 
over  enormous  areas. 

Other  Hydrozoa  are  the  jellyfish,  of  which  recognizable  casts 
have  been  found  in  large  numbers.  Stromatopora  formed  reefs 
in  some  of  the  Cambrian  limestones. 

It  is  still  a  question  whether  Corals  were  present  in  the  Cam- 
brian; certain  fossils  (Archceocyathus,  I,  7,  8,)  which  by  some 
authorities  are  called  corals,  are  by  others  regarded  as  sponges. 
Though  sufficiently  abundant  in  some  parts  of  the  West  to  form 
reefs,  the  genus  has  only  a  few  species,  and,  except  locally,  they 
are  not  conspicuous  elements  in  the  fauna. 

Echinodermata. — The  Echinoderms  are  rare,  and  belong  to 
the  Cystoids,  a  very  primitive  grade  of  the  type. 

Worms.  — The  presence  of  marine  worms  is  abundantly  in- 
dicated by  tubes,  tracks,  and  borings  in  the  sands  which  have 
now  consolidated  into  hard  rocks.  Hyolithes,  a  worm-tube 
(I,  18),  is  very  common. 

Arthropoda.  — The  only  known  Cambrian  Arthropods  are  the 
Crustacea,  and  of  these  much  the  most  abundant  group  is  that 
of  the  Trilobita,  which  are  altogether  confined  to  the  Palaeozoic 
rocks,  and  are  by  far  the  most  important  of  Cambrian  fossils.  It 


556 


PALEOZOIC  ERA 


is  only  within  recent  years  that  the  systematic  position  of  the 
Trilobites  has  been  established  through  the  fortunate  discovery 
of  specimens  with  their  appendages  attached  (see  PI.  VII,  Figs,  i, 
i  a).  Trilobites  have  a  more  or  less  distinctly  three-lobed  body,  at 
one  end  of  which  is  the  head-shield,  usually  with  a  pair  of  fixed 
compound  eyes;  at  the  other  end  is  the  tail-shield,  and  between  the 
two  shields  is  a  ringed  or  jointed  body  made  up  of  a  variable 
number  of  movable  segments.  The  Trilobites  display  an  ex- 


PLATE  II.  —  CAMBRIAN  TRILOBITES 


Fig.  x,  Holmia  broggeri  Wale.,  X 
(Walcott) 


L.  C.     2,  Olenellus  thompsoni  Hall,  X  V2  L.  C. 


traordinary  variety  in  form  and  size,  in  the  proportions  of  the 
head-  and  tail-shields,  in  the  number  of  free  segments,  and  in  the 
development  of  spines.  Already  in  the  Cambrian  this  wealth  of 
forms  is  notable,  though  far  less  so  than  it  became  in  the  Or- 
dovician.  As  compared  with  those  of  later  times,  the  Cam- 
brian Trilobites  are  marked  by  the  (usually)  very  small  size  of 
the  tail-shield,  the  large  number  of  free  segments,  and  their  in- 


PLATE  III.  — CAMBRIAN  CRUSTACEA 

Fig.  i,  Ptychoparia  kingi  Meek,  X  %  M.  C.  2,  P.  a ntiquitata  Salter,  X  %,M.  C.  3, 
Crepicephalus  texanus  Shumard,  X  %,  M.  C.  4,  Mesonacis  vermontana  Wale.,  x  %,  L. 
C.  5,  Zacanthoides  typicalis  Wale.,  x  i,  M.  C.  6,  Paradoxides  harlani  Green,  X  %, 
M.  C.  7,  Dorypyge  curticei  Wale.,  x  V2»  M.  C.  8,  Atops  trilineatus  Emmons,  X  %,  L. 
C.  q,Agnostus  tnterstrictus  White,  x3/2,  M.  C.  10,  Microdiscus  speciosus  Ford,  X  i, 
L.  C.  ii.  Hipponicharion  eos  Matthew,  x  4,  L.  C.  12,  Aristozoe  rotundata  Wale-, 


558  PALEOZOIC   ERA 

ability  to  roll  themselves  up.  The  large  Trilobites  with  long 
eye-lobes  are  very  distinctively  Cambrian.  Some  of  them,  like 
Paradoxides  (III,  6),  are  very  large  (from  10  inches  to  2  feet  in 
length).  Olenellus  (II,  2)  and  Holmia  (II,  i)  also  have  large 
species,  while  Agnostus  (III,  q),Microdiscus  (III,  10),  and  Atops 
(III,  8)  are  small  and  without  eyes. 

The  great  importance  of  'the  Trilobites  for  Cambrian  strati- 
graphy is  indicated  by  the  fact  that  the  three  divisions  of  the 
system  are  named  for  the  three  dominant  genera  of  these  crus- 
taceans, Olenellus,  Paradoxides,  and  Dikellocephalus. 

Two  other  divisions  of  the  Crustacea  are  found  in  the  Cam- 
brian: the  Ostracoda,  little  bivalve  forms,  whose  shells  look 
deceptively  like  those  of  molluscs;  and  the  Phyllocarida,  which 
have  a  large  shield  on  the  head  and  thorax,  and  a  many-jointed 
abdomen,  with  terminal  spine. 

Brachiopoda.  — These  are  among  the  most  abundant  of  Cam- 
brian fossils;  most  of  them  belong  to  the  lower  order  of  the  class 
(Inarticulata) ,  in  which  the  shells  are  mostly  horny  and  the  two 
valves  are  not  articulated  together  by  a  hinge.  The  horny- 
shelled  types,  Linnarssonia  (I,  n),  Lingulepis  (I,  10),  and  Lin- 
gulella  are  of  great  interest,  as  they  differ  but  little  from  certain 
brachiopods  which  still  exist.  The  second  order  of  Brachiopods, 
the  Articulata,  which  have  calcareous  shells  connected  by  an 
elaborate  hinge,  were  more  common  in  the  Upper  Cambrian. 
In  subsequent  periods  they  became  vastly  more  numerous  than 
the  Inarticulata,  and  throughout  the  post-Cambrian  divisions 
of  the  Palaeozoic  their  shells  are  found  in  incalculable  numbers. 

The  Mollusca  are  already  represented  by  their  principal  divi- 
sions. The  Pelecypoda,  or  Bivalves  (I,  15),  are  of  very  small 
size  and  found  very  scantily;  their  variety  and  relative  importance 
have  gone  on  increasing  ever  since  Cambrian  times.  Gastropoda 
(I,  1 6,  17)  occur  in  small  numbers,  especially  in  the  Upper  Cam- 
brian. Fossils  formerly  referred  to  the  Pteropoda,  but  now, 
generally  regarded  as  worm-tubes,  are  among  the  most  frequent 
of  shells  found  in  these  rocks,  but  display  no  great  variety.  The 


CAMBRIAN   LIP^E  559 

Cephalopoda,  which  are  the  highest  group  of  molluscs,  are  per- 
haps represented  in  the  Cambrian  by  shells  which  are  rare  and 
minute  in  size,  and  almost  confined  to  the  uppermost  part  of 
the  system ;  that  is,  assuming  that  the  tiny  Volborthella  found  in 
Russia  and  New  Brunswick  is  really  a  cephalopod,  but  this  is 
not  certain. 

The  Cambrian  fauna  displays  steady  progress,  being  distinctly 
more  advanced  in  the  upper  than  in  the  lower  division. 


CHAPTER   XXVII 
THE    ORDOVICIAN    (OR    LOWER    SILURIAN)    PERIOD 

ORDOVICIAN   SYSTEM 

WALES  NEW  YORK 

f  Richmond  Stage  (Medina) 
_  .  Cmcmnatian    T  0 

Bala  Q    .          \  Lorraine  Stage 

[Utica  Stage 

f  Trenton  Stage 
Llandeilo  Mohawkian    filack  ^ 

Llanvirn  Series       [  Lowville  Stage 

Arenig  Canadian  J  Chazy  Stage 

Tremadoc  Series     |  Beekmantown  Stage 

SIR  RODERICK  MURCHISON  divided  his  great  Silurian  system  pri- 
marily into  two  parts,  Upper  and  Lower.  This  method  of  classi- 
fication is  generally  followed  even  at  the  present  day,  although 
it  is  widely  recognized  that  the  most  decided  break  in  the  entire 
Palaeozoic  group  is  the  one  between  these  divisions.  In  1879 
Professor  Lapworth  proposed  to  give  due  emphasis  to  this  dis- 
tinction by  erecting  the  Lower  Silurian  into  a  separate  system, 
the  Ordovician.  The  name  is  taken  from  the  Ordovici,  an  ancient 
British  tribe  which  dwelt  in  Wales  during  Roman  times.  Lap- 
worth's  example  is  now  largely  followed  in  England  and  the 
United  States,  but  on  the  continent  of  Europe  the  name  Silurian 
is  still  retained  for  both  systems. 

The  classification  and  subdivision  of  the  American  Ordovician 
were  first  worked  out  in  the  State  of  New  York,  and  consequently 

560 


DISTRIBUTION  OF  ORDOVICIAN   ROCKS  $6 1 

the  New  York  scale  serves  as  a  standard  of  reference  for  the  rest 
of  the  continent. 

In  the  preceding  table  the  classification  lately  issued  by  the 
New  York  Survey  is  given  in  comparison  with  that  of  the  Welsh 
Ordovician.  It  is  not  to  be  supposed,  however,  that  the  subdi- 
visions in  the  two  continents  are  exactly  equivalent,  but  merely 
that  they  correspond  to  one  another  in  a  general  way. 

DISTRIBUTION  OF  ORDOVICIAN  ROCKS 

American.  —  The  passage  from  Cambrian  to  Ordovician  was 
gradual,  without  any  marked  physical  break.  Only  where  the 
Upper  Cambrian  is  sandy,  as  in  New  York,  is  there  a  decided 
change  in  the  character  of  sedimentation.  In  the  latter  part  of 
the  Cambrian  a  great  inland  sea  had  been  established  over  what 
is  now  the  Mississippi  valley  and,  with  frequent  fluctuations  in 
depth  and  modifications  in  form,  it  was  to  persist  for  long 
periods  as  one  of  the  salient  features  of  Palaeozoic  geography. 
This  sea  was  separated  from  the  Atlantic  by  the  land  mass  called 
Appalachia,  and  on  the  western  side  it  was  demarcated  from  the 
Pacific  by  islands  of  undetermined  size.  A  generalized  repre- 
sentation of  the  arrangement  of  land  and  water  in  Ordovician 
North  America  is  presented  in  the  map,  Fig.  260.  Such  a  map, 
which  can  only  be  a  rude  approximation  to  the  truth,  is  con- 
structed by  marking  as  water  all  those  areas  where  Ordovician 
rocks  are  known,  or  confidently  inferred  to  be  present,  even  though 
concealed  by  overlying,  newer  strata,  and  as  land  those  areas 
where  the  Ordovician  is  wanting.  Frequently,  however,  it  is  im- 
•  possible  to  determine  whether  the  absence  of  the  strata  is  due 
to  their  never  having  been  present  at  the  given  point,  or  to  their 
removal  by  denudation.  On  the  other  hand,  the  deep-lying  and 
buried  extensions  of  the  strata  may  be  subject  to  many  interrup- 
tions, of  which  there  is  no  surface  indication,  for  ancient  islands 
and  peninsulas  may  be  covered  over  and  concealed  by  newer 
strata.  Hence,  the  shore-lines  appear  unduly  simple,  for  the 


562 


THE  ORDOVICIAN  PERIOD 


DISTRIBUTION   OF  ORDOVICIAN   ROCKS  563 

details  have  been  destroyed  by  erosion,  or  hidden  by  deposition. 
Most  attempts  to  reconstruct  on  a  map  the  long-vanished  geo- 
graphy of  some  ancient  period  probably  err  in  the  direction  of 
not  making  sufficient  allowance  for  the  removal  of  the  strata  by 
denudation.  An  example  will  make  this  clear.  At  Elmhurst, 
fifteen  miles  west  of  Chicago,  from  which  the  nearest  known 
exposure  of  Devonian  rocks  is  distant  eighty  miles,  and  where 
the  surface  is  made  by  a  Silurian  limestone  (Niagara),  was  found 
a  fissure  containing  Devonian  fossils,  brachiopods,  and  fish  teeth. 
This  proves  that  the  Devonian  once  covered  that  whole  region, 
but  has  been  entirely  swept  away,  leaving  hardly  a  trace  behind. 
"  The  presence  of  this  Upper  Devonian  fauna  at  Elmhurst,  buried 
as  it  is  deep  down  in  the  Niagara  limestone,  indicates  with  cer- 
tainty that  during  the  greater  part  of  Devonian  time,  the  region 
now  known  as  northern  Illinois  was  above  sea-level.  ...  At  a 
later  period,  near  the  close  of  the  Devonian,  the  sea  again  occu- 
pied the  region,  sand  was  sifted  down  into  these  open  joints,  and 
with  it  the  teeth  of  fishes."  (Weller.) 

The  Cambrian  subsidence  continued  into  the  earliest  Ordo- 
vician,  "  when  more  of  the  continent  was  under  water  and  the 
sea  probably  deeper  than  at  any  subsequent  period."  (Ulrich 
and  Schuchert.)  At  the  end  of  the  Beekmantown  stage,  extensive 
geographical  changes  occurred;  two  parallel  folds,  extending 
from  Alabama  to  Newfoundland  and  following  the  trend  of  the 
future  Appalachian  Mountains  and  the  Cambrian  trough,  were 
raised  as  low  barriers,  which  played  a  very  important  part  in 
separating  the  Atlantic  from  the  Interior  Sea.  Inclosed  between 
these  parallel  folds  was  a  narrow  basin,  which  was  frequently 
invaded  by  the  sea,  though  rarely  completely  submerged  from 
end  to  end.  At  the  same  time  there  was  a  widespread  elevation, 
which  greatly  reduced  the  area  and  depth  of  the  Interior  Sea,  but 
comparatively  soon  there  followed  a  less  extensive  submergence 
(Chazy)  both  at  the  south  and  in  the  north.  In  the  latter  region 
a  long,  narrow  gulf  extended  up  the  valley  of  the  St.  Lawrence, 
dividing  at  the  Adirondack  Mountains  and  sending  one  arm. 


564  THE  ORDOVICIAN   PERIOD 

where  now  is  the  valley  of  the  Ottawa,  and  the  other  over  the 
present  site  of  Lake  Champlain.  Another  narrow  body  of  water 
called  the  Levis  channel,  and  believed  to  be  separate  from  the 
Chazy  bay,  though  very  near  to  it,  extended  from  Newfoundland 
into  the  northeastern  corner  of  New  York.  From  the  south, 
the  invasion  extended  into  Kentucky  and  Tennessee  and  persisted 
for  a  longer  time. 

In  the  upper  Mississippi  valley,  the  lower  Ordovician  limestones 
are  followed  by  a  very  extensively  developed  sandstone  (the  St- 
Peter),  which  occurs,  at  the  surface  or  underground,  over  nearly 
the  whole  of  Illinois,  Iowa,  and  Missouri,  much  of  Wisconsin, 
Minnesota,  and  Michigan  and  Indiana,  and  smaller  parts  of 
other  states  from  Kansas  to  Oklahoma. 

The  Middle  Ordovician  (Mohawkian  series)  was  a  time  of  lime- 
stone-making on  an  extraordinarily  wide  scale,  which  implies  that 
the  Interior  Sea  received  less  terrigenous  sediment  than  before, 
and  this,  in  turn,  probably  indicates  that  the  surrounding  lands 
were  low  and  flat,  and  that  the  sluggish  streams  carried  but  small 
loads  of  fine.  silt.  During  the  Chazy  age  the  Interior  Sea  had  been 
reduced  in  size,  and  nearly  all  of  New  York  was  above  water,  but 
a  renewed  submergence  again  extended  the  sea  over  most  of 
New  York  and  reopened  the  northeastern  connection  with  the 
Atlantic.  The  Mohawkian  limestones,  especially  the  Trenton, 
occur  in  New  Brunswick,  New  York,  Canada,  over  the  upper 
Mississippi  valley,  in  the  Black  Hills,  Bighorn,  Rocky, 
Wasatch,  and  Uinta  Mountains,  and  the  Great  Basin.  In 
Kentucky  and  Tennessee  unconformities  point  to  oscillations  of 
level,  emergence  and  submergence  alternating. 

The  Upper  Ordovician  consists  largely  of  a  thick  mass  of  shales 
( Utica  and  Lorraine)  formed  from  the  terrigenous  silts  spread 
widely  over  the  sea-bottom,  due  perhaps  to  an  elevation  of  the 
land,  which,  rejuvenating  the  streams,  increased  the  loads  carried 
to  the  sea,  and  perhaps  also  to  a  concomitant  shoaling  of  the  sea. 
These  shales  and  slates  are  thickest  toward  the  east,  and  extend 
along  the  Appalachian  line  from  the  St.  Lawrence  tQ  Tennessee 


DISTRIBUTION  OF  ORDOVICIAN   ROCKS  565 

and  westward  into  Indiana.  The  northeastern  passage  to  the 
Atlantic  was  more  freely  opened,  and  a  fauna  with  strong  European 
affinities  invaded  the  Interior  Sea,  though  not  for  long.  The 
immensely  thick  mass  of  shales  and  slates,  which  occurs  in  the 
valley  of  the  Hudson  near  Albany,  and  follows  the  trend  of  the 
Appalachian  Mountains  to  Tennessee,  was  once  regarded  as  a 
distinct  series  (Hudson  River}  and  placed  at  the  top  of  the  Ordo- 
vician.  It  is  now  known,  however,  to  be  a  separate  facies,  repre- 
senting at  least  the  Trenton,  Utica,  and  Lorraine  stages,  and 
possibly  the  whole  of  the  Ordovician. 

The  uppermost  of  the  Ordovician  stages  (Richmond)  is  most 
typically  developed  in  Ohio  and  Indiana,  with  a  littoral,  and  per- 
haps partly  continental,  facies  in  central  New  York,  and  along 
the  Appalachians,  the  Oneida  conglomerate  and  Medina  sand- 
stone, which,  until  very  lately,  have  been  regarded  as  the  base 
of  the  Silurian.  The  "eastern  Oneida,"  or  Shawangunk  grit, 
has  recently  been  proved  to  belong  to  the  upper  part  of  the  Silu- 
rian, and  is  therefore  of  much  later  date  than  the  Oneida-Medina 
of  central  New  York.  "Ulrich  has  reexamined  the  Medina 
deposits  of  the  Appalachian  region,  rnore  especially  in  Penn- 
sylvania, Virginia,  and  Tennessee,  and  has  concluded  that  they 
are  the  eastern  shore  deposits  equivalent  to  the  Richmond  series 
of  the  Ohio  and  Mississippi  valleys.  This  result  therefore  forces 
stratigraphers  to  place  the  line  separating  the  Siluric  from  the 
Ordovicic,  not  at  the  base  of  the  Medina  formation  of  the  New 
York  standard  section,  but  at  its  uppermost  limit  and  beneath 
the  Clinton."  (Schuchert.) 

In  the  western  portion  of  the  continent  the  Ordovician  is  not 
so  well  known  as  in  the  eastern,  because  it  is  so  generally  buried 
under  newer  strata,  and,  over  great  areas  where  it  probably  once 
existed,  it  has  been  removed  by  denudation.  In  the  Lower  Ordo- 
vician great  areas  of  the  northwest  were  land,  but  these  were 
very  extensively  submerged  in  the  Middle  Ordovician,  when  the 
sea  probably  covered  most  of  the  Great  Plains  and  much  of  the 
plateau  region.  The  Upper  Ordovician  is  much  more  restricted, 


566  THE  ORDOVICIAN   PERIOD 

and  in  many  places  lies  unconformably  upon  the  Middle,  pointing 
to  emergences  and  prolonged  denudation.  The  Great  Basin,  or 
that  portion  of  it  known  as  the  Nevada  trough,  seems  to  have  been 
submerged  throughout  the  Ordovician,  as  it  had  been  in  the 
Cambrian,  and,  indeed,  during  nearly  the  whole  Palaeozoic  era. 
Ordovician  rocks  fringe  the  western  side  of  the  great  northern 
pre-Cambrian  area  and  occur  in  the  islands  of  the  Arctic  Sea. 

Aside  from  the  slow  and  gentle  oscillations  of  level  above 
mentioned,  the  Ordovician  was  a  period  of  tranquillity,  generally 
speaking,  without  violent  diastrophic  movements,  nor  have  any 
signs  of  volcanoes  of  that  date  been  discovered  in  North  America. 
Igneous  intrusions  are  rare,  though  they  have  been  found  in  New 
York  and  in  the  Wichita  Mountains  of  Oklahoma,  where  the 
deposition  of  Trenton  beds  was  followed  by  the  upheaval  of  the 
mountains  and  the  intrusion  of  a  great  mass  of  granite,  which 
has  metamorphosed  the  overlying  sedimentaries. 

Foreign.  —  In  Europe  the  Ordovician  rocks  appear  to  have 
been  laid  down  in  two  distinct  seas  separated  by  a  ridge  of  land. 
The  northern  area  extends  from  Ireland  far  into  Russia,  while 
the  southern  is  represented  by  numerous  scattered  masses.  These 
rocks  cover  a  much  wider  surface  than  do  the  Cambrian.  In 
Great  Britain,  especially  in  Wales,  they  form  very  thick  masses 
of  shales  and  slates,  with  but  little  limestone,  intercalated  with 
much  volcanic  lava  and  tuff,  the  volcanic  activity  being  in  very 
marked  contrast  to  the  quiet  of  North  America.  In  Scandinavia 
these  rocks  are  nearly  horizontal  limestones  and  shales,  and  in 
Russia  they  cover  very  large  areas  and  are  so  perfectly  undis- 
turbed that  many  are  still  incoherent  sediments.  In  the  southern 
sea  were  laid  down  the  Ordovician  strata  of  Bohemia,  German' 
northwestern  and  central  France,  Spain,  Portugal,  Sardinia,  ?nd 
Morocco. 

The  very  marked  difference  between  the  fossils  of  the  two  great 
European  areas,  and  the  fact  that  the  Ordovician  fossils  of  other 
continents  agree  with  those  of  northern  Europe,  while  those  of 
the  southern  district  are  peculiar,  indicate  that  the  latter  region 


DISTRIBUTION  OF  ORDOVICIAN   ROCKS  567 

was  a  partially  closed  sea,  which  occupied  the  Mediterranean 
basin,  though  extending  far  beyond  its  present  limits. 

Asia  was  largely  above  water  in  Ordovician  times,  but  a  broad 
Indo-Chinese  sea  covered  much  of  the  eastern  coast,  and  in  north- 
ern Siberia  are  great  areas  of  Ordovician  strata,  the  upper  mem- 
bers of  which  are  red  sandstones  with  gypsum  and  salt.  This 
points  to  an  arid  climate.  Marine  rocks  of  Ordovician  date  are 
found  in  north  Africa,  but  the  equatorial  and  southern  regions 
are  highly  peculiar  among  the  continents  in  the  very  subordinate 
part  taken  by  marine  rocks  of  any  period,  the  land  being  built 
up  almost  entirely  of  continental  rocks.  In  South  Africa  a  thick 
series  of  barren  sandstones  underlies  marine  Devonian,  and  prob- 
ably some  of  these  are  referable  to  the  Ordovician.  Ordovician 
rocks  are  found  in  New  Zealand,  Tasmania,  and  the  southern 
part  of  Australia.  In  South  America  they  are  not  extensively 
developed,  but  have  been  found  in  Argentina  and  Peru. 

The  Climate  of  the  Ordovician,  so  far  as  at  present  known, 
was  uniformly  mild  and  equable,  as  appears  from  the  fossils  of 
the  Arctic  lands.  No  glacial  deposits  have  yet  been  discovered, 
though  arid  conditions  obtained  in  northern  Asia. 

Close  of  Ordovician.  —  At  the  end  of  the  period  came  a  time 
of  widespread  disturbance,  upheaval,  and  mountain-making,  the 
traces  of  which  are  still  plain  in  North  America  and  Europe,  espe- 
cially along  the  Atlantic  slope  of  each  continent.  In  Nova  Scotia 
and  New  Brunswick  the  Silurian  strata  lie  unconformably  upon 
the  upturned  Ordovician.  Along  the  line  between  New  York 
and  New  England  the  Taconic  range  was  upheaved,  its  rocks 
greatly  compressed,  plicated,  faulted,  and  metamorphosed.  Many 
of  the  crystalline  schists  of  this  region,  it  has  been  proved,  were 
derived  from  the  metamorphosis  of  Cambrian  and  Ordovician 
sedimentary  rocks.  Evidences  of  this  disturbance  have  been 
traced  as  far  south  as  Virginia.  The  effects  of  the  upheaval  were 
not  felt  in  the  northern  part  of  the  Gulf  of  St.  Lawrence,  for  on 
Anticosti  Island  the  great  limestone,  which  was  begun  in  Ordo- 
vician times,  continued  without  a  break  into  the  Silurian.  The 


568  THE  ORDOVICIAN   PERIOD 

disturbance  was  along  a  line  of  especially  thick  accumulationsv 
as  appears  from  the  comparative  measurements  of  the  same  strata 
in  different  areas.  The  Interior  Sea  appears  to  have  been  entirely 
drained;  at  all  events  no  deposits  transitional  to  the  Silurian  are 
known  from  that  region.  In  the  West  and  Northwest  large  areas 
remained  land  for  long  periods,  but  the  Interior  Sea  was  soon 
reestablished  in  the  Mississippi  valley.  Some  narrow  strips  of 
land  were  added  to  the  margin  of  the  Cambrian  coasts,  and  on  a 
line  running  through  southern  Ohio,  Kentucky,  and  Tennessee 
a  low,  broad  arch,  the  formation  of  which  appears  to  have  begun 
early  in  the  Ordovician,  was  forced  up  by  lateral  compression. 
This  is  called  the  "  Cincinnati  anticline  or  axis." 

In  Europe  the  disturbances  which  brought  the  Ordovician  to  a 
close  produced  their  maximum  effects  in  England,  Wales,  and 
the  Highlands  of  Scotland,  where  the  thickness  of  the  sediments 
is  greatest.  In  these  regions  the  Ordovician  beds  are  folded  and 
often  greatly  metamorphosed,  the  Silurian  strata  lying  upon  their 
upturned  edges. 

THE  LIFE  OF  THE  ORDOVICIAN 

Ordovician  life  displays  a  notable  advance  over  that  of  the  Cam- 
brian, becoming  not  only  very  much  more  varied  and  luxuriant, 
but  also  of  a  distinctly  higher  grade.  During  the  long  ages  of 
the  period  also  very  decided  progress  was  made,  and  when  the 
Ordovician  came  to  its  close,  all  of  the  great  types  of  marine  inver- 
tebrates and  most  of  their  more  important  subdivisions  had  come 
into  existence.  In  a  general  way  the  life  of  the  Ordovician  is 
an  expansion  of  that  of  the  Cambrian,  though  but  little  direct 
connection  between  the  two  can  yet  be  traced,  and  evidently  there 
were  great  migrations  of  marine  animals  from  some  region  which 
cannot  yet  be  identified.  Several  groups  of  invertebrates  attained 
their  culmination  and  began  to  decline  in  the  Ordovician,  becom- 
ing much  less  important  in  subsequent  periods.  Thus  the  Grap- 
tolites,  the  Cystoidean  order  of  Echinoderms,  the  straight-shelled 


PLATE  IV. —  ORDDVICIAN  SPONGES,  CORALS,  ETC. 

Fig.  i,  Zittelella  typicalis  Ulrich  and  Everett,  x  %,  Trenton.  2,  Strobilospongia  tuber- 
osa  Beecher,  x  %,  Trenton.  3,  Cyathophycus  reticulatus  Wale.,  X  %,  Utica.  4,  Recep- 
taculites  fungosus  Hall,  X  %,  Trenton.  5,  Petraia pro/undo.  Conrad,  X  %,  Trenton. 
5«,  The  same,  top  view.  5  3,  The  same,  vertical  section.  6,  Columnaria  stellata  Hall, 
X  ^,  Trenton.  7,  Romingeria  trentonensis  Weller,  X  %,  Trenton.  8,  Malocystites  em- 
monsi  Hudson,  x  2,  Chazy.  9,  Pleurocystites  filitextus  Bill.,  x  i,  Trenton.  10,  Lepi- 
dodiscus  cincinnatiensis  Hall,  x  1/2,  Richmond.  IT,  Glyptocrinus  dyeri  Meek,  X  %, 
Richmond.  12,  Blastoidocrinus  carcharieedens,  Bill  ,  x  %,  Chazy.  12  *r,  TA^  same, 
basal  view.  12  £,  7"^^  same,  side  view.  13,  Palceasterina  stellata  Bill.,  x  %,  Trenton. 


5/O  THE  ORDOVICIAN   PERIOD 

Cephalopods  (orthoceratites)  among  Molluscs,  and  the  Trilobites, 
were  never  so  abundant  and  so  varied  as  during  this  period. 

Plants.  —  In  America  no  plants  above  the  grade  of  seaweeds 
and  coralline  Algae  have  been  discovered,  but  in  Europe  a  few  of 
the  higher  Cryptogams  are  doubtfully  reported.  The  flora  of  the 
Devonian,  however,  renders  it  highly  probable  that  land  plants 
were  already  well  advanced  in  the  Ordovician,  and  their  remains 
may  be  discovered  at  any  time.  This  must  remain  a  matter  of 
accident,  for  the  known  Ordovician  rocks  are  almost  all  marine, 
which  is  not  a  favourable  circumstance  for  the  preservation  of 
land  plants.  Such  discoveries  have,  indeed,  already  been  re- 
ported, but  the  evidence  for  them  is  not  satisfactory. 

Foraminifera  and  Radiolaria  have  been  found  in  a  few  regions 
in  great  numbers,  sufficient  to  prove  that  they  were  abundant 
in  the  Ordovician  seas. 

Spongida.  —  Sponges  are  much  more  numerous  and  varied 
than  |n  the  Cambrian.  Of  course  it  is  only  those  sponges  with 
skeletons  of  lime  or  flint  which  can  be  well  preserved  in  the  fossil 
state,  and  of  these  the  Ordovician  has  many  (PL  IV,  Figs.  1-4). 
The  horny  sponges,  of  which  the  common  bath  sponge  is  a 
familiar  example,  are  necessarily  much  rarer  and  less  satisfac- 
tory as  fossils. 

Ccelenterata  —  The  Graptolites  are  very  numerous  and  varied, 
wherever  conditions  are  favourable  to  their  preservation,  as  in 
fine-grained  rocks  with  smooth  bedding-planes.  The  Ordovician 
is  the  time  of  their  culmination  and  is  especially  characterized  by 
the  double  forms,  with  rows  of  cells  on  both  sides  of  the  stem 

Beekmantown.  6,  Con  stellar  ia  polystomella,  Whitfield,  x  %,  Richmond.  7,  Stictoporella 
cribrosa  Ulrich,  x  %.  Trenton,  ja,  The  same,  a  fragment,  x  9.  8.  Stomatopora  in- 
jflata  Hall,  on  a  brachiopod  shell,  x  ya>  Richmond.  8a,  The  same,  a  portion  x  4.  9,  Tre- 
matis  ottawaensis  Bill.,  +  1/2,  Trenton.  10,  Schizambon  canadensis  Ami.,  x  T,  Utica. 
ii,  Orthts  tricenaria  Conrad,  x  %,  Trenton.  \ia,  The  same,  lateral  view.  12,  Platy- 
strophia  lynx  Eichw.,  X  %,  Trenton.  13.  Hebertella  sinuata  Hall,  X  %,  Richmond. 
130,  The  same,  inner  side  of  ventral  valve.  13^,  The  same,  inner  side  of  dorsal  valve.  14, 
Dalmanella  testudinaria  Dalman,  x  y2,  Trenton.  140;,  The  same,  inner  side  of  ventral 
valve.  14^,  The  same,  inner  side  of  dorsal  valve.  15,  Plectambonites  sericeus  Sowerby, 
x  i,  Richmond.  16,  Rafinesquina  alternata  Conrad  X  %,  Richmond.  i6a,  The  same, 
longitudinal  section.  17,  Strophomena  planumbonum  Hall,  X  %,  Richmond,  ija,  The 
same,  longitudinal  section.  18,  Zygospira  modesta  Say,  X  i,  Richmond.  i8«,  The  same, 
from  the  side.  IQ,  Rhynchotrema  capax  Conrad.  X  %,  Richmond.  io#,  The  same,  ante, 
rior  view.  20,  Triplecia  extans  Emmons,  X  %,  Trenton.  2oa,  The  same,  anterior  view. 


PLATE  V.  —  ORDOVICIAN  GRAPTOLITES,  BRACHIOPODS,  ETC. 

Fig.  i,  Climacograptus  bicornis  Hall,  x  i,  Utica. 
X  i/2,  Beekmantown.  3,  Climacograptus  pungens  Rue 
tus  postremus  Rued.,  x  %,  Beekmantown.  5,  Phyllograptus  augustifolius  Hall,  X  a, 


Utica.     2,   Tetragraptusfruticosus   Hall, 
ns  Ruedemann,  x  5/2.  Chazy.    4,  Goniograp- 


572  THE  ORDOVICIAN   PERIOD 

(see  PL  V,  Figs.  1-5).  So  abundant  are  the  Graptolites  that 
in  many  parts  of  the  system  they  are  almost  the  only  fossils,  and 
are  employed  to  divide  the  substages  into  zones.  Graptolite 
zones,  with  the  same  or  closely  similar  species,  and  in  'the  same 
order  of  succession,  are  found  in  Great  Britain,  the  St.  Lawrence 
and  Champlain  valleys,  and  in  Australia.  Hydroid  corals,  Stro- 
matocerium,  are  abundant,  and  form  reefs  in  the  Chazy  and  Black 
River  limestones.  The  few  and  doubtful  Cambrian  Corals  are 
succeeded  by  a  considerable  number  of  Ordovician  genera  and 
species.  Like  other  Palaeozoic  Corals,  these  are  characteristically 
different  from  the  reef-builders  of  the  present  day  in  showing  a 
marked  bilateral  symmetry  and  having  the  septa  arranged  in 
multiples  of  four  (Tetracoralla).  Solitary  cup-corals,  like  Strep- 
telasma  and  Petraia  (IV,  5),  and  compound  colonies,  like  Colum- 
naria  (IV,  6),  are  examples  of  the  range  of  differences  among 
these  early  corals. 

The  Echinodermata  have  greatly  increased  in  -importance,  and 
all  the  main  subdivisions  of  the  group  are  represented,  all  of 
which,  except  the  Cystoids,  first  appear  in  the  Ordovician.  The 
Cystoidea,  which  we  have  already  found  in  the  Cambrian,  attain 
their  greatest  development  in  the  Ordovician.  In  these  curious 
animals  the  body  is  either  irregularly  shaped,  or  symmetrical, 
with  a  short,  tapering  stem,  by  which  the  animal  was  attached 
to  the  sea-floor,  and  weakly  developed  arms.  The  body,  or  calyx, 
is  made  up  of  a  number  of  calcareous  plates;  when  these  plates 
are  very  numerous,  they  are  of  irregular  size  and  arrangement 
(IV,  8,  10),  while  the  forms  with  few  plates  have  them  of  a  definite 
number,  size,  and  shape  (IV,  9).  Some  of  the  more  regular 
Cystoidea  have  much  resemblance  to  the  true  Crinoids.  The  latter 
are  not  rare,  though  less  abundant  than  they  afterward  became. 
These  aninials  (IV,  n)  have  a  symmetrical  calyx,  with  long, 
branching  arms;  the  number  and  arrangement  of  the  compo- 
nent plates  are  definite  and  characteristic  for  each  genus.  Most, 
but  not  all,  of  the  Crinoids  have  a  long,  jointed  stem,  by  which 
they  are  attached  to  the  sea-bottom.  The  earliest  Blastoidea, 


THE  LIFE  OF  THE  ORDOVICIAN  573 

represented  by  Blastoidocrinus  (IV,  12),  appear  in  the  Chazy. 
This  genus,  a  very  primitive  form  which  retains  notable  cystoid- 
ean  characters,  is  the  first  member  of  a  group  which  was  to  become 
important  in  a  long  subsequent  period,  the  Carboniferous.  As- 
teroids  (starfishes)  and  Ophiuroids  (brittle-stars)  are  found,  but 
cannot  be  called  common,  or  abundant.  The  Echinoidea,  or 
sea-urchins,  are  represented  by  very  primitive  forms. 

Arthropoda. — The  Trilobites  (PL  VII)  increase  very  greatly 
in  the  number  of  genera  and  species,  and  most  of  the  Cambrian 
genera  are  replaced  by  new  ones.  This  is  the  period  in  which 
the  group  of  Trilobites  attains  its  highest  development,  gradually 
declining  afterward  and  becoming  extinct  with  the  close  of  the 
Palaeozoic.  The  most  characteristic  and  widely  spread  genera 
of  Ordovician  Trilobites  are:  Asaphus,  Isotelus  (VII,  7),  Bu- 
mastus  (VII,  2),  Triarthrus  (VII,  i),Calymmene  (VII,  8),  Cerau- 
rus  (VII,  6),  Trinucleus  (VII,  4),  Addas  pis  (VII,  3),  Bronteus 
(VII,  5),  Pterygometopus,  etc.  'These  genera  differ  in  aspect 
from  those  of  the  Cambrian  in  their  much  larger  tail-shields,  in 
their  ability  to  roll  themselves  up  (see  VII,  2  a,  So),  and  in  their 
rounder  and  better-developed,  faceted  eyes. 

Other  Crustacea  mark  advances  in  the  Ordovician.  Thus, 
the  Eurypterida,  a  group  which  dates  from  the  Algonkian,  and 
was  destined  to  a  remarkable  development  in  the  Silurian  and 
Devonian,  is  represented,  though  not  abundantly.  Ostracoda 
and  Phyllocarida  undergo  no  marked  change.  That  terrestrial 
animal  life  had  already  begun  is  demonstrated  by  the  occurrence 
of  an  Insect,  Protocimex,  in  Scandinavia.  From  this  we  may  be 
assured  that  terrestial  vegetation  was  already  established  and 
that  the  atmosphere  was  fitted  for  the  existence  of  air-breathers. 

Brachiopoda.  — These  shells  increase  very  largely  in  abundance 
and  variety,  the  genera  with  hinged  calcareous  shells  (Articulata) 
now  gaining  the  upper  hand  and  reducing  the  horny-shelled  kinds 
to  comparative  insignificance.  The  most  important  genera  are: 
Orthis  (V,  n);  Platystrophia  (V,  12);  Dalmanella  (V,  14); 
Plectambonites  (V,  15);  Rafinesquina  (V,  16);  Leptana,  Stro- 


574 


THE  ORDOVICIAN   PERIOD 


PLATE  VI.  — ORDOVICIAN   MOLLUS 


Fig.  i,  Byssonych ia  radiata  Hall.  X  %,  left  valve,  Trenton.  2,  Ambonychia  planistri- 
ala  Hall,  X  %,  left  valve.  3,  Opisthoptera  fissicosta  Meek, 'x  V2>  right  valve,  Richmond. 
4,  Pterinea  demissa  Conrad,  x  %,  left  valve,  Trenton.  5,  Cyrtodonta  huronensis 
Bill.,  x  %,  right  valve,  Lowville.  6,  Cymatonota  attenuata  Ulrich,  x  %,  right  valve, 
Richmond.  7,  Cyclonema  humerosum  Ulrich,  x  ^,  Lorraine.  8,  Eotomaria  supracin- 
gulata  Bill.,  x  %  o,  Trochonema  umbilicatum  Hall,  x  %,  Trenton.  10.  Hortnotoma 
gracilis  Hall.x  \'2,  Trenton  u,  Cyrtolites  ornatus  Conrad,  x  %,  Lorraine.  12,  Proto- 
•warthia  cancdlata  Hall,  x  %,  Black  River.  13,  Maclurea  logani  Salter,  x  %,  Trenton. 
14,  Ophileta  compacta  Salter,  x  ^,  Beekmantown  15,  Conularia  trentonensis  Hall, 
X  %,  Trenton.  16,  Orthoceras  mnlticameratnm  Hall.  X  %,  Lowville.  17,  Cyrtoceras 
juvenalis  Bill.,  x  i/2,  Trenton.  18,  Eurystomites  occidentalis  Hall,  X  %.  19,  Schrae- 
deroceras  eatoni  Whitfield,  x  ya. 


THE  LIFE  OF  THE  ORDOVICIAN  575 

phomena  (V,  17),  and  RJtynchotrema  (V,  19).  Spine-bearing 
shells  begin  in  Zygospira  (V,  18). 

Bryozoa.  — This  is  a  group  which  has  yet  yielded  no  repre- 
sentatives from  the  Cambrian,  but  appears  abundantly  in  the 
Ordovician  (V,  6-8).  The  genera  differ  little  from  those  which 
live  in  the  modern  seas. 

Mollusca.  —  One  of  the  most  striking  differences  between  the 
Cambrian  and  the  Ordovician  is  the  great  advance  made  by  the 
Molluscs  in  the  latter  period.  The  Bivalves  (Pelecypoda)  are 
larger,  more  numerous,  and  more  like  modern  forms  (see  PL  VI, 
Figs.  1-6).  The  Gastropoda  likewise  increase  notably  in  size  and 
in  numbers,  especially  the  spirally  coiled  shells.  Important 
genera  are:  Eotomaria  (VI,  8);  Hormotoma  (VI,  10);  Pro- 
towarthia  (VI,  12);  Trochonema  (VI,  9);  Maclurea  (VI,  13). 
Neither  Bivalves  nor  Gastropods  had  anything  like  the  relative 
importance  which  they  possess  in  modern  times;  the  latter  all 
had  the  mouth  of  the  shell  forming  a  complete  ring  (holosto- 
mate). 

Much  the  most  significant  change  in  the  Mollusca,  however, 
is  the  great  expansion  of  the  Cephalopoda,  a  few  of  which  perhaps 
appear  in  the  uppermost  Cambrian,  but  in  the  Ordovician  have 
become  one  of  the  predominant  elements  in  the  marine  life  of 
the  times.  The  Cephalopods,  which  are  the  highest  group  of 
molluscs,  are  in  modern  times  represented  by  two  suborders; 
in  one,  the  squids  and  cuttlefishes  (Dibranchiata) ,  the  shell  is 
rudimentary  and  internal;  while  in  the  other  (Tetrabranchiata) 
the  shell  is  external.  Such  an  external  shell  is  divided  by  trans- 
verse septa  into  chambers,  which  are  connected  by  means  of  a  tube, 
the  siphunde,  the  animal  living  only  in  the  terminal  chamber  at 
the  mouth  of  the  shell,  the  remainder  of  which  is  empty.  The 
only  existing  representative  of  the  Tetrabranchiata  is  the  Pearly 
Nautilus,  but  .throughout  the  Mesozoic  and  most  of  the  Palaeozoic 
eras  there  was  an  extraordinary  variety  of  these  chambered  shells. 
In  the  Ordovician  the  Cephalopods  were  all  Nautiloids,  most 
nearly  allied  to  the  modern  Pearly  Nautilus,  with  chambered 


PLATE  VII.  —  ORDOVICIAN  TRILOBITES 

Fig  i,  irt.  Triarthrus  becki  Green,  x  3/2.  Utica.  Restoration  by  Beecher  of  dorsal  and 
ventral  sides.  2,  Bumastus  trentonetisis  "Emmons,  x  l/2,  Trenton,  za,  The  same,  from 
the  side,  rolled  up.  3.  Acidaspis  crosotus  Locke,  x  4,  Richmond.  4,  Tnnncleus  concen- 
irtcus  Eaton,  X  i,  Trenton.  5,  Bronteus  lunatus  Bill.,  X  i,  Trenton  6,  Ceranrns  pleu- 
rexanthmus  Green,  x  i,  Trenton.  7,  Isotelus  ntaximus  Locke,  X  i,  Trenton.  8,  Lalym- 
Kene  callicephala  Green,  x  i,  Richmond.  8«,  The  same,  rolled  up,  from  the  side. 


THE   LIFE   OF  THE   ORDOVICIAN  577 

shells,  divided  internally  by  simple  septa.  The  commonest 
shell  of  this  type  is  Orthoceras,  which  is  a  straight  and  very  elon- 
gate cone  (VI,  1 6)  and  sometimes  attains  a  length  of  10  feet;  the 
genus  persists  throughout  the  Palaeozoic  and  into  the  Mesozoic. 
Endoceras,  which  likewise  has  a  straight  shell,  with  a  curiously 
complex  siphuncle,  is  confined  to  the  Ordovician.  Besides  these 
straight  forms  we  find  curved  shells  like  Cyrtoceras  (VI,  17),  shells 
like  Eurystomites  (VI,  18),  and  Schmderoceras  (VI,  19),  which 
have  the  young  shell  coiled  and  the  portion  formed  in  old  age 
straight,  resembling  an  Orthoceras  with  its  smaller  end  rolled  up 
into  a  coil.  Others  again,  like  Trocholites,  have  the  shell  coiled 
in  a  close,  flat  spiral. 

A  peculiar  shell,  Conularia  (VI,  15),  which  has  a  four-sided, 
pyramidal  shape,  with  four  triangular  pieces  to  close  the  mouth, 
is  a  genus  referred  to  the  Pteropoda. 

Vertebrata. — The  curious,  mail-clad  Ostracoderms,  primitive 
vertebrates  which  somewhat  resemble  the  fishes  in  appearance, 
have  been  found  in  the  Ordovician  sandstones  of  Colorado  and 
Wyoming.  As  these  remains  are  imperfect,  description  of  the 
Ostracoderms  will  be  deferred  till  a  later  chapter. 


2P 


CHAPTER  XXVIII 


THE   SILURIAN    (UPPER   SILURIAN)    PERIOD 

THE  name  Silurian,  like  Cambrian  and  Ordovician,  refers  to 
Wales.  The  term  was  proposed  by  Murchison  in  1835  for  a  great 
system  of  strata  older  than  the  Devonian,  and  was  taken  from  the 
Silures,  another  ancient  tribe  of  Britons  which  inhabited  part  of 
Wales.  Murchison  gave  great  extension  to  his  Silurian  system, 
including  in  it  most  of  Sedgwick's  Cambrian,  but,  as  already 
pointed  out,  the  present  tendency  is  to  divide  this  vast  succession  of 
rocks  into  three  systems  of  equivalent  rank.  It  is  unfortunate,  and 
even  unjust,  that  Murchison's  term  should  not  have  been  retained 
for  the  more  important  and  widely  developed  lower  division, 
now  called  the  Ordovician,  rather  than  for  the  upper  division. 

As  in  the  Ordovician  and  Devonian,  the  New  York  classification, 
given  in  tabular  form  below  in  comparison  with  that  of  Wales,  is 
the  standard  of  reference  for  the  American  Silurian :  — 


SILURIAN   SYSTEM 


WALES 

Ludlow, 
or  Clunian 

Wenlock, 

or  Salopian 
May  Hill, 

or  Valentian 


NEW  YORK 

Manlius  Stage 


Cayugan 
Series 


Niagaran 
Series 


Rondout  Stage 
Cobleskill  Stage 
Salina  Stage 
Guelph  Stage 
Lockport  Stage 
Rochester  Stage 
Clinton  Stage 


N.B.  Until  very  lately  a  basal  series,  the  Oswegan,  with  the 
Oneida  conglomerate  and  Medina  sandstone,  has  been  regarded 
as  the  lowest  member  of  the  Silurian,  but  this  is  now  referred  to 
the  Ordovician.  (See  p.  565.) 

578 


DISTRIBUTION  OF  THE   SILURIAN  ROCKS  579 


DISTRIBUTION  OF  THE  SILURIAN  ROCKS 

American.  —  The  general  disturbance  which  closed  the  Ordo- 
vician  period  appears  to  have  greatly  increased  the  extent  of  the 
continent.  A  relatively  narrow  strip  of  coast  lands  had  been 
added  to  the  northern  pre-Cambrian  area,  converting  much  of 
Minnesota,  Wisconsin,  and  the  province  of  Ontario,  northern 
New  York  and  New  Jersey,  and  western  New  England  into  land. 
Southern  Ohio  and  central  Kentucky  and  Tennessee  had  been 
raised  into  the  Cincinnati  anticline,  but  it  is  doubtful  whether 
they  remained  as  islands  in  the  Silurian  sea.  Much  of  the  Interior 
Sea  had  withdrawn,  but  the  emergence  was  not  long,  geologically 
speaking,  and  the  sea  was  soon  reestablished,  but  with  entirely 
different  boundaries  and  connections.  What  changes  affected  the 
land  masses  of'  the  West  and  Southwest  cannot  yet  be  definitely 
determined,  but  the  absence  of  the  Silurian  from  extensive  areas 
where  the  Ordovician  is  found  indicates  that  these  masses  were 
greatly  enlarged.  How  much  of  this  enlargement  came  at  a 
later  date  and  how  far  the  absence  of  the  Silurian  is  the  result  of 
denudation,  there  is  no  present  means  of  finding  out. 

The  Silurian  rocks  are  far  thicker  in  the  East,  especially  along 
the  Appalachian  range,  than  in  the  interior  or  western  regions, 
where  they  thin  out  and  are  wanting  over  large  areas. 

An  important  feature  in  the  Silurian  geography  of  eastern 
North  America  was  the  establishment  of  the  Cumberland  Basin, 
or  "  Appalachian  Mediterranean,"  as  it  has  been  called.  This 
large  sea  lay  to  the  eastward  of  the  Interior  Sea,  from  which  it 
would  seem  to  have  been  either  completely  separated,  or  so 
nearly  so  that  the  species  of  marine  animals  inhabiting  the  two 
bodies  of  water  were  very  different.  The  Cumberland  Basin 
was  east  of  the  Catskill-Helderberg  line  in  New  York,  and  its 
western  shore  crossed  New  Jersey  and  curved  westward  beyond 
the  centre  of  Pennsylvania,  whence  it  ran  southwest  more  or 
less  parallel  with  the  Appalachian  line,  toward  which  it  curved 


58o 


THE  SILURIAN   PERIOD 


FIG.  261.  —  Generalized  map  of  North  America  in  the  Silurian.     Black  areas  = 
known  exposures.     Lined  areas  =  sea.    White  areas  =  land,  or  unknown 


DISTRIBUTION  OF  THE  SILURIAN   ROCKS  58 1 

eastward  in  southern  West  Virginia.  This  basin  began  apparently 
in  western  Maryland  and  adjoining  areas  very  early  in  Silurian 
times,  and  continued  to  grow  larger  and  deeper  until  the  Devonian 
was  well  advanced.  The  Interior  Sea  underwent  a  succession 
of  oscillations  much  like  those  which  had  affected  it  during  the 
Ordovician;  it  was  apparently  closed  at  the  south,  but  extended 
northwestward  to  the  Arctic  Sea,  while  its  east-west  diameter 
had  been  greatly  reduced  from  that  of  the  Ordovician. 

The  submergence  which  inaugurated  the  Silurian  period 
(i.e.  on  the  assumption  that  the  Medina  is  properly  referable  to 
the  Ordovician)  brought  the  Interior  Sea  up  to  the  narrow  barrier 
which  separated  it  from  the  Cumberland  Basin,  and  in  it  were 
laid  down  the  sediments  of  the  Clinton  stage,  shales  in  the  east 
passing  westward  into  limestones,  which  extend  through  New 
York  to  Indiana,  and  perhaps  also  through  Illinois  to  Missouri. 
In  the  Cumberland  Basin  the  Clinton  shales  followed  the  trend 
of  the  Appalachians  to  Alabama.  It  must  be  remembered, 
however,  that  the  Appalachian  Mountains  were  not  then  in  exist- 
ence, as  such,  but  they  were  foreshadowed  by  structural  lines 
of  depression  and  low  folding  which  exerted  a  definite  control 
of  the  coast-lines  and  basins  through  most  of  the  Palaeozoic  era. 
Northeastward,  the  Clinton  recurs  in  Nova  Scotia  and  at  other 
points  in  eastern  Canada,  but  is  not  always  readily  identifiable. 
In  many  places  interstratified  concretionary  haematites  are  found 
in  the  Clinton,  especially  along  the  Appalachian  line,  but  also 
in  Wisconsin,  New  York,  and  Nova  Scotia. 

A  time  of  limestone-making  on  a  great  scale  (the  Lockport  and 
Guelph  stages),  preceded  in  New  York  by  the  Rochester  shales, 
followed  the  Clinton.  In  the  East  this  great  limestone  has  but 
a  limited  extension  southward,  but  southwestward  it  stretches 
for  nearly  1000  miles,  to  Wisconsin  and  thence  across  Illinois, 
Iowa,  Missouri,  and  western  Tennessee.  Many  scattered  outliers 
in  Manitoba  and  the  region  west  of  Hudson's  Bay  indicate  the 
probable  former  extension  of  the  limestone  unbrokenly  to  the 
Arctic  shores  and  islands.  .  Rocks  of  corresponding  date,  laid 


582  THE   SILURIAN   PERIOD 

down  in  the  Cumberland  Basin,  but  with  marked  faunal  differences 
in  the  fossils  from  those  of  the  Interior  Sea,  are  found  in  western 
Maryland  and  Virginia,  New  England,  New  Brunswick,  and 
Nova  Scotia.  East  Tennessee,  on  the  other  hand,  was  elevated 
at  the  end  of  the  Clinton  stage  and  remained  as  a  land-surface 
till  the  middle  of  the  Devonian. 

Little  is  known  of  the  Silurian  of  the  West,  for,  as  already 
pointed  out,  there  is  reason  to  believe  that  nearly  all  of  that  region 
was  then  land.  However,  the  Nevada  trough  continued  to  be 
submerged,  presumably  forming  a  gulf  from  the  Pacific,  and 
here  the  Niagara  series  is  represented  by  the  upper  part  of  a 
thick  mass  of  limestone  which  extends  upward  unbrokenly  from 
the  Trenton,  the  great  limestone  of  the  Ordovician. 

The  limestone  of  the  Niagara  epoch  (Lockport)  is  very  largely 
made  up  of  corals,  and  in  some  places,  as  in  eastern  Wisconsin 
and  areas  to  the  south,  distinct  coral  reefs  may  be  observed, 
the  most  ancient  which  have  as  yet  been  found.  As  we  have 
seen,  corals  flourished  abundantly  in  the  Ordovician,  but,  so  far, 
no  definite  reefs  have  been  noted  in  the  rocks  of  that  period. 

The  next  change  (Salina  stage)  was  the  separation,  along  the 
northern  part  of  the  northeastern  arm  of  the  Interior  Sea,  of  a 
series  of  salt  lagoons,  in  which  were  deposited  red  marls  and 
shales,  interstratified  with  gypsum  and  rock-salt,  from  which 
are  obtained  the  brines  of  New  York,  Ontario,  and  Ohio.  In 
part  contemporaneous  with  these  is  a  hydraulic  limestone,  called 
the  Water-lime,  which  has  much  the  same  distribution  as  the  salt- 
bearing  beds,  but  is  thickest  where  they  are  thin.  The  Water- 
lime  indicates  the  freshening  of  the  Salina  lagoons  and  has 
preserved  a  remarkable  assemblage  of  Crustacean  fossils,  belonging 
to  the  Eurypterida.  The  rocks  of  the  Salina  stage  are  thickest 
in  New  York  and  Pennsylvania,  thinning  to  the  westward.  In 
the  Shawangunk  Mountains  of  eastern  New  York  the  Salina  is 
represented  by  thick  conglomerates  of  quartz  pebbles,  which  were 
formerly  referred  to  the  Oneida,  but  are  now  known  to  contain, 
in  interstratified  shales,  the  remarkable  Eurypterida  of  the  Water- 


DISTRIBUTION   OF  THE   SILURIAN   ROCKS  583 

lime.  This  formation  extends  along  the  Appalachian  line  to 
Tennessee  as  a  very  thick  mass  of  sandstones  and  conglomerates. 
The  beds  of  salt  and  gypsum  are  strong  evidence  that  the  climate 
of  Salina  times,  at  least  in  the  northeastern  part  of  the  conti- 
nent, was  arid,  but  how  far  this  aridity  was  local,  cannot  be 
determined. 

Throughout  the  Salina  age  the  Interior  Sea  had  been  growing 
shallower,  and  shortly  after  the  close  of  that  stage  the  whole  interior 
of  the  continent  became  land  and  remained  so  for  a  considerable 
period,  but  the  Cumberland  Basin  persisted,  and  in  it  the  lime- 
stones of  the  closing  part  of  the  Cayugan  epoch  and  Silurian 
period  were  accumulated,  the  Cobleskill,  Rondout,  and  Manlius, 
which  are  variously  distributed  in  different  parts  of  the  basin, 
due  probably  to  differential  movements  of  the  sea-floor,  but  all 
occur  in  succession  in  New  York  and  in  western  Maryland,  and 
for  some  distance  north  and  south  of  the  latter. 

The  Silurian  rocks  of  North  America  have  yielded  no  sign  of 
volcanic  material;  in  a  few  places  they  are  traversed  by  igneous 
intrusions,  most  of  which  may,  however,  be  of  much  later  date. 
In  Maine  are  some  igneous  rocks  which  decidedly  appear  to  be 
Silurian,  and  the  same  may  be  true  of  certain  areas  in  New  Bruns- 
wick and  Nova  Scotia. 

Foreign.  —  The  division  of  Europe  into  northern  and  southern 
areas  which  we  found  in  the  Ordovician  was  maintained  in  Silurian 
times,  and  the  southern  sea  was  as  peculiar  in  its  animal  life  as  it 
had  been  before,  the  northern  being  the  typical  Silurian  which 
is  found  in  the  other  continents.  In  the  west  of  Ireland,  Wales, 
northern  England,  and  Scotland,  Silurian  beds  accompany  and 
overlie  the  Ordovician,  but  the  much  greater  development  of  lime- 
stone points  to  a  deepening  of  the  water  in  those  seas  or  a  lowering 
of  the  surrounding  lands.  The  volcanic  materials,  so  conspicuous 
in  the  Ordovician,  are  no  longer  found.  The  Wenlock  limestone 
of  Great  Britain,  which  corresponds  to  the  American  Niagara 
(Lockport-Guelph)  is,  like  the  latter,  largely  coralline.  In  Scan- 
dinavia also  there  is  a  great  development  of  Silurian  limestones, 


584  THE  SILURIAN   PERIOD 

which  extend  far  into  Russia.  In  the  latter  country  the  sea 
had  retreated  much  from  its  extension  in  the  Ordovician,  except 
toward  the  southeast,  where  it  was  carried  into  Bessarabia.  Most 
of  the  Russian  Silurian  was  formed  in  an  interior  sea,  connected 
with  that  of  southern  Europe.  In  the  southern  European  countries, 
which  display  the  Bohemian  fades,  the  Silurian  rocks  have  nearly 
the  same  general  distribution  as  the  Ordovician.  The  two  systems 
are  also  associated  in  the  Arctic  islands,  in  China,  north  Africa, 
South  America,  and  Australia.  In  South  Africa  the  Silurian  is 
probably  represented  in  some  of  the  barren  Table  Mountain 
sandstones.  In  all  of  these  areas,  as  also  in  North  America,  the 
fossils  resemble  those  of  the  northern  European  region,  rather 
than  those  of  the  southern.  In  general  the  Silurian  rocks  are  less 
extensively  exposed  at  the  surface  than  the  Ordovician. 

Climate. — The  Silurian  climate  seems  to  have  been  like  that 
of  the  Ordovician,  very  uniform  over  the  earth  and  with  no  indica- 
tion of  climatic  zones.  The  aridity  of  the  New  York  region  in  the 
Salina  age,  which  has  already  been  mentioned,  corresponds  to 
the  similar  conditions  in  the  Ordovician  of  Siberia.  Both  were 
probably  local. 

Close  of  the  Silurian.  —  In  parts  of  North  America  the  Silurian 
passed  so  gradually  and  gently  into  the  Devonian,  that  it  is  difficult 
to  draw  the  line  between  the  two  systems.  Some  disturbances, 
however,  took  place  in  Ireland,  Wales,  and  the  north  of  England, 
for  in  these  localities  the  Devonian  lies  unconformably  upon  the 
Silurian.  In  other  parts  of  Europe  the  transition  was  gradual. 

THE  LIFE  OF  THE  SILURIAN 

Silurian  life  is  the  continuation  and  advance  of  the'same  organic 
system  as  flourished  in  the  Ordovician,  certain  groups  diminishing, 
others  expanding;  and  some  new  groups  now  make  their  first 
appearance 

PLATE  V 1 1 1 .  —  ( Continued) 

sonoceras  americanum  Foord,  x  %,  Guelph.  17,  Trochoceras  desplainese  McChesney, 
X%,  Guelph.  19,  Lichas  Ireviceps  Hall,  X  %,  Lockport.  20,  Staurocephalus  murchi- 
soni,  Barrande,  x  r,  Lccknort.  21.  Deiphon  forbesi  Barrande,  x  i,  Niagaran  of  Arkansas. 


PLATE  VIII.  — SILURIAN  FOSSILS 

Fig.  i,Astrceospongia  meniscus  Roemer,  x  i,  Lockport.  2,  Favosites  occidental™  Hall, 
Xi,Lockport.  3,  Halysitrs  catenulatus  Linn.,  x  i,  Lockport.  4,  Holocystites  cylindricus 
Hall,  x  %,  Guelph.  5,  Caryocrinus  ornatus  Say,  X  i,  Lockport.  6,  Eucalyptocrinus 
crassns  Hall,  X  y2,  Lockport.  7,  Tentaculites  gyracanthns,  Eaton, xn/2,  Manlius.  8,  Mono- 
merella  noveboracum  Clarke  and  Rued.,  x  %,  Guelph.  Sa,  The  same,  interior  of  ventral 
valve.  9,  Spirifer  radiatus  Sowerhy,  x  %,  Lockport.  10,  S.  crispus  Hisinger,  X  i, 
Lockport.  n,  Meristina  nitida  Hall  X  %,  Lockport.  12,  Rhynchotreta  americana 
Hall,  xi,  Lockport.  13,  Pentamerns  obloiigus  Sowerby,  x  %.  Clinton.  14,  Strophostylus 
cyclostomus  Hall-  x  i/L  Lockport.  15.  Trematonotus  alpkeus  Hall,  x  %,  Guelph.  16,  Daw- 


586  THE  SILURIAN   PERIOD 

Plants.  —  Our  knowledge  concerning  the  land  vegetation  of  the 
Silurian  is  not  much  more  definite  than  concerning  that  of  the 
Ordovician.  Most  of  the  remains  referred  to  land  plants  are  of 
disputable  character;  the  best  authenticated  is  a  fern  (Neuro- 
pteris)  from  the  Silurian  of  France. 

Spongida  are  still  common.  Well-known  forms  are  Astylo- 
spongia  and  Astraospongia  (PL  VIII,  Fig.  i);  both  are  almost 
restricted  to  Tennessee. 

Coelenterata.  —  The  Graptolites  have  greatly  diminished,  es- 
pecially the  branching  forms  and  those  with  two  or  more  rows 
of  cells.  Those  that  persist  are,  for  the  most  part,  straight  and 
simple.  The  Hydroid  Corals,  on  the  other  hand,  such  as  Heliolites, 
Stromatopora,  etc.,  become  important  elements  of  marine  life  and 
in  the  formation  of  the  reefs.  The  true  Corals  likewise  increase 
largely,  and  play  a  more  important  role  than  in  the  preceding 
period.  The  increase  is  principally  in  the  enlarged  number  of 
species  belonging  to  much  the  same  genera.  Favosites  (VIII,  2) 
is  a  characteristic  new  genus,  and  Holy  sites  (VIII,  3),  the  chain 
coral,  Syringopora,  and  others  are  much  commoner  than  before. 

Echinodermata.  —  In  this  group  we  observe  a  great  expansion 
of  the  Cystoidea,  which  are  very  abundant  in  the  Chicago  lime- 
stone (Niagara)  and  the  Manlius  of  Maryland  and  West  Virginia. 
Holocystites  and  Caryocrinus  (VIII,  4,  5)  are  typical  of  the  fauna. 
There  is  also  a  marked  increase  of  the  Crinoids  ;  Eucalyptocrinus 
(VIII,  6)  is  a  good  example.  Starfishes  also  have  grown  more 
abundant.  The  Blastoidea,  which  originated  in  the  Ordovician 
and  became  extinct  at  the  end  of  the  Palaeozoic  era,  remain  rare 
in  the  Silurian  and  Devonian,  first  becoming  important  in  the 
Carboniferous.  The  Echinoids,  or  sea-urchins,  which  were 
commoner  than  before,  have  no  arms,  but  a  closed  spheroidal  or 
discoidal  test,  made  up  of  calcareous  plates,  which  in  all  the 
modern  sea-urchins  are  arranged  in  just  twenty  vertical  rows, 
and  are  closely  fitted  together  by  their  edges,  like  a  mosaic  pave- 
ment. In  the  Palaeozoic  sea-urchins  the  number  of  rows  of  plates 
is  either  more  or  less  than  twenty;  in  some  of  the  Silurian  genera 
the  plates  are  loosely  fitted,  and  slightly  overlapping,  like  fish-scales. 


THE   LIFE   OF  THE   SILURIAN 


587 


Arthropoda.  —  Among  the  Crustacea  the  Trilobites  are  still 
numerous,  though  decidedly  less  so  than  they  were  in  the  Ordovi- 
cian;  they  represent,  for  the  most  part,  new  species  of  genera 
which  have  survived  from  the  preceding  period.  The  common- 
est genera  are  Calymmene,  Illcenus,  Dalmanites  Lichas  (VIII,  19), 
Phacops,  Proetus,  Encrinums,  etc.  Especially  characteristic 
are  the  genera  Staurocephalus  (VIII,  20)  and  Deiphon  (VIII,  21) 
and  the  large  number  of 
spiny  forms.  Eurypterids 
continue  to  increase  in 
numbers  and  size,  though 
not  reaching  their  maxi- 
mum in  either  respect.  In 
these  extraordinary  Crus1 
tacea  the  head  is  quadrate 
and  is  followed  by  a  long, 
tapering  body,  composed 
of  thirteen  movable  seg- 
ments; the  last  segment  is 
either  a  pointed  spine,  as 
in  Euryptems  (see  Fig.  262), 
or  a  broad  tail-fin,  as  in 
Pterygotus.  Five  pairs  of 
appendages  are  attached  to 
the  head,  the  bases  of  four 
of  which,  on  each  side  of 
the  mouth,  form  the  jaws, 
as  in  the  existing  horse- 
shoe crab.  The  first  pair 
of  appendages  are  either  short  and  simple  (Eurypterus,  Stylonurus), 
or  are  much  elongated,  and  armed  with  pincers  (Pterygotus).  The 
fifth  pair  are  either  very  long,  or  enlarged  to  serve  as  swimming 
paddles.  The  first  body-segment  carries  a  pair  of  apron-like 
appendages,  with  a  narrow  median  extension,  but  the  other 
segments  have  no  appendages.  The  horse-shoe  crabs  find  their 


FlG.  262.  —  Eurypterus^  fischeri  Eichw.,  Island 
of  Oesel,  Russia.     (Schmidt) 


588 


THE  SILURIAN   PERIOD 


most  ancient  representative  in  the  genus  Hemiaspis  of  the  Euro, 
pean  Silurian.  The  Eurypterida  would  appear  not  to  have  been 
marine  animals  but  to  have  lived  in  the  freshening  lagoons  of 
the  upper  Salina  and  to  have  been  entombed  in  the  shoal-water 
calcareous  muds  of  the  Water-lime.  Other  Crustacea  are  much 
as  in  the  preceding  period. 

Scorpions  have  been  found  in  the  Silurian  of  Europe  and  Amer- 
ica, and  some  remains  of  Insects  in  the  former  continent.  These 
animals  prove  the  existence  of  a  contemporaneous  land  vegetation, 
and  confirm  the  doubtful  evidence  of  the  Ordovician  and  Silurian 
plants. 

Bryozoa  decline,  but  are  still  quite  abundant,  and  contribute 
in  an  important  way  to  the  growth  of  the  coral  reefs  as  well  as 
forming  reefs  by  themselves  in  the  Clin- 
ton and  Lockport  limestones,  but  they 
are  less  important  reef-builders  than 
they  had  been  in  the  Ordovician,  chiefly 
because  the  massive  kinds  are  so  largely 
replaced  by  delicate,  lace-like  colohies. 

Brachiopoda  continue  to  be  present 
in  multitudes,  but  with  a  distinct  change 
in  dominant  genera  from  those  which 
were  commonest  in  the  Ordovician. 
Especially  characteristic  is  the  increase 
in  the  families  of  the  Spiriferidce  and 
Pentameridce,  both  of  which  continue 
prominent  in  the  Devonian.  The  most 
important  genera  are  A  try  pa,  Spirifer 
(VIII,  9,  10),  Pentamerus  (VIII,  13), 
Rhynchotreta  (VIII,  12),  Leptana,  Strep- 
tis,  and  Whitfieldella.  A  curious  family 
of  Inarticulate  Brachiopods  is  the 
Trimerellidae,  several  forms  of  which  are  found  in  the  coral  reef 
dolomites  of  the  Guelph.  Monomer ella,  a  genus  of  this  family,  is 
figured  (VIII,  8). 


FlG.  263.  —  Lanarkia  sp.; 
a  primitive  Ostracodertn. 
(Traquair.)  Restored 


THE  LIFE  OF  THE   SILURIAN 


589 


Mollusca. — The  Bivalves  show  no  very  significant  changes 
from  the  Ordovician,  but  the  Gastropods,  especially  such  forms 
as  Capulus,  increase  decidedly;  other  well-represented  genera  of 
these  shells  are  Platyostoma,  Cydonema,  Strophostylus  (VIII,  14), 
and  Trematonotus  (VIII,  15).  Pteropods  are  smaller  and  less 
numerous  than  before;  a  very  common  form  is  the  little  nail- 
shaped  shell,  Tentaculites  (VIII,  7),  which  is  doubtfully  referred 
to  this  group,  but  may  belong  to  the  Worms.  Among  the  Cepha- 
lopods  Orthoceratites  are  less  frequent  than  in  the  Ordovician, 
but  are  more  commonly  ornamented  by  rings  or  longitudinal 
ridges  (VIII,  16),  and  Endoceras  has  disappeared,  while  low  tur- 
reted  shells,  like  Trochoceras  (VIII,  17),  are  characteristic.  The 
shells  with  curiously  contracted  mouth  openings,  like  Phragmo- 
ceras,  are  more  commonly  found  than  in  the  Ordovician. 

Vertebrata. — The  remains  of  Ostracoderms  of  a  primitive 
kind  have  been  found  in  the  uppermost  Silurian  of  Scotland; 
they  are  of  small  size  and  very  peculiar  appearance,  but  are  re- 
lated to  the  genera  which  were  to  attain  such  prominence  in  the 
Devonian.  Sharks,  doubtless  of  extremely  primitive  character, 
existed  in  the  Silurian  seas,  but  very  little  is  known  about  them. 


FIG.  264.  —  Upper  figure,  Birkenia  etegans  Traq.,  X  8/2'     Lower  figure,  Lasanius pro6- 
lemct(ict(S  Traq.,  enlarged-    Both  figures  restored.     (Traquair) 


CHAPTER    XXIX 


THE   DEVONIAN   PERIOD 


DEVONIAN   SYSTEM 


RHINE-BELGIUM  SECTION 


NEW  YORK 


' 

Chautauquan 

Chemung  and 

Cypridina  Slates 

Series 

Catskill  Stage 

Upper      ^ 

Clymenia  Limestone 

Devonian 

Senecan        (PcrtaSe  StaSe 

Series          1  Genesee  StaSe 

Cuboides  Zone 

|_Tully  Stage 

Stringocephalus 
Limestone 

Erian          (  Hamilton  Stage 
Series          (  Marcellus  Stage 

Middle 

Calceola 
Beds 

Devonian 

Ulsterian      {Onondaga  Stage 

<.,    .             i  Schohane  Stage 

[Esopus  Stage 

• 

Oriskanian 

Oriskany  Stage 

Coblenzian 

Series 

Series 

Port  Ewen  Stage 

Gedinnian 

i      Lower 
Devonian 

Helderbergian 
Series 

Becraft  Stage 
New  Scotland 

Series 

Stage 

- 

Coeymans  Stage 

THE  name  Devonian,  taken  from  the  English  county  Devon- 
shire, was  proposed  by  Sedgwick  an.d  Murchison  in  1839;  it  has 
found  universal  acceptance  and  has  passed  into  the  geological 
literature  of  all  languages. 

As  in  the  Ordovician  and  Silurian,  and  for  the  same  reason,  the 
divisions  of  the  New  York  Devonian  are  taken  as  the  standard  of 

59° 


DISTRIBUTION   OF  THE  DEVONIAN  591 

reference  for  North  America,  but  the  general  standard  is  no 
longer  to  be  found  in  Great  Britain,  in  spite  of  the  first  description 
of  these  rocks  in  Devonshire.  This  is  because  these  rocks  in 
England  are  so  much  disturbed,  metamorphosed,  and  faulted, 
that  the  order  of  succession  among  them  could  not  be  determined 
until  the  Devonian  of  the  Rhine  and  Belgium,  the  largest  and 
best-developed  Devonian  area  of  western  Europe,  had  first  been 
made  out.  Hence,  the  Rhenish  section  is  widely  employed  as 
a  standard  for  the  different  continents. 

DISTRIBUTION  OF  THE  DEVONIAN 

American.  —  In  certain  areas,  notably  in  that  of  the  Cumber- 
land Basin,  the  transition  from  Silurian  to  Devonian  is  so  gradual 
that  the  boundary  between  them  remained  long  in  doubt  and  has 
been  shifted  more  than  once.  The  Helderbergian  series  has, 
until  quite  recently,  been  very  generally  referred  to  the  Silurian, 
and  at  one  time  even  the  Oriskanian  was  included  in  the  same 
system. 

At  the  beginning  of  the  Devonian  (Helderbergian  epoch)  most 
of  the  continent  west  of  the  Cumberland  Basin  was  land,  but  the 
Basin  itself  continued  to  deepen  and  enlarge,  and  subsidence  in 
the  South  brought  the  sea  in  over  western  Tennessee  into  Mis- 
souri and  southern  Illinois,  reaching  into  Oklahoma.  The  lime- 
stones of  the  Helderbergian  series,  which  were  laid  down  in  the 
Cumberland  Basin,  extend,  with  some  interruptions,  from  south- 
western Virginia  along  the  line  of  the  Appalachians  to  Albany,  N.  Y. 
The  Gaspe  Peninsula,  which  forms  part  of  the  west  coast  of  the 
Gulf  of  St.  Lawrence,  was  submerged  at  this  time,  for  here  we 
find  1500  feet  of  limestone  representing  the  Helderbergian  and 
Oriskanian  series.  "  The  St.  Lawrence  tidal  waters  of  this  period 
must  have  extended  westward  to  the  border  of  Vermont  and 
Montreal  and  southward  along  the  Connecticut  valley."  (Dana.) 
Northern  and  southern  New  Brunswick,  northern  Nova  Scotia, 
northern  Maine  and  part  of  its  coast  were  under  the  Helder- 


THE  DEVONIAN  PERIOD 


FIG  265.  -  Map  of  North  America  in  the  Devonian  period.     Black  areas  =  known 
exposures;  white  areas  =  land,  or  unknown;  horizontal  lines  =  sea 


DISTRIBUTION  O*    THE  DEVONIAN  593 

bergian  sea,  and  the  New  England  troughs,  the  Connecticut 
valley,  and  the  Gaspe- Worcester  trough  were  submerged.  In 
the  West  the  Helderbergian  has  been  identified  only  in  the  Nevada 
trough,  where  marine  sedimentation  continued  uninterruptedly 
from  the  Silurian,  while  in  the  far  North  on  the  shore  of  Kennedy 
Channel,  80°  N.  lat.,  Helderbergian  fossils  have  been  found. 

The  Oriskanian  epoch  witnessed  some  geographical  changes. 
The  rocks  of  this  series,  which  are  prevailingly  sandstones  laid 
down  in  the  Cumberland  Basin,  are  thickest  in  western  Maryland, 
and  thin  away  both  north  and  south  from  that  region;  to  the 
south  the  beds  are  chiefly  lower  Oriskany  and  to  the  north  mostly 
upper  Oriskany,  indicating  oscillations  of  the  Basin  floor.  In 
central  New  York  the  Oriskanian  waters  extended  themselves 
westward  across  the  state,  breaking  through  the  barrier  in  the 
southeast.  In  the  Mississippi  valley,  the  Oriskany  has  about 
the  same  limits  as  the  Helderbergian,  but  it  has  not  been  found 
in  Oklahoma,  while,  on  the  other  hand,  the  sea  transgressed  over 
northern  Georgia  and  Alabama,  where  the  Oriskany  sandstone, 
only  20  feet  thick,  successively  overlaps  the  older  formations  from 
the  Middle  Cambrian  to  the  late  Ordovician.  In  addition  to  the 
Oriskanian  limestones  of  the  Gaspe  Peninsula,  rocks  of  this  stage 
are  found  in  Nova  Scotia  and  New  Brunswick  and  northern  Maine, 
in  which  State  they  are  enormously  thick,  5000  feet. 

The  Lower  Devonian  of  the  Maritime  Provinces,  and  especially 
of  Maine,  shows  clear  indications  of  an  invasion  of  the  Coblenzian 
fauna  of  Europe,  from  which  may  be  inferred  the  existence  of 
a  land  bridge  across  the  North  Atlantic,  affording  the  necessary 
conditions  for  the  migration  of  the  shoal-water  animals.  "The 
evidence  then  is  fairly  conclusive  that  during  the  period  repre- 
sented by  the  Coblenzian-Oriskany,  the  arenaceous  epicontinental 
sediments  was  the  ground  traversed  by  the  Coblenz  fauna  west- 
ward along  the  North  Atlantic  continent."  (Clarke.) 

Very  extensive  changes  characterized  the  Middle  Devonian; 
the  Cumberland  Basin  was  elevated  into  land  at  the  end  of  the 
Oriskany,  and  in  eastern  New  York  we  have  only  the  coarse  sands 
2  o 


594  THE   DEVONIAN   PERIOD 

and  grits  of  the  Esopus  (which  may  be  a  phase  of  the  Oriskany) 
and  the  Schoharie,  but  the  Interior  Sea  was  once  more  established, 
with  a  restricted  area,  and  not  improbably  connecting  with  the 
Gulf  of  St.  Lawrence  by  way  of  the  Connecticut  valley  trough. 
In  it  was  accumulated  the  great  Onondaga  limestone,  which 
extends  from  the  Hudson  River  across  New  York  into  Michigan, 
and  around  what  may  have  been  the  islands  of  the  Cincinnati 
anticline  into  Indiana,  Illinois,  and  Kentucky.  Interpretations 
differ  concerning  the  form  and  size  of  this  Mississippi  valley  sea; 
according  to  one  view,  it  extended  southward  to  the  Gulf  of  Mexico, 
while  another  makes  it  completely  enclosed  on  the  south  and 
postulates  a  northern  extension  across  Hudson's  Bay  to  the  Arctic 
Sea.  Another  great  invasion,  called  the  Dakota  Sea,  which  may 
have  had  no  direct  communication  with  the  former,  opened  from 
the  Arctic  Ocean,  where  now  is  the  mouth  of  the  Mackenzie 
River,  and  crossed  British  America,  the  Dakotas,  Nebraska, 
and  Kansas,  sending  a  gulf  into  Iowa,  Minnesota,  and  northern 
Illinois  and  southern  Wisconsin,  and  reaching  or  perhaps  passing 
through  Texas.  "This  channel  was  bounded  on  the  west  by  the 
extensive  Archaean  islands  or  edges  of  land  constituting  the  eastern 
axis  of  the  present  Rocky  Mountains."  (Williams.)  However, 
another  interpretation  separates  the  Dakota  Sea  from  the  Mississip- 
pian  by  the  width  of  the  Great  Plains,  with  a  possible  cross-con- 
nection in  southern  Canada,  and  includes  all  the  eastern  Onondaga 
in  the  Interior  Sea.  A  third  sea,  the  Cordilleran,  covered  much 
of  the  Great  Basin,  probably  extending  to  the  Pacific  and  con- 
necting on  the  north  with  the  Dakota  Sea.  In  the  northeastern 
part  of  the  continent  the  Gaspe  area  was  converted  into  a  coastal 
lagoon,  into  which  great  masses  of  sand  were  swept  by  rapid 
streams;  these  sands  contain  numerous  land  plants.  A  remnant 
of  one  of  the  old  channels  is  marked  by  a  coralline  limestone  in 
northern  Vermont. 

The  Onondaga  limestone  is  largely  a  coral  formation,  and  in 
some  places  the  reefs  are  still  beautifully  preserved,  as  in  the 
famous  example  at  the  Falls  of  the  Ohio,  above  Louisville,  Ky. 


DISTRIBUTION   OF  THE  DEVONIAN  595 

Limestone  formation  on  such  a  scale  implies  the  very  general 
absence  of  terrigenous  sediments  from  the  epicontinental  seas^ 
which  may  be  interpreted  as  due  in  part  to  the  base-levelling 
of  the  surrounding  lands. 

A  change  of  conditions  in  the  northeastern  bay  of  the  Interior 
Sea,  probably  the  elevation  of  the  land  and  resulting  rejuvenation 
of  the  streams,  checked  the  accumulation  of  limestone  and  brought 
in  great  quantities  of  mud  and  silt  (Marcellus  and  Hamilton), 
though  in  the  Mississippi  valley  limestone-making  still  went  on, 
and  even  in  New  York  thin  limestones  occur  here  and  there  in 
the  great  mass  of  the  Hamilton  shales.  At  Gaspe  continen- 
tal sedimentation  continued,  and  there  the  Erian  series  is  repre- 
sented by  a  very  thick  mass  of  sandstones,  bearing  fossils  of  land 
plants,  and  showing  occasional  brief  incursions  of  the  sea.  .  In 
New  Brunswick  and  Nova  Scotia  are  sandstones  and  shales. 
Western  Maryland  and  the  adjoining  parts  of  the  Virginias  were 
again  submerged,  but  this  time  apparently  by  an  expansion  of 
the  Interior  Sea,  that  seems  to  have  covered  part  of  the  old  Cum- 
berland Basin,  which  had  emerged  at  the  end  of  the  Oriskanian. 
In  the  West  the  seas  remained  much  as  they  had  been  during 
the  Onondaga;  indeed,  the  difference  in  interpretations  of  the 
latter  stage,  referred  to  on  the  preceding  page,  is  chiefly  a  question 
of  distinguishing  Hamilton  from  Onondaga.  A  depression  sub- 
merged the  coast  of  British  Columbia  up  to  Alaska. 

The  Upper  Devonian  has  much  the  same  distribution  as  the 
Middle,  but  the  Interior  Sea  appears  to  have  lost  its  connection 
with  the  Gulf  of  Mexico  and  to  have  become  joined  with  the 
Dakota  Sea,  while  here  and  there  in  the  Mississippi  valley,  the 
Upper  Devonian  overlaps  older  rocks  where  the  Lower  and 
Middle  are  absent.  The  Tully  stage  is  a  locally  developed  lime- 
stone in  New  York,  which  is  of  interest  as  corresponding  to  the 
Cuboides  zone  in  the  Rhenish  section;  both  contain  the  very  wide- 
spread brachiopod  species,  Hypothyris  cuboides,  a  very  useful 
guide  or  index  fossil,  as  marking  the  base  of  the  Upper  Devonian. 
The  Genesee  is  a  mass  of  black  shales,  which  increases  in  thickness 


596  THE  DEVONIAN  PERIOD 

from  Lake  Erie  to  central  Pennsylvania,  where  it  is  300  feet  thick, 
and  is  succeeded  by  the  Portage,  which  is  largely  arenaceous. 
The  Portage  exceeds  1000  feet  in  thickness.  In  western  New 
York  both  the  Genesee  and  the  Portage  have  embedded  in 
them  a  limestone  which  carries  a  highly  interesting  assemblage 
of  animals  called  the  "  Naples  Fauna."  This  has  very  little  in 
common  with  that  of  the  Hamilton,  but  is  well  marked  in  many 
parts  of  the  world.  (Figs.  255-6,  pp.  522-3.)  "  The  migration  path 
of  this  pelagic  fauna  has  been  traced  toward  the  Northwest  through 
Manitoba  into  Siberia,  thence  through  Russia  into  Westphalia.  .  .  . 
In  New  York,  where  its  fauna  became  extensive,  it  was  alien  and 
short-lived."  (Clarke.) 

The  Chemung  is  a  great  mass  of  sandstones  and  conglomerates, 
which  reaches  its  maximum  thickness  (8000  feet)  in  Pennsyl- 
vania, thinning  greatly  to  the  westward.  Indeed,  the  stages 
of  the  Upper  Devonian  given  in  the  New  York  scale,  can  seldom 
be  recognized  except  in  that  State  and  in  Pennsylvania.  In  Ohio 
the  whole  Upper  Devonian  is  represented  by  the  Ohio  Shale, 
which  varies  from  300  to  2600  feet  in  thickness.  The  Catskill, 
which  was  originally  regarded  as  a  distinct  series,  is  a  very  thick 
mass  of  sandstones,  representing  a  facies  of  the  Upper  Devo- 
nian extending  through  the  Senecan  and  Chautauquan  epochs 
and,  in  Pennsylvania,  into  the  earliest  Carboniferous.  These 
beds  are  believed  to  have  been  accumulated  in  a  long  and  narrow 
estuary,  running  from  eastern  New  York  into  Pennsylvania, 
where  the  beds  reach  a  thickness  of  7500  feet,  and  containing  fresh 
water  in  part  of  its  course.  Areas  of  similar  rocks,  with  fresh  or 
brackish  water  fossils,  occur  in  the  Portage  and  Genesee  and 
represent  deposition  in  coastal  lagoons. 

The  Western  Devonian  indicates  such  a  different  succession 
of  physical  events,  that  its  subdivisions  can  seldom  be  correlated 
with  those  of  the  East.  Devonian  strata  are  not  known  to  underlie 
the  Great  Plains,  and  the  Front  Range  of  Colorado  appears  to 
have  been  a  land  area,  for  there  Carboniferous  strata  overlap 
and  rest  upon  Cambrian  and  Ordovician.  On  the  other  hand,  in 


DISTRIBUTION  OF  THE  DEVONIAN  597 

the  plateau  region,  from  Arizona  to  Montana  and  along  the 
Canadian  Rockies  are  many  Devonian  .outcrops,  indicating  that 
much  or  all  of  this  region  was  submerged  at  one  epoch  or  another 
of  the  period.  Oscillations  of  level  are  also  shown,  as  in  the 
Grand  Canon  region,  where  thin  and  worn  patches  of  Devonian, 
evidently  remnants  of  a  much  thicker  and  more  widespread  mass, 
lie  unconformably  upon  Cambrian  and  below  Carboniferous 
beds..  In  the  Wasatch  Mountains  of  eastern  Utah  the  Devonian 
is  represented  by  2400  feet  of  quartzites  and  limestones,  and  in 
the  Nevada  trough,  where  deposition  seems  to  have  been  unbroken, 
2000  feet  of  shale  and  6000  feet  of  limestone  are  assigned  to  this 
system.  Though  the  faunas  differ  notably  from  those  of  the  East 
and  have  more  affinity  with  those  of  Europe  and  Asia,  correspond- 
ences with  the  Helderberg,  Onondaga,  and  Hamilton  have  been 
observed.  Affinities  with  the  Old  World  are  also  shown  by  the 
Devonian  of  northern  and  southern  California,  British  Columbia, 
and  Alaska. 

Like  the  rocks  of  the  other  Palaeozoic  systems  in  North  America, 
those  of  the  Devonian  are  quite  free  from  igneous  intrusions, 
except  in  several  widely  separated  localities.  Contemporaneous 
lava  flows  are  interbedded  with  the  Lower  Devonian  shales  of 
northern.  New  Brunswick,  pointing  to  volcanic  eruptions  in  that 
region,  and  the  same  is  true  regarding  the  Devonian  of  Nova 
Scotia,  and  that  of  northern  California  has  beds  of  tuff  and  lava 
sheets.  In  central  New  York  and  various  places  in  the  West  the 
Devonian  strata  are  cut  by  intrusions,  but  these  may  be  post- 
Devonian  in  date. 

Comparing  the  rocks  of  the  Ordovician,  Silurian,  and  Devonian, 
as  these  are  developed  in  the  Appalachian  and  adjoining  regions, 
a  certain  rhythmic  or  periodic  recurrence  of  events  may  be  dis- 
covered among  them.  Each  system  is  characterized  by  a  great 
and  very  widespread  limestone  formation,  the  Trenton,  Lockport- 
Guelph,  and  Onondaga,  respectively,  and  in  each  the  limestone 
is  succeeded  by  shales  or  other  clastic  rocks,  the  Utica,  Salina,  and 
Portage,  due  to  an  increase  of  terrigenous  material,  and  each  was 


THE  DEVONIAN   PERIOD 

closed  by  a  more  or  less  widespread  emergence  of  the  sea-bottom. 
Each  began  with  a  subsidence  which  gradually  extended  to  a 
maximum  at  the  time  when  the  great  limestone  was  formed. 
The  parallelisms  are  not  exact,  but  they  are  certainly  suggestive. 

Foreign.  — The  European  Devonian  appears  in  three  different 
fades;  one  of  these  is  the  "  Old  Red  Sandstone,"  which  is  largely 
of  continental  origin,  and  lies  to  the  north  The  second  facies  is 
of  marine,  shoal-water  deposits  and  runs  from  Devonshire,  through 
Belgium,  the  northern  part  of  the  lower  Rhenish  and  the  Hartz 
Mountains,  to  Poland;  and  the  third,  extending  from  northwestern 
France,  through  Germany  to  Bohemia,  was  laid  down  in  deeper 
water.  On  the  other  hand,  great  changes  took  place  in  the  extent 
and  depth  of  the  Devonian  seas,  those  of  the  Lower  Devonian 
being  far  less  extensive  than  those  of  the  Middle  and  Upper  parts 
of  the  period,  as  is  also  true  of  North  America. 

The  period  began  in  Europe  with  an  advance  of  the  sea  over 
the  land  in  many  places,  reaching  its  maximum  extension  in  the 
latter  part  of  the  period,  but  beginning  to  retire  before  the  open- 
ing of  the  Carboniferous.  This  subsidence  removed  the  barrier 
which  in  Ordovician  and  Silurian  times  had  separated  the  northern 
and  southern  seas,  but  was  accompanied  by  the  formation  of 
closed  basins  farther  to  the  north.  Europe  then  was  largely  an 
open  sea  with  many  islands,  and  where  the  waters  were  sufficiently 
clear  and  free  from  terrigenous  sediment,  coral  reefs  were  ex- 
tensively formed. 

The  marine  Devonian  occurs  in  the  southwest  of  England, 
over  large  areas  of  Germany,  in  northwestern  and  southern  France, 
and  on  an  enormous  scale  in  Russia.  During  the  Silurian  the  sea 
had  withdrawn  almost  entirely  from  Russia  west  of  the  Ural 
Mountains.  In  the  Lower  Devonian  the  sea  broke  in  from  the 
north  over  Siberia,  reaching  far  into  central  Asia.  In  the  Middle 
Devonian  a  great  basin  was  formed  by  the  depression  of  central 
Russia,  the  sea  advancing  from  the  north  and  the  east.  Devonian 
limestones  and  great  coral  reefs  occur  in  the  Alps,  as  do  lime- 
stones, shales,  and  sandstones  of  the  same  period  in  Spain  and 
Portugal. 


THE   LIFE  OF  THE  DEVONIAN  599 

The  "  Old  Red  Sandstone  "  is  of  particular  interest,  because, 
owing  to  the  peculiar  circumstances  of  its  formation,  it  has  pre- 
served a  record  of  Devonian  land  life,  which,  though  fragmentary, 
is  far  more  complete  than  anything  we  possess  from  the  more 
ancient  periods.  These  strata  were  laid  down  in  closed  basins 
(sometimes,  perhaps,  in  fresh-water  lakes),  which  had  only  a 
restricted  communication  with  the  sea,  and  it  may  be  that  these 
accumulations  were  partly  made  by  the  wind,  though  there  is  no 
gypsum  or  salt  in  the  beds  to  indicate  the  prevalence  of  desert 
conditions.  The  Old  Red  is  found  in  south  Wales  and  the  adjoin- 
ing part  of  England,  and  on  a  much  larger  scale  in  Scotland; 
also  in  the  Baltic  provinces  of  Russia,  where  it  is  interstratified 
with  beds  of  the  marine  Devonian;  in  Spitzbergen  and  Green- 
land the  same  formation  recurs.  These  sandstones  are  said  to  be 
10,000  feet  thick,  but  according  to  some  authorities,  the  lower- 
most part  of  them  is  Silurian.  The  Catskill  of  New  York  is  very 
like  the  Old  Red,  and  contains  similar  fossils,  and  the  Old  Red 
facies  is  found  in  northern  New  Brunswick  on  Chaleur  Bay. 

The  European  Devonian  is  full  of  the  evidences  of  volcanic 
activity,  in  the  shape  of  great  lava-flows  and  tuffs.  In  central 
Scotland  the  volcanic  accumulations  exceed  6000  feet  in  thickness. 
•  Besides  the  Devonian  areas  already  mentioned  in  Asia,  rocks 
of  this  system  are  found  in  China,  the  Altai,  and  in  Asia  Minor. 
They  recur  in  northern  Africa.  The  Bokkeveld  beds  of  South 
Africa  are  among  the  rare  marine  formations  of  that  region,  and 
these,  which  are  Lower  Devonian,  thin  away  northward  and  die 
out  within  a  hundred  miles  of  the  coast.  Below  the  Bokkeveld, 
in  the  upper  part  of  the  Table  Mountain  sandstone,  is  a  boulder 
clay  with  striated  boulders  of  evidently  glacial  origin,  pointing 
to  the  establishment  of  rigorous  climatic  conditions  in  South 
Africa  in  the  earliest  Devonian,  or  perhaps  late  Silurian.  In 
South  America  occurred  a  great  transgression  of  the  sea,  and 
Devonian  strata  form  larger  areas  of  the  surface  than  those  of 
any  other  Palaeozoic  system.  Shallow-water  deposits  are  found 
in  Peru,  Bolivia,  over  large  parts  of  Brazil,  especially  the  basin 


600  THE   DEVONIAN   PERIOD 

of  the  Amazon,  and  in  the  Falkland  Islands.    The  Bolivian  Devo- . 
nian,  which  belongs  to  the  lower  and  middle  parts  of  the  system, 
contains  a  very  similar  fauna  to  that  of  North  America,  and  connects 
the  latter  with  Brazil,  the  Falkland  Islands,  and  South  Africa. 

Kayser  distinguishes  two  great  marine  provinces  during  the 
Devonian:  (i)  the  Eurasian,  which  extended  from  Europe  east- 
ward over  northern  and  central  Asia  to  Manitoba,  Canada,  and 
(2)  the  American,  which  reached  from  the  United  States  to  South 
America  and  thence  to  South  Africa. 

Climate.  —  With  the  exception  of  South  Africa,  the  distribu- 
tion of  Devonian  fossils  leads  us  to  infer  that  the  climate  of  the 
period  was,  like  that  of  the  Ordovician  and  Silurian,  generally 
uniform  over  the  earth  and  without  distinction  of  zones. 

DEVONIAN  LIFE 

The  life  of  the  Devonian  is,  in  its  larger  outlines,  very  like  that 
of  the  Silurian,  but  with  many  significant  differences,  which  are 
due,  on  the  one  hand,  to  the  dying  out  of  several  of  the  older 
groups  of  animals,  and  on  the  other,  to  the  great  expansion  of 
forms  which  in  the  Silurian  had  played  but  a  subordinate  role. 

Plants.  —  The  fossils  show  that  in  Devonian  times  the  land  was 
already  clothed  with  a  varied,  rich,  and  luxuriant  vegetation  of 
the  same  general  type  as  that  whose  scanty  traces  are  found  in 
Silurian  strata.  All  the  higher  Cryptogams  are  represented,  and 
by  large,  tree-like  forms,  as  well  as  by  small  herbaceous  plants. 
The  bulk  of  the  flora  is  composed  of  Ferns,  Lycopods,  or  Club- 
Mosses  (especially  the  great  tree-like  Lepidodendrids) ,  and  Equi- 
setales,  or  Horsetails.  The  highly  important  extinct  groups,  the 
Sphenophyllales  and  Cycadofilices,  which  were  already  in  existence, 
will  be  described  from  the  more  complete  Carboniferous  fossils. 
Besides  these  Cryptogams,  we  find  representatives  of  the  lower 

Helderberg.  10,  Tropidoleptus  carinatns  Conr,  X  %,  Hamilton.  ii,Ckonetes  coronatus 
Conr  ,  x  %,  Hamilton.  12,  Stropheodonta  demissa  Conr.,  x  %,  Hamilton.  13,  Pterinea 
flabellum  Conr.jX1/^,  Hamilton.  14,  Platyceras  dumosnm  Conr.,  x  %,  Onondaga. 
15,  Manticoceras  oxy  Clarke,  x  1/6,  Portage.  16,  Phacops  rana  Green,  x  V4,  Hamilton. 
17,  Odontocephalus selenurns  Eaton,  x  V4,  Onondaga.  18,  Terataspis  grand iSttati,  x  Vi2» 
Schoharie.  19,  Echinocaris punctatus  Hall,  x  %,  Hamilton. 


w 

PLATE  IX.  — DEVONIAN  FOSSILS 

Fig.  i,  Edriocrinus  sacculus  Hall,  x  %,  Oriskany.  2,  Nucleocrinus  verneuli  Troost, 
X  i,  Onondaga.  3,  Sprifer  tnacropleurns  Conr  ,  X  ^,  Helderberg.  4,  S.  disjunctus 
Sowerby,  X  %;  internal  cast  of  ventral  valve,  Chemung.  5,  S.  mucronatus  Conr.,  x  %; 
inner  view  of  dorsal  valve,  showing  arm-supports,  Hamilton.  6,  Vitulina  pustulosa  Hall, 
x  :i/2,  Hamilton.  7,  Atrypa  reticularis  Linn.,  x  %,  Hamilton.  8,  Hypothyris  vt nustula 
Half,  x  %,  Tully.  8 a,  The  same,  anterior  view.  9,  Gypidula  galeata  Dalman,  X  ^ 


602 


THE  DEVONIAN   PERIOD 


kinds  of  flowering  plants  jn  the  Gymnosperms,  which  presumably 
grew  upon  the  higher  lands.  We  shall  meet  this  same  flora  in 
richer  and  more  varied  display  in  the  Carboniferous  period. 

Sponges  are  conspicuous  elements  of  the  Devonian  fauna,  as, 
for  example,  the  Dictyospongida  in  the  Chemung. 

Crelenterata.  —  The  Graptolites,  which  were  so  abundant  in 
the  Ordovician  and  had  become  much  less  common  in  the  Silu- 
rian, are  now  almost  extinct,  only  a  few  simple 
species  occurring  in  the  Lower  Devonian.  The 
Corals,  on  the  contrary,  expand  and  multiply 
enormously  both  in  numbers  and  in  size.  Most 
of  the  Silurian  genera  persist  (though  the  chain- 
coral  Holy  sites  has  become  extinct),  and  many 
new  forms  are  added.  Heliophyllum  (Fig.  266) 
is  an  example  of  the  solitary  corals,  and  Phittips- 
astrcea  and  Acervularia  (Fig.  267)  of  the  reef- 
builders. 

Echinodermata. — The  Cystoids  have  become 
much  rarer  than  before,  and  are  on  the  point  of 
extinction;  the  Blastoids  (PI.  IX,  Fig.  2)  are  still 
in  the  background,  though  locally  abundant  in  a  few  places,  and 
the  Echinoids  have  not  yet  become  common;  but  the  Crinoids 
and  Star-fishes  have  greatly  in- 
creased in  number  and  variety. 
Important  genera  of  the  former 
group  are  Cupressocrinus,  Platy- 
crinus,  Actinocrinus ,  Dolatocrinus, 
etc.  Edriocrinus  (IX,  i),  a  free- 
swimming  crinoid,  without  a  stem, 
occurs  in  the  Helderbergian  and 
Oriskany.  The  multitude  of  the 
crinoids  contributed  largely  to  the  FIG. 
building  up  of  the  calcareous  sea- 
bottom  on  which  they  flourished. 
Arthropoda. — The  Trilobites  had  already  begun  to  decline 


FIG.  266.  —  He- 
liophyllum hal- 
li  E.  &  H.,  X 
%.  Hamilton 
of  Michigan. 
(Rominger) 


.  —  Acervularia  davidsom 
E.  &  H.,  X  %.  Hamilton  of  Michi- 
gan. (Rominger) 


THE   LIFE  OF  THE  DEVONIAN  603 

in  the  Silurian,  while  in  the  Devonian  the  decline  becomes  much 
more  marked,  though  they  are  still  far  from  rare.  New  species 
of  Silurian  genera,  like  Phacops  (IX,  16),  Homalonotus,  Lichas, 
Addas  pis,  Odontocephalus  (IX,  17),  etc.,  are  the  commonest. 
Terataspis  (IX,  18),  is  one  of  the  largest  and  most  curious  of 
Trilobites.  A  characteristic  of  the  Devonian  Trilobites  is  the 
ornamentation  of  the  margin  of  the  head,  as  well  as  the  extra- 
ordinary development  of  spines  which  many  display  on  the  head- 
and  tail-shields.  (IX,  17,  18.) 

The  other  Crustacea  make  notable  progress  in  this  period. 
The  Phyllocarida  are  abundant  in  the  Upper  and  Middle  Devonian. 
The  first  of  the  Isopoda  and  of  the  long-tailed  Schizopoda 
make  their  appearance  in  the  Devonian.  The  Eurypterids 
now  attain  their  culmination  in  size,  being  actually  gigantic 
for  Crustacea,  and  some  of  them  are  as  much  as  six  feet  long. 
The  genera  (Eurypterus,  Stylonurus,  and  Pterygotus)  are  the 
same  as  in  the  Silurian.  Insects,  though  still  rare  as  fossils,  are 
very  much  commoner  than 'in  the  Silurian;  they  represent  both 
Orthopters  and  Neuropters,  which  are  among  the  primitive  groups. 

Brachiopoda.  —  As  in  the  Silurian,  Brachiopods  continue  to 
be  the  most  abundant  fossils,  both  in  species  and  individuals; 
indeed,  the  Devonian  is  the  culminating  period  of  brachiopod 
abundance  and  variety;  in  North  America  at  least  it  has  many 
more  genera  than  any  other.  Many  Silurian  genera  have  died 
out,  but  of  others,  like  Orthis,  Stropheodonta  (IX,  12),  Ortho- 
thetes,  Atrypa  (IX,  7),  Chonetes  (IX,  n),  and  Productella,  the 
species  are  more  numerous.  The  most  characteristic  shells  are 
those  belonging  to  the  genera  Spirifer,  especially  the  very  broad 
"winged"  species  (IX,  4,  5),  Hypothyris  (IX,  8),  Athyris,  Gypi- 
dula  (IX,  9),  Tropidoleptus  (IX,  10),  Vitulina  (IX,  6),  and  those 
belonging  to  the  still  existing  family  Terebratulida,  of  which 
Renssellaeria  and  Stringocephalus  are  Devonian  genera. 

Mollusca.  —  Bivalves  and  Gastropods  are  much  as  in  the  Silu- 
rian: examples  of  the  former  are  Pterinea  (IX,  13),  Conocar- 
ditim,  and  Grammysia,  while  larger  species  of  the  Gastropod 


604  THE  DEVONIAN   PERIOD 

Euomphalus,  and  spiny  Platycerids  (IX,  14),  are  characteristic. 
A  minute  Pteropod,  Styliolina,  is  extraordinarily  abundant  in 
the  Upper  Devonian,  forming  limestone  masses.  The  Cephalo- 
pods  have  been  revolutionized;  the  wealth  of  Nautiloid  shells 
which  we  found  in  the  Silurian  has  been  much  diminished,  though 
Orthoceras,  Phragmoceras,  Gomphoceras,  and  Cyrtoceras  still  per- 
sist abundantly  in  the  Lower  Devonian,  while  many  other  genera 
have  disappeared.  More  significant  is  the  first  appearance  of  the 
Ammonoid  division  of  the  Tetrabranchiate  Cephalopods,  a  group 
of  shells  which  was  destined  to  attain  extraordinary  development 
in  the  Mesozoic  era.  The  Ammonpids  are  distinguished  by  the 
complexity  of  the  "  sutures,"  or  lines  made  by  the  junction  of 
the  septa  with  the  outer  wall  of  the  shell.  In  the  Devonian  Am- 
monoids,  of  which  the  Goniatites  (IX,  15)  are  the  common  forms, 
the  sutures  are  much  less  complex  than  in  the  Mesozoic  shells. 
Another  member  of  the  group  which  is  far  more  abundant  in 
Europe  than  in  America  is  Clymenia,  the  only  Ammonoid  in 
which  the  siphuncle  is  on  the  inner  side  of  the  spiral.  Bactrites 
has  a  straight  shell,  like  that  of  Orthoceras,  but  with  the  complex 
sutures  which  show  it  to  be  an  Ammonoid. 

Vertebrata.  —  One  of  the  most  characteristic  features  of  Devo- 
nian life  is  the  great  development  of  the  aquatic  Vertebrates, 
which  is  so  striking  that  the  period  is  often  called  the  "  Age  of 
Fishes."  So  numerous  and  so  finely  preserved  are  these  fossils 
that  a  satisfactory  account  may  be  given  of  the  structure  and 
systematic  position  of  many  of  the  genera.  This  great  assemblage 
of  fishes  and  fish-like  forms,  it  should  be  remembered,  was  not 
someth;ng  entirely  new  in  the  earth's  history,  but  was  rather  the 
wondenul  expansion  of  types  which  during  the  Ordovician  and 
Silurian  had  remained  rare  and  obscure. 

Of  the  Devonian  Vertebrates  none  are  more  peculiar  and  char- 
acteristic than  the  Ostracoderms,  which,  though  generally  called 
fishes,  really  belong  to  a  type  much  below  the  true  fishes,  being  de- 
void of  true  jaws  and  of  paired  fins.  The  head  and  more  or  less 
of  the  body  are  sheathed  in  heavy  plates  of  bone,  and  the  remainder 


THE  LIFE  OF  THE  DEVONIAN 


605 


of  the  body  and  the  tail  are  covered  with  scales.  No  trace  of  the 
internal  skeleton  is  preserved,  and  it  evidently  was  not  ossified. 
The  genus  Cephalaspis  of  this  group  is  curiously  like  a  Trilobite 
in  appearance,  though,  of  course,  the  resemblance  is  entirely 
superficial.  The  head-shield  is  formed  of  a  single  great  plate 
of  bone,  shaped  like  a  saddler's  knife,  with  rounded  front  edge 
and  with  the  hinder  angles  drawn  out  into  spines;  the  eyes  are 
on  the  top  of  the  head  and  very  close  together.  The  body  is 
covered  with  large,  angular  plates  of  bone,  arranged  in  rows; 
a  small  median  dorsal  fin  and  a  larger  triangular  tail-fin  make 
up  the  locomotor  apparatus. 


FIG.  268.  —  Pterichthys  testudinarius.     Restored.      Old  Red  Sandstone.      (Dean 
after  Smith  Woodward) 

Pteraspis  has  a  bony  plate  over  the  snout,  a  large  shield  on 
the  back,  and  another  on  the  belly,  with  rhomboidal  scales  cover- 
ing the  rest  of  the  body. 

A  most  extraordinary-looking  creature  is  Pterichthys  (Fig.  268), 
in  which  the  head  and  most  of  the  body  are  encased  in  heavy 
plates,  the  remainder  in  overlapping  scale-like  bones;  the  eyes  are 
even  closer  together  than  in  Cephalaspis.  Dorsal  and  tail-fins 
are  present  and  what  appear  to  be  pectoral  fins.  The  pair  of 
appendages  referred  to  doubtless  acted  as  fins,  but  they  are  not 
comparable  to  the  paired  fins  of  the  true  fishes,  being  merely 


6o6 


THE  DEVONIAN   PERIOD 


jointed  extensions  of  the  head-shield.  These  three  genera,  Cephal* 
aspis,  Pteraspis,  and  Pterichthys,  have  been  selected  as  types  of 
the  Ostracoderms,  each  one  of  which  has  several  allies,  differing 
from  it  in  one  or  other  particular. 

Of  the  true  Fishes  there  is  great  variety  in  the  Devonian.    The 
Selachians  are  well  represented,  one  of  which  is  Cladoselache 


FIG.  269.  —  Cladoselache  newberryi.     Restored,  X  Vs-     Ohio  shale.     (Dean) 


(Fig.  269),  a  small  shark,  from  two  to  six  feet  in  length,  and  the 
most  primitive  known  member  of  the  group.  The  Dipnoi,  or 
Lung-fishes,,  were  important  elements  of  the  Devonian  fish  fauna. 
Dipterus  (Fig.  270),  an  example  of  this  group,  is  very  like  the 
modern  lung-fishes,  which  have  dwindled  to  three  genera,  one 


FlG.  270.  —  Dipterus  valenciennesi  Sedg.  and  Murch.     Restored.     Old  Red  Sand- 
stone.    (Smith  Woodward  after  Traquair) 

in  South  America,  one  in  Africa,  and  one  in  Australia.  A  re- 
markable series  of  fishes,  the  Arthrodira,  is  very  characteristically 
Devonian.  One  of  the  best-known  members  of  this  group  is 
the  European  genus  Coccosteus  (Fig.  271),  in  which  the  head, 
back,  and  belly  are  covered  with  bony  plates,  but  the  rest  of  the 
body  is  naked.  This  bony  armour  gives  the  fish  something  of 


THE   LIFE  OF  THE  DEVONIAN 


607 


the  appearance  of  the  Ostracoderms,  with  which  group  it  is  often, 
though  erroneously,  classified.  The  backbone  is  represented  by 
an  unsegmented  rod  (the  notochord,  N,  Fig.  271),  to  which  arches 
of  bone  are  attached  (N,  H,  Fig.  271).  Paired  ventral  fins  were 
present,  but  pectorals  have  not  been  found.  The  jaws  were  pro- 


FiG.  271. —  Coccosteus  decipiens  Ag.    Restored.    Old  Red  Sandstone.    (Dean,  after 
Smith  Woodward) 

vided  with  teeth,  which  fuse  into  plates.  In  the  uppermost  Devo- 
nian of  Ohio  are  found  some  huge  fishes  allied  to  Coccosieus,  but 
much  larger  and  more  formidable.  Tne  most  important  of  these 
are  Dinichthys  and  Titanichthys,  the  latter  attaining  a  length  of 
25  feet. 


FIG.  272.  —  Holoptychius  nobilissimus  Ag.     Restored.     Old  Red.     (Smith  Wood- 
ward after  Traquair.)     The  ornamentation  of  the  scales  is  not  shown 

A  higher  type  of  Devonian  fish  is  that  of  the  Crossopterygii,  an 
ancient  group  of  which  but  two  representatives  remain  at  present, 
both  of  them  African.  These  fishes,  like  the  Dipnoans,  have 
"  lobate  "  paired  fins  (see  Fig.  272),  i.e.  the  part  of  the  fin  belong- 


608  THE  DEVONIAN   PERIOD 

ing  to  the  internal  skeleton  is  large  and  covered  with  scales,  form- 
ing a  lobe  to  which  the  fin-rays  are  attached.  Most  of  the  Devo- 
nian members  of  the  group  have  massive  rhomboidal  scales,  but 
in  others,  like  Holoptychius,  the  scales  are  thinner,  rounded,  and 
overlapping. 

The  most  advanced  fishes  of  the  period  are  the  Ganoid  mem- 
bers of  the  Actinopteri,  which  from  the  Devonian  until  nearly 
the  end  of  the  Mesozoic  era  continue  to  be  the  dominant  type  of 
fishes.  Nearly  all  of  them  have  thick,  shining  scales  of  rhom- 
boidal shape. 

The  Devonian  fish  fauna  (using  that  term  in  a  very  compre- 
hensive sense)  is  thus  seen  to  be  a  rich  and  varied  one,  includ- 
ing Ostracoderms,  Sharks,  Lung-fishes,  Arthrodirans,  Crossop- 
terygians  and  Actinopteri,  each  with  many  representatives  and 
mostly  of  very  curious  and  bizarre  forms.  While  thus  varied 
and  plentiful,  this  assemblage  differs  from  the  modern  fish  fauna 
in  the  primitive  character  of  the  groups  which  are  represented, 
and  in  the  entire  absence  of  the  Bony  Fishes  (Teleosts),  which 
now  make  up  the  vast  majority  of  fishes,  both  fresh-water  and 
marine. 

Amphibia.  —  Certain  footprints  which  have  been  reported  from 
the  Upper  Devonian  of  Pennsylvania,  show  that  the  Amphibia, 
the  lowest  of  air-breathing  vertebrates,  had  already  begun  their 
career;  that  is,  if  the  correlation  of  the  rocks  in  which  these  foot- 
prints occur  has  been  correctly  made. 


CHAPTER   XXX 
THE   CARBONIFEROUS   PERIOD 

THE  name  Carboniferous  was  given  in  the  early  part  of  the 
last  century,  when  it  was  supposed  that  every  geological  system 
was  characterized  by  the  presence  of  some  peculiar  kind  of  rock. 
We  now  know  that  this  conception  is  erroneous,  and  that  work- 
able coal  seams  have  been  formed  in  all  the  periods  since  the 
Carboniferous.  It  still  remains  true,  however,  that  the  latter 
contains  much  the  most  important  share  of  the  world's  supply 
of  mineral  fuel,  upon  which  the  whole  fabric  of  modern  industrial 
civilization  is  founded.  The  great  economic  importance  of  the 
coal  measures  has  caused  them  to  be  most  carefully  surveyed  in 
all  civilized  lands,  a  process  greatly  assisted  by  the  innumerable 
shafts  and  borings  which  penetrate  these  rocks.  One  result  of 
this  gigantic  work  is,  that  the  history  and  life  of  the  Carbonifer- 
ous are  better  known  than  those  of  any  other  Palaeozoic  period, 
though  our  knowledge  is  still  very  far  from  complete. 

The  Carboniferous  rocks  are  displayed  in  very  different  aspects 
or  facies  in  the  various  parts  of  the  continent  and  even  in  contigu- 
ous regions.  New  York  no  longer  gives  the  standard  scale,  for 
that  state  has  very  little  that  is  newer  than  the  Devonian.  For 
the  eastern  part  of  the  country  the  sequence  of  strata  in  Penn- 
sylvania serves  as  the  scale  of  reference,  while  a  very  different 
one  is  needed  for  the  Mississippi  valley.  In  the  Rocky  Mountain 
region,  again,  the  character  of  deposition  deviated  markedly 
from  what  occurred  in  the  East,  and  all  over  the  far  West  the 
Carboniferous  is  almost  entirely  marine,  without  coal.  Even  in 
this  region,  however,  the  distinction  between  the  Lower  and 
Upper  Carboniferous  may  be  drawn.  The  following  table  gives 
?R  609 


6io 


THE  CARBONIFEROUS  PERIOD 


the  succession  in  England,  Pennsylvania,  and  the  middle  West, 
Illinois,  Missouri,  Iowa,  etc. 


ENGLAND 

Ardwick  Series 
Middle  Coal 
Gannister  Series 
Millstone  Grit 
Yoredale  Series 
Scaur  Limestone 

Series 
Limestone 
Shales 


CARBONIFEROUS   SYSTEM 


PENNSYLVANIA 

fMonongahela  Stage 
Upper      I  Conemaugh  Stage 
Carbonif.  1  Allegheny  Stage 
[Pottsville  Stage 

f  Mauch  Chunk 
Lower      I  '       Stage 
Carbonif.  j  (Greenbrier) 
tPocono  Stage 


MISSISSIPPI  VALLEY 


Pennsyl-   ( Coal  Measures 
vanian    1  Millstone  Grit 


Kaskaskia  Stage 
St.  Louis  Stage 

Osage      f 


Missis- 
sippian 


Stage 


<  Keokuk 


[  Burlington 
Kinderhook  Stage 


DISTRIBUTION  OF  THE  CARBONIFEROUS  ROCKS 

American.  —  The  Carboniferous  is  divisible  into  two  sharply 
marked  portions,  the  Lower,  or  Mississippian,  and  Upper,  or 
Pennsylvanian,  a  distinction  which  is  applicable  in  all  the  con- 
tinents in  which  the  strata  of  this  period  have  been  carefully 
studied. 

In  most  parts  of  North  America  the  Devonian  passed  so  grad- 
ually into  the  Carboniferous  that  there  is  no  definite  line  of  division 
between  them,  but  at  Gaspe,  and  in  Nova  Scotia,  New  Brunswick 
and  Maine,  there  was  a  time  of  upheaval  and  erosion  toward  the 
end  of  the  Devonian,  followed  by  a  depression,  in  consequence  of 
which  there  is  an  unconformity  between  the  two  systems.  When 
the  Carboniferous  period  began,  most  of  New  York,  New  England, 
and  eastern  Canada  were  above  sea-level,  but  the  Gulf  of  St. 
Lawrence  covered  western  Newfoundland,  most  of  New  Bruns- 
wick, and  part  of  Nova  Scotia. 

The  Interior  Sea  expanded  widely,  probably  covering  nearly 
the  whole  of  the  Great  Plains,  and  most  of  the  old  land  areas  of 
the  West  and  Southwest,  which  had  persisted  through  more  or 
less  of  the  Silurian  and  Devonian,  were  extensively  submerged, 


DISTRIBUTION   OF  THE  CARBONIFEROUS   ROCKS       6ll 


FIG.  273.  —  Map  of  North  America  in  the  Lower  Carboniferous.     Black  areas 
known  exposures ;  white  areas  =  land,  or  unknown ;  horizontal  lines  =  sea 


6l2  THE  CARBONIFEROUS   PERIOD 

probably  including  all  of  Mexico  and  the  northern  part  of  Central 
America.  West  of  the  Rocky  Mountains  the  Carboniferous  is 
much  the  most  widely  extended  of  any  of  the  Palaeozoic  systems, 
the  sea  reaching  through  British  Columbia,  on  both  sides  of  the 
Gold  Range  into  southeastern  Alaska. 

In  consequence  of  this  great  transgression,  the  Carboniferous 
strata  rest  unconformably,  or  sometimes  in  apparent  conformity, 
upon  all  the  older  systems  from  the  Pre-Cambrian  to  the  Devo- 
nian. The  Arctic  coast  of  Alaska  was  submerged  and  several 
islands  of  the  Arctic  Sea,  but  the  main  portion  of  Alaska,  which 
has  great  areas  of  Carboniferous  rocks,  appears  not  to  have  been 
invaded  by  the  sea  till  Upper  Carboniferous,  or  Pennsylvanian 
times.  According  to  Girty  none  of  the  known  Alaskan  faunas, 
except  from  the  north  coast,  "  can  be  confidently  referred  to  the 
Lower  Carboniferous.  The  typical  Mississippian  is  certainl)) 
absent  as  far  as  evidence  has  come  to  hand,  and  but  one  occur- 
rence of  a  fauna  definitely  related  to  the  Lower  Carboniferous 
of  California  (Baird)  has  been  found."  (Girty.)  The  fossils 
of  the  Baird  stage  in  California  are  entirely  different  from  those 
of  the  Interior  Sea,  and  if  actually  contemporaneous  with  the 
latter,  must  have  been  separated  from  them  by  some  barrier. 
The  Great  Basin  sea  appears  also  to  have  been  separated  for  a  time 
from  the  Interior  Sea,  but  communication  was  established  before 
the  end  of  the  Lower  Carboniferous.  Over  the  Central  States 
and  the  West,  the  Mississippian  rocks  are  almost  uniformly  lime- 
stones, showing  that  this  vast  sea  was  clear  and  free  from  terrige- 
nous sediments,  but  probably  of  moderate  depth. 

The  northeastern  portion  of  the  Interior  Sea  was  divided  by 
the  islands  of  the  Cincinnati  anticline  into  two  bays,  the  eastern 
one  of  which  covered  most  of  Ohio,  western  Pennsylvania  and 
Maryland;  and  the  western  bay  occupied  the  southern  peninsula 
of  Michigan  and  had  but  a  narrow  communication  with  the  first. 
The  Appalachian  valley  trough  has  in  its  middle  third  a  nearly 
or  quite  complete  succession  of  the  Devonian,  but  lacks  the  earliest 
Mississippian,  indicating  a  brief  elevation  of  this  region.  In  the 


DISTRIBUTION  OF  THE   CARBONIFEROUS   ROCKS       613 

South  the  eastern  edge  of  the  Interior  Sea  followed  the  line  of 
the  Appalachian  fold  probably  into  Virginia  and  there  broke 
across  the  barrier  and  sent  off  some  narrow  sounds  southward 
into  the  Appalachian  Valley. 

In  the  Acadian  province,  comprising  Nova  Scotia,  New  Bruns- 
wick and  Newfoundland,  the  Lower  Carboniferous  is  remark- 
ably like  that  of  Great  Britain.  In  Nova  Scotia,  the  Horton 
sandstone  corresponds  to  the  Calciferous  of  Scotland,  and  contains 
thin  seams  of  coal;  it  is  followed  by  the  Windsor  stage  of  marine 
limestone  (=  the  British  Scaur,  or  Mountain,  Limestone)  which 
contains  beds  of  gypsum.  The  presence  of  gypsum  shows  not 
only  that  occasionally  bodies  of  sea-water  were  shut  off  in  closed 
lagoons,  but  also  that  an  arid  climate  prevailed  in  the  region. 

In  eastern  Pennsylvania  the  Lower  Carboniferous  has  a  maxi- 
mum thickness  of  4000  feet,  but  thins  rapidly  southward  and 
westward.  The  Pocono  is  a  thick,  hard  sandstone,  which  caps 
many  of  the  mountain  ridges;  it  follows  down  the  Appalachian 
line,  thinning  as  it  goes.  The  area  of  maximum  sedimentation  may 
have  received  largely  continental  deposits.  Under  different  local 
names,  the  Pocono  extends  to  eastern  Kentucky.  The  Mauch 
Chunk  shales  form  the  remainder  of  the  Lower  Carboniferous 
in  northeastern  Pennsylvania,  where  the  thickest  portion  not 
improbably  represents  a  great  delta,  and  the  prevalence  of  sun 
cracks  is  indicative  of  an  arid  climate,  such  as  probably  prevailed 
in  Nova  Scotia.  In  Maryland,  West  Virginia,  and  Kentucky, 
part  of  this  series  is  composed  of  a  marine  limestone  (Greenbrier, 
Newman).  The  Lower  Carboniferous  of  Virginia  contains  some 
workable  beds  of  coal,  the  "  false  Coal  Measures."  Ohio  was  a 
region  of  slow  sedimentation;  here  the  Lower  Carboniferous  is 
formed  by  the  Waverly  series,  which  is  divisible  into  seven  stages, 
some  of  them  carrying  marine  faunas,  and  is  only  700  feet  thick. 
In  the  Michigan  bay  were  deposited  the  sandstones,  grits,  and 
shales  of  the  Marshall  series,  followed  by  shales  with  some  lime- 
stone and  gypsum,  whence  we  may  infer  that  the  bay  was  for  a 
time  converted  into  a  salt  lake  and  that  the  climate  was  dry,  as 


6 14  THE  CARBONIFEROUS  PERIOD 

in  Nova  Scotia  and  Pennsylvania.  The  bay  was  soon  again  in- 
vaded by  the  sea,  for  a  marine  limestone  overlies  the  gypsiferous 
beds. 

Southwest  of  these  more  or  less  completely  enclosed  bays,  the 
Interior  Sea  was  clear  and  free  from  terrigenous  material,  so  that 
in  it  were  deposited  great  masses  of  limestone  (1500  feet  maximum 
thickness)  formed  from  a  most  luxuriant  growth  of  corals,  brachi- 
opods  and  crinoids.  In  the  Central  States  many  different  stages 
and  substages  may  be  distinguished  in  these  limestones,  and 
evidences  are  not  wanting  of  fluctuating  shore-lines.  The  Kin- 
derhook  extends  farther  north  than  the  Osage,  while  the  St.  Louis 
sea  again  extended  northward.  The  Osage  series  is  remarkable 
for  the  extraordinary  abundance  and  variety  of  its  crinoids,  un- 
equalled, perhaps,  in  the  world.  This  produces  a  peculiar  facies 
which  is  nearly  confined  to  the  Mississippi  valley,  but  was  ex- 
tended to  the  southwest,  into  New  Mexico.  "  But  this  condition 
appears  not  to  have  invaded  other  western  parts  of  the  Missis- 
sippian  sea,  where  I  believe,  under  uniform  conditions,  the  Kin- 
derhook  faunas  persisted  through  Burlington  and  Keokuk  [i.e. 
Osage]  time  without  feeling,  save  in  a  subordinate  degree,  the 
influences  which  helped  to  differentiate  the  early  Mississippian 
faunas  of  the  Mississippi  Valley."  (Girty.) 

The  Lower  Carboniferous  was  brought  to  a  close  by  a  very 
widespread  upheaval,  which  removed  nearly  the  whole  Interior 
Sea  and  resulted  in  a  very  general  unconformity  between  the 
Mississippian  and  the  overlying  Pennsylvanian.  In  only  a  few 
areas  does  there  appear  to  be  a  continuity  of  sedimentation  be- 
tween them  and  in  some  of  these  there  is  reason  to  think  that  the 
conformity  is  apparent,  not  real.  The  Kaskaskia ,  faunas  are 
entirely  wanting  in  the  western  portion  of  the  Mississippian  sea, 
but  this  should  probably  be  interpreted  to  mean  that  the  upper- 
most beds  of  the  Lower  Carboniferous  were  stripped  away  by 
denudation  during  the  interval  between  the  two  formations,  when 
so  much  of  the  continent  was  land.  If  this  was  not  the  case,  the 
upheaval  must  have  affected  the  western  portion  of  the  Interior 


DISTRIBUTION  OF  THE  CARBONIFEROUS   ROCKS       6l$ 

Sea  considerably  before  it  drained  the  central  and  eastern  portions. 
Some  folding  accompanied  the  upheaval  in  certain  areas,  as  in 
Iowa  and  northeastern  Pennsylvania. 

After  a  time  of  quite  prolonged  erosion  over  a  great  part  of  the 
continent,  a  new  series  of  events  was  inaugurated.  In  Penn- 
sylvania an  orogenic  movement  took  place,  raising  the  low-lying 
mud  flats  of  the  Mauch  Chunk,  but  down-warping  their  eastern 
border  into  a  long  and  narrow  trough  which  extended  to  Alabama, 
and  in  this  trough  a  rapid  sedimentation  occurred,  forming  great 
bodies  of  gravel  and  sand,  the  Pottsville  stage.  In  southwestern 
Virginia  there  is,  apparently  at  least,  continuity  of  deposition 
from  the  Mauch  Chunk  into  the  Pottsville.  During  the  latter 
epoch  the  trough  continued  to  subside  under  its  accumulating 
load  of  sediment  and,  from  time  to  time,  to  transgress  westward, 
in  which  direction  the  higher  members  of  the  series  extend.  The 
subsidence  of  the  trough  was  intermittent,  and  fresh-water  peat- 
bogs were  established  upon  its  surface,  resulting  in  the  formation 
of  coal-beds,  especially  in  the  southern  Appalachians,  where  the 
Pottsville  is  6000  feet  thick.  The  water  which  filled  the  trough 
varied  in  character;  in  the  middle  of  the  epoch  marine  faunas 
extend  as  far  north  as  central  West  Virginia,  but  the  northern  por- 
tion appears  to  have  been  an  estuary.  Over  the  Mississippi  valley 
the  Millstone  Grit  and  various  sandstones  with  local  names  repre- 
sent the  Pottsville,  but  the  evidence  of  the  fossil  plants  shows  that 
the  Millstone  Grit  is  not  a  single  bed,  but  that  in  different  places 
it  corresponds  to  different  levels  of  the  Pottsville.  At  the  end  of  the 
age  the  sea  covered  much  of  the  Mississippi  valley,  perhaps  con- 
necting with  the  Michigan  basin,  though  this  is  doubtful,  but  it 
spread  over  western  Pennsylvania  and  a  great  part  of  Ohio.  "  It 
is  highly  probable  that  by  this  time  the  Pottsville  sea  swept  across 
the  Cincinnati  arch  in  southern  Kentucky  and  Tennessee  so  as 
to  connect  with  the  interior  region."  (D.  White.) 

In  Arkansas  the  Pottsville,  as  shown  by  the  fossils,  is  represented 
by  a  series  of  limestones,  shales,  and  sandstones,  which  have  till 
very  lately  been  placed  in  the  Lower  Carboniferous,  and  so  many 


6i6 


THE  CARBONIFEROUS   PERIOD 


FIG.  274.  —  Map  of  North  America  in  the  Upper  Carboniferous.  Black  areas  = 
known  exposures;  white  areas  =  land,  or  unknown;  horizontal  lines  =  sea; 
dotted  area  ^=  Permian  of  Kansas  and  Texas 


DISTRIBUTION  OF  THE  CARBONIFEROUS   ROCKS       617 

localities  of  Pottsville  marine  animals  have  been  found  over  the 
West  as  to  make  it  probable  that  the  sea  extended  to  Nevada 
(Girty),  while  in  that  state  the  same  stage  appears  to  be  repre- 
sented by  beds  which  contain  a  fauna  transitional  between  the 
Lower  and  Upper  Carboniferous. 

Though  coal  accumulation  had  begun  in  the  Pottsville  and 
even  in  theMississippian,  the  time  of  its  formation  on  the  greatest 
scale  was  in  the  second  half  of  the  Upper  Carboniferous.  Vast 
areas  of  low-lying  swamps  bordered  the  Interior  Sea,  and  in  these 
vegetation  flourished  most  luxuriantly.  A  very  slow  subsidence, 
often  intermittent,  allowed  great  thicknesses  of  vegetable  material 
to  accumulate,  but  frequently  a  more  rapid  sinking  brought  in 
the  sea,  or  bodies  of  fresh  water  over  the  bogs,  killing  the  trees 
which  grew  there.  We  cannot  yet  determine  how  far  the  different 
coal  regions  represent  separate  basins,  and  how  far  their  separa- 
tion is  due  to  the  subsequent  removal  of  connecting  strata,  but 
even  in  connected  areas  we  find  great  differences  in  the  nature 
and  thickness  of  the  beds.  This  indicates  that  oscillations  of 
level  of  different  amounts  took  place  in  particular  parts  of  the 
same  basin.  Thus,  in  one  portion  may  occur  a  coal  seam  of  great 
thickness,  divided  into  two  or  more  layers  by  exceedingly  thin 
"  partings  "  of  shale.  As  we  trace  the  coal  seam  in  the  proper 
direction,  the  partings  gradually  grow  thicker,  until,  perhaps, 
they  become  strata,  that  intervene  between  very  distinct  and 
quite  widely  separated  coal  seams,  each  of  which  is  continuous 
with  the  corresponding  portion  of  the  thick  seam.  The  meaning 
of  such  a  structure  is,  that  while  one  part  of  the  bog  subsided 
very  slowly,  permitting  the  almost  uninterrupted  accumulation 
of  vegetable  matter,  other  portions  sank  more  rapidly  and  were 
inundated  with  water,  which  deposited  mechanical  sediments  on 
the  surface  of  the  submerged  bog. 

Hardly  more  than  2%  of  the  thickness  of  the  coal  measures 
consists  of  workable  coal.  The  strata  are  mostly  sandstones, 
shales,  clays,  and  in  some  regions  limestones,  interstratified  with 
numerous  seams  of  coal  of  \tery  varied  thicknesses.  This  alter- 


6l8  THE  CARBONIFEROUS   PERIOD 

nation  of  coal  with  mechanical  deposits  does  not  necessarily,  or 
even  probably,  imply  oft-repeated  oscillations  of  level,  but  may 
be  explained  by  assuming  a  general,  slow,  but  intermittent  sub- 
sidence. After  each  submergence,  we  may  suppose,  the  move- 
ment was  nearly  or  quite  arrested,  and  the  shallow  water  was 
filled  up  with  sediment,  until  a  bog  could  again  be  formed.  Doubt- 
less, movements  of  elevation  also  occurred  at  times,  but  the  general 
movement  was  downward.  In  the  Nova  Scotia  field  are  76  dis- 
tinct coal  seams,  each  of  which  implies  the  formation  of  a  separate 
bog.  Beneath  most  coal  seams  occurs  what  miners  call  the  "  seat- 
stone  "  or  "  underclay,"  which  is  ordinarily  a  fire-clay,  or  it  may 
be  siliceous,  but  is  always  evidently  an  ancient  soil.  The  under- 
clay is  filled  with  fossil  roots,  from  which  often  rise  the  stumps 
of  trees  that  penetrate  the  coal  seam,  or  may  even  extend  many 
feet  above  it.  The  rock  which  lies  on  a  coal  seam  is  usually  a 
shale,  stained  black  by  organic  matter,  but  may  be  a  sandstone 
or  even  a  limestone,  according  to  the  depth  of  water  over  the  sub- 
merged bog. 

That  coal  is  of  vegetable  origin  is  no  longer  questioned.  Such 
a  mode  of  origin  is  directly  proved  by  microscopical  examination, 
which  shows  that  even  the  hardest  anthracite  is  a  mass  of  car- 
bonized but  determinable  vegetable  fibres.  On  the  other  hand, 
there  has  been  much  difference  of  opinion  concerning  the  way  in 
which  such  immense  masses  of  vegetable  matter  were  brought 
together.  Much  the  most  probable  view  is,  that  the  coal  was 
formed  in  position  in  great  peat-bogs,  added  to,  no  doubt,  by 
more  or  less  drifted  material.  The  evidence  for  this  view  is  to 
be  found:  (i)  In  the  great  extent  and  uniform  thickness  and 
purity  of  many  coal  seams,  which  we  cannot  account  for  in  any 
other  way.  Had  the  vegetable  matter  been  largely  drifted  to- 
gether, it  must  have  been  contaminated  with  sediment  and  could 
not  have  been  spread  out  so  evenly  over  great  areas.  This 
objection  to  the  "  driftwood  theory  "  becomes  all  the  stronger 
when  it  is  remembered  that  the  process  of  converting  vegetable 
matter  into  coal  greatly  reduces  its  bulk,  a  given  thickness  of  coal 


DISTRIBUTION   OF  THE  CARBONIFEROUS   ROCKS       619 

representing  only  about  7%  of  the  original  thickness  of  vegetable 
substance.  Thus  a  20-foot  seam  of  coal  implies  the  accumula- 
tion of  nearly  300  feet  of  plants,  and  it  is  highly  improbable  that 
such  a  mass  could  have  been  evenly  spread  as  drift  over  hundreds 
(or  even  thousands)  of  square  miles,  without  a  large  admixture 
of  mud  or  sand.  (2)  The  very  general  presence  of  the  underclay 
beneath  coal  seams  points  to  the  same  conclusion.  An  underclay, 
as  we  have  seen,  is  an  ancient  soil,  and  is  of  just  the  same  char- 
acter as  that  which  we  find  under  such  modern  peat-bogs  as  the 
Great  Dismal  Swamp. 

The  subsidence  of  the  bogs  and  the  deposition  of  sediments 
upon  them  gradually  built  up  the  great  series  of  strata  which  are 
called  the  coal  measures.  The  peat  was  thus  subjected  to  the 
steadily  increasing  pressure  of  the  overlying  masses,  which  greatly 
aided  in  the  transformation  of  the  vegetable  accumulations  into 
coal.  Where  the  coal  measures  have  been  folded,  the  still  greater 
pressure,  aided  by  heat,  and  perhaps  by  steam,  has  resulted  in  the 
formation  of  anthracite.  The  greater  number  of  the  Carboniferous 
bogs  appear  to  have  been  covered  by  fresh  water,  though  some 
were  coast  swamps,  extending  out  into  brackish  or  even  salt  water. 

On  the  other  hand,  it  must  be  admitted  that  there  are  not  a 
few  cases  to  which  the  peat-bog  theory  does  not  apply,  for  example, 
to  the  small  coal  basins  in  the  central  plateau  of  France.  The 
famous  basin  of  Commentry  is,  according  to  Fayol,  explained 
without  difficulty  by  regarding  it  as  a  delta  formation  in  a  large 
lake.  The  coarse  gravel  was  deposited  in  inclined  beds  imme- 
diately at  the  mouth  of  a  swift  stream,  the  finer  sediment  was 
carried  farther  out  and  the  floating  masses  of  vegetation  still 
farther.  The  vegetable  matter  became  water-logged  and  sank 
to  the  lake-bottom,  where  it  was  free  from  sediment.  Such  a 
case,  however,  has  little  bearing  upon  the  great  coal-fields. 

The  workable  coal-fields  of  North  America,  belonging  to  the 
Carboniferous  system,  are  found  in  several  distinct  areas  some 
of  which  were  doubtless  separate  basins  of  accumulation,  while 
others  have  become  disconnected  by  denudation. 


620  THE  CARBONIFEROUS  PERIOD 

(1)  In  the  Acadian  province  the  coal  measures  occur  in  the 
island  of  Cape  Breton,  Nova  Scotia,  and  New  Brunswick;    in 
Nova  Scotia  they  are  of  immense  thickness,  7000  feet,  with  6000 
feet  of  underlying  conglomerate.    The  Coal  Measures  of  Nova 
Scotia,  the  Upper  Carboniferous,  strongly  resemble  the  type  of 
development  in  central   England.    The  immensely  thick  basal 
conglomerate  is  the  Millstone  Grit.     A  second  basin  of  this  prov- 
ince is  near  Worcester  (Mass.),  and  a  third  extends  through  Rhode 
Island  into  southeastern  Massachusetts     The  latter  basins  are 
metamorphic  and  yield  a  very  hard  anthracite. 

(2)  The  great  Appalachian  field  has  an  area  of  more  than 
50,000  square  miles.     It  covers  most  of  central  and  western  Penn- 
sylvania, eastern  Ohio,  western  Maryland  and  Virginia,  and  West 
Virginia,  eastern  Kentucky  and  Tennessee,  to  northern  Alabama. 
In  this  field  the  measures  are  thinner  than  in  Nova  Scotia;    the 
beds  are  thickest  along  the  Appalachian  shore-line,  about  4000  feet 
in  western  Pennsylvania  and  6000  in  Alabama,  thinning  much 
to  the  westward. 

(3)  In  Michigan  the  measures  are  only  about  300  feet  thick, 
and  were  doubtless  laid  down  in  an  isolated  basin. 

(4)  The  Indiana-Illinois  field,  which  extends  into  Kentucky, 
is  from  600  to  1000  feet  thick. 

(5)  The  Iowa-Missouri  field   extends  southward  around  the 
Palaeozoic  island  of  southern  Missouri  into  Arkansas  and  Texas. 
In  Arkansas  the  Carboniferous  system  attains   a   greater  thick- 
ness than  anywhere  else  in  North  America,  and  all  but  an  insig- 
nificant amount  of  this  is  Pennsylvanian. 

The  two  latter  fields  are  separated  by  a  very  narrow  interval, 
and  almost  certainly  were  once  continuous;  the  Indiana-Illinois 
field  was  probably  also  connected  with  the  Appalachian  area 
across  western  Kentucky  and  Tennessee. 

As  the  coal  measures  are  traced  westward  into  Kansas,  Nebraska, 
and  adjoining  states,  we  find  them  dipping  beneath  strata  of  a 
very  much  later  date.  When  they  once  more  return  to  the  surface, 
as  in  the  Rocky  Mountain  region,  they  appear  under  an  entirely 


DISTRIBUTION  OF  THE  CARBONIFEROUS   ROCKS       621 

new  aspect,  being  here  altogether  marine  and  containing  no 
coal. 

After  the  Pottsville  age  and  during  the  formation  of  the  Coal 
Measures,  the  Interior  Sea  was  greatly  restricted  in  the  Mississippi 
valley  by  the  broad,  surrounding  fringe  of  swamps  and  bogs, 
which  the  sea  periodically  invaded.  The  same  succession  of 
great  swamps  followed  the  Appalachian  line  into  northeastern 
Pennsylvania  and  probably  into  southern  New  York.  In  eastern 
Pennsylvania  the  sea  rarely  came  in  during  the  formation  of  the 
coal  measures,  but  once,  at  least,  penetrated  to  Wilkes-Barre. 
Westward  the  Interior  Sea  probably  did  not  extend  to  Nevada, 
as  that  of  Pottsville  time  had  done,  but  ended  farther  east  along 
a  line  not  yet  determined.  A  shore-line  in  Colorado  is  indicated 
by  the  generally  sandy  and  conglomeratic  character  of  the  Pennsyl- 
vanian  rocks  in  that  state,  which  have  an  eastern  type  of  fauna. 
On  that  account,  Girty  has  "  tentatively  assumed  that  the  line  of 
division  between  the  Eastern  and  Western  provinces  passes  through 
western  Texas,  central  or  eastern  New  Mexico,  western  Colorado, 
and  so  on  upward,  in  a  northwesterly  direction,  following  nearly 
the  trend  of  the  Rocky  Mountains." 

The  northwestern  arm  of  the  Interior  Sea  was  shifted  eastward 
from  the  position  it  had  occupied  in  the  Lower  Carboniferous,  and 
apparently  joined  the  Arctic  Ocean  instead  of  the  Pacific,  sub- 
merging nearly  the  whole  of  Alaska,  except  a  broad  belt  on  the 
Pacific  side,  this  belt  of  land  continuing  southward  through 
British  Columbia  to  Oregon.  The  area  of  the  sea  was  somewhat 
diminished  in  southern  and  westernMexico  and  in  Central  America. 

The  Carboniferous  period  in  North  America  was,  on  the  whole, 
a  time  of  tranquillity,  with  oscillations  of  level  and  shifting  of 
the  boundaries  of  land  and  sea  from  time  to  time,  such  as  have 
been  described  in  the  foregoing  pages.  A  very  general  upheaval 
of  the  continent  brought  the  Mississippian  to  a  close  and  the  suc- 
ceeding time  of  erosion  was  followed  by  some  folding  in  the  Ap- 
palachian region,  the  formation  of  the  Pottsville  trough  and  the 
renewed  transgression  of  the  sea.  Volcanic  action  appears  to 


622  THE  CARBONIFEROUS   PERIOD 

have  been  confined  to  the  Pacific  coast  region;  the  Lower  Car- 
boniferous of  British  Columbia  is  largely  made  up  of  volcanic 
material,  and  vulcanism  was  manifested  in  the  Upper  Carbon- 
iferous of  the  coast  from  Alaska  to  California.  In  northwestern 
Kentucky  and  southern  Illinois  the  Carboniferous  rocks  are  cut 
by  dykes,  but  these  may  have  been  formed  at  a  long-subsequent 
time. 

Foreign.  —  In  Europe  the  Carboniferous  system  is  developed 
in  a  very  interesting  way.  In  the  western  and  central  parts  of 
the  continent  (and  in  Great  Britain)  the  succession  of  strata  is 
very  similar  to  that  of  the  eastern  half  of  North  America,  while 
in  Russia  it  has  more  analogy  with  the  western  half  of  our  con- 
tinent. The  changes  of  level  which  opened  the  period  converted 
much  of  the  Devonian  sea- bed  into  land,  but  at  the  same  time 
the  sea  broke  in  over  many  of  the  closed  basins  in  which  the  Old 
Red  Sandstone  had  been  laid  down.  From  the  west  of  Ireland 
to  central  Germany,  a  distance  of  750  miles,  stretched  a  clear  sea, 
free  from  terrigenous  sediments,  in  which  flourished  an  incredible 
number  of  corals,  crinoids,  and  other  calcareous  organisms.  From 
their  remains  was  constructed  an  immense  mass  of  limestone, 
having  a  thickness  of  6000  feet  in  the  northwest  of  England  and 
of  2500  feet  in  Belgium.  Above  this  great  "  Mountain  Limestone," 
as  it  is  called  in  England,  come  the  coal  measures.  In  Scotland 
the  limestone  is  replaced  by  shore  and  shallow-water  formations, 
such  as  sandstones,  with  some  coal.  In  the  southwest  of  England 
and  east  of  the  Rhine  in  Germany,  the  Lower  Carboniferous  is 
represented,  not  by  a  limestone,  but  by  a  series  of  sandstones 
and  slates,  called  the  Culm,  with  the  coal  measures  above.  In 
Russia  the  order  of  succession  is  reversed,  the  productive  coal 
beds  being  below  and  the  great  bulk  of  the  limestone  above,  but 
there  is  some  productive  coal  interstratified  in  the  marine  lime- 
stones of  the  Donjetz  basin  in  the  south.  This  younger  Carbon- 
iferous limestone  is  principally  composed  of  shells  of  Foraminifera. 
Great  areas  of  southern  and  eastern  As*'4  are  covered  by  this 
limestone,  which  is  also  largely  developed  in  western  North 


DISTRIBUTION  OF  THE  CARBONIFEROUS   ROCKS       623 


America,  extending  as  far  east  as  Illinois.  In  southern  Europe 
Spain,  the  south  of  France,  the  Alps,  and  the  Balkan  peninsula, 
the  Lower  Carboniferous  is  partly  limestone  and  partly  Culm, 
while  the  Upper  is  largely  made  up  of  the  foraminiferal  limestone 
associated  with  clastic  rocks.  In  the  Arctic  Sea,  Nova  Zembla, 
Bear  Island,  Spitzbergen,  and  Greenland  have  Upper  Carbonif- 
erous limestones. 

The  following  table,  from  Kayser,  displays  the  relations  of  the 
Carboniferous  beds  in  eastern  and  western  Europe:  — 


LITTORAL  AND 
LACUSTRINE  FACIES 

MARINE  FACIES 

Upper 
Carboniferous. 

Productive 
Coal  Measures 
(Western  Europe). 

Younger  Carboniferous  or 
Fusulina  Limestone 
(Russia,  etc.). 

Lower 
Carboniferous. 

Productive 
Coal  Measures 
(Russia,  etc.). 

Lower   Carboniferous 
Limestone 
(Western  Europe). 

Culm 

(Germany). 

In  western  Europe  the  Carboniferous  period  did  not  run  such 
a  tranquil  course  as  in  North  America,  but  was  broken  by  dis- 
turbances, of  which  the  greatest  were  at  the  close  of  the  Lower 
Carboniferous  epoch,  when  the  rocks  were  folded  and  upturned 
over  extensive  regions.  These  movements  were  accompanied  and 
followed  by  volcanic  outbursts,  especially  in  Scotland,  France,  and 
Germany,  and  great  eruptions  occurred  in  China  at  the  end  of  the 
period. 

In  Asia  are  large  areas  of  Lower  Carboniferous  limestone  and 
Culm,  and  of  the  Upper  Carboniferous  both  the  foraminiferal 
limestone  and  productive  coal  measures.  China  is  one  of  the 
richest  countries  in  the  world  in  supplies  of  coal.  Foraminiferal 
limestones  of  the  Upper  Carboniferous  are  found  in  Japan, 
Borneo,  and  Sumatra. 

Africa. — Carboniferous  limestones  are  found  in  Morocco 
and  the  Sahara  and  Egypt.  In  the  southern  part  of  the  conti- 


624  THE  CARBONIFEROUS   PERIOD 

nent  no  marine  rocks  of  the  period  are  known.  In  Cape  Colony 
the  Witteberg  quartzites,  which  overlie  the  Devonian,,  have  Lower 
Carboniferous  plants,  and  in  the  Zambesi  district  near  the  east 
coast  is  a  coal  basin  of  Upper  Carboniferous  age,  which  has  a  flora 
like  that  of  the  higher  Coal  Measures  of  Europe. 

According  to  Freeh,  the  Carboniferous  of  Australia  is  confined 
to  the  Lower  division  and  appears  in  the  eastern  half  of  the  con- 
tinent, and  in  Tasmania.  In  the  second  half  of  the  period  was 
a  time  of  elevation  and  erosion. 

In  South  America  the  Carboniferous  is  not  nearly  so  extensive 
as  the  Devonian;  the  Lower  Carboniferous  is  principally  com- 
posed of  sandstones,  which  in  Argentina  contain  plants  so  simi- 
lar to  those  of  South  Africa  and  Australia  as  to  indicate  the 
probability  of  a  land  connection  between  these  continents;  lime- 
stones of  this  date  have  been  reported  from  Chili.  The  Upper 
division,  largely  of  limestones,  has  been  found  in  Peru,  Bolivia, 
and  Brazil;  in  the  latter  it  has  a  great  extension  in  the  Amazon 
valley  and  belongs  to  the  uppermost  part  of  the  system. 

Climate. — The  striking  uniformity  of  the  climate  during  the 
Carboniferous  is  indicated  by  the  distribution  of  the  fossils,  more 
especially  of  the  plants,  which  are  almost  the  same  in  the  Arctic 
and  Tropical  regions.  The  formation  of  coal  in  vast  peat-bogs 
does  not  imply  a  tropical  climate,  but  rather  conditions  of  moisture 
and  moderate  temperature.  In  the  Lower  Carboniferous,  aridity 
prevailed  in  northeastern  America,  and  in  the  Upper,  gypsum 
and  rock-salt  were  formed  east  of  the  Ural  Mountains.  There  is 
some  evidence  of  an  American  ice-epoch  at  the  end  of  the  Lower 
Carboniferous. 


CARBONIFEROUS  LIFE 

The  life  of  this  period  is  thoroughly  Palaeozoic  and  continues 
along  the  lines  already  marked  out  in  the  Devonian,  but  there 
are  some  notable  changes  and  advances  which  look  toward  the 
Mesozoic  order  of  things. 


CARBONIFEROUS   LIFE  625 

Plants.  — The  Carboniferous  vegetation  is  of  very  much  the 
same  character  as  that  of  the  Devonian,  but  owing  to  the  peculiar 
physical  geography  of  the  times,  the  plants  were  preserved  as  fos- 
sils in  a  much  more  complete  state  and  in  vastly  larger  numbers. 
The  flora  is  composed  entirely  of  the  higher  Cryptogams  and  the 
Gymnosperms,  no  plant  with  conspicuous  flowers  having  come 
into  existence,  so  far  as  we  yet  know.  By  far  the  most  abundant 
of  Carboniferous  plants  are  the  Ferns  (Filicales)  which  flourished 
in  multitudes  of  species  and  individuals,  both  as  tall  trees  and  as 
lowly,  herbaceous  plants.  Many  of  these  ferns  cannot  yet  be 
compared  with  modern  ones,  because  the  organs  necessary  for 
trustworthy  classification  have  not  been  recovered,  and  such  are 
named  in  accordance  with  the  venation  of  the  leaves.  In  other 
cases  the  comparison  with  existing  ferns  may  be  definitely 
made,  and  these  remains  show  that  many  of  the  modern  families 
(Marattiacece,  Ophioglossacece,  etc.)  had  representatives  in  the 
Carboniferous  forests  and  swamps. 

Even  more  conspicuous,  though  much  less  varied,  were  the 
Lycopods  (Lycopodiales)  the  remarkable  character  of  which  has 
been  elucidated  by  the  long-continued  and  laborious  efforts  of 
many  investigators.  While  the  Ferns  have  remained  an  important 
group  of  plants  to  the  present  time,  the  Lycopods  have  dwindled 
to  a  few  insignificant  herbaceous  forms,  but  in  Carboniferous 
times  they  were  the  abundant  and  conspicuous  forest  trees,  at 
least  of  the  swampy  lowlands.  One  of  the  most  characteristic 
of  these  trees  was  Lepidodendron  (PI.  X),  of  which  many  species 
have  been  found  in  the  coal  measures.  These  great  club-mosses 
had  trunks  of  2  or  3  feet  in  diameter  and  50  to  75  feet  high,  which 
possessed  the  remarkable  quality,  for  a  Cryptogam,  of  an  annual 
growth  in  thickness.  At  a  considerable  height  above  the  ground 
the  trunk  divides  into  two  main  branches,  each  of  these  again  into 
two,  and  so  on  (dichotomous  division).  The  younger  parts  of 
the  tree  are  covered  with  long,  narrow,  stiff,  and  pointed  leaves, 
while  the  older  parts  are  without  leaves,  which  have  dropped  off, 
making  conspicuous  scars,  arranged  in  spiral  lines  around  the 

2S 


PLATE  X.  — CARBONIFEROUS  VEGETATION 

Lepidodendron,  central  tree  with  cones.  Sigillaria,  each  side  of  middle,  with  leafy 
trunks.  Calamites,  right  side.  Cordaites,  left  side  on  raised  ground.  Cycadofilices, 
fern-like  growth  in  foreground. 


CARBONIFEROUS  LIFE  62; 

stem.  At  the  ends  of  the  twigs  in  some  species,  or  on  the  sides 
of  the  trunk  and  larger  branches,  in  others,  are  found  the  spore- 
bearing  bodies,  which  have  much  the  appearance  of  pine-cones. 
The  stem  was,  to  a  large  extent,  filled  with  loose  tissue  and  had 
only  a  relatively  small  amount  of  wood. 

Another  very  characteristic  and  abundant  tree  is  Sigillaria 
(PL  X) ;  it  is  closely  allied  to  Lepidodendron,  but  has  a  very 
different  appearance.  The  trunk  is  quite  short  and  thick,  rarely 
branching,  and  with  a  pointed  or  rounded  tip,  much  as  in  the 
great  Cactus;  the  leaves  are  similar  to  those  of  Lepidodendron, 
but  are  arranged  between  vertical  ridges.  Sigillaria  also  possessed 
the  power  of  annual  increase  in  diameter.  Both  Lepidodendron 
and  Sigillaria  are  provided  with  branching  rhizomes,  or  under- 
ground stems,  which  carry  finger-like  appendages  inserted  into 
pits.  Before  the  nature  of  these  rhizomes  was  understood,  they 
were  regarded  as  distinct  plants  and  named  Stigmaria. 

A  third  group  of  Cryptogams,  the  Equisetales,  or  Horsetails, 
were  of  great  importance  in  the  Carboniferous  forests.  The 
Calamites  were  decidedly  superior  to  the  existing  horsetails,  not 
only  in  size,  but  in  many  features  of  organization  as  well.  These 
plants  had  tall,  slender  stems  divided  by  transverse  joints,  with 
a  soft  inner  pith,  surrounded  by  a  ring  of  woody  tissue,  which 
grew  annually  in  thickness.  The  shape  and  arrangement  of  the 
leaves  differ  much  in  the  various  genera,  and  even  in  different 
parts  of  the  same  plant;  for  example,  they  are  needle-like  in 
Astrophyllites,  while  in  Annularia  they  are  broad  and  at  the  base 
united  into  a  ring  around  the  stem,  but  some  species  of  Annularia, 
at  least,  are  probably  merely  the  branches  of  larger  calamites. 
The  shape,  size,  and  position  of  the  spore-bearing*  organs  likewise 
differ  in  the  different  genera,  but  often  resemble  those  of  the 
modern  horsetails.  The  base  of  the  stem  tapers  abruptly,  and 
is  either  connected  with  a  horizontal  rhizome  or  gives  off  a  bundle 
of  roots.  Fragments  of  calamite  stems  are  among  the  commonest 
fossils  of  the  coal  measures. 

The  three  preceding  groups  of  Cryptogams  all  have  representa- 


628  THE  CARBONIFEROUS  PERIOD 

tives  in  the  modern  world,  and  one  of  them,  the  Ferns,  is  still 
abundant  and  varied.  In  addition  to  these,  Carboniferous 
vegetation  had  two  other  cryptogamic  classes  of  great  interest, 
which  are  now  extinct,  and  are  not  known  to  have  passed  beyond 
the  Palaeozoic  era.  Of  these,  the  first  is  the  class  Sphenophyllales, 
a  group  of  very  slender,  probably  climbing  and  trailing  plants, 
with  small  leaves  varying  in  shape  in  different  plants  and  different 
parts  of  the  same  plant.  Some  of  the  leaves,  which  are  always 
small,  are  wedge-shaped,  others  are  divided  and  others  again  are 
narrow  and  simple.  The  great  interest  of  the  class  lies  in  the 
fact  that  it  is  intermediate  between  the  horsetails  and  club-mosses, 
and  doubtless  its  Carboniferous  representatives  were  the  sur- 
vivors of  an  ancient  group  which  was  ancestral  to  both  club- 
mosses  and  horsetails. 

Even  more  remarkable  is  the  class  Cycadofilices,  which  was 
extremely  abundant  in  the  Carboniferous  forests  and  swamps, 
and  which  affords  the  long-sought  transition  between  the  flower- 
less  and  the  flowering  plants,  connecting,  as  its  name  implies, 
the  ferns  and  cycads.  In  external  appearance  of  stem  and  foliage 
these  plants  most  resembled  tree-ferns. 

The  Flowering  Plants  are  still  represented  only  by  the  Gymno- 
sperms,  of  which  the  dominant  group  is  the  Cordaitea  (see  PI.  X), 
which  were  slender,  very  tall  trees,  "  with  trunks  rising  to  a  great 
height  before  branching,  and  bearing  at  the  top  a  dense  crown, 
composed  of  branches  of  various  orders,  on  which  simple  leaves 
of  large  size  were  produced  in  great  abundance."  (D.  H.  Scott.) 
The  centre  of  the  trunk  was  occupied  by  a  large  soft  pith,  and  the 
leaves,  with  their  parallel  venation  resembling  those  of  lilies  and 
grasses,  were  long,  broad  in  most  species,  narrow  in  others,  and 
either  sharply  pointed  or  bluntly  rounded.  The  Cordaiteae  had 
affinities  with  each  of  the  three  existing  orders  of  Gymnosperms, 
the  Cycads,  Conifers,  and  Gingkos,  but  is  not  referable  to  any  of 
them.  The  three  orders  named  may  all  have  existed  in  the 
Carboniferous,  but  this  is  not  definitely  known. 

The  Carboniferous  flora  is  merely  the  Devonian  flora  somewhat 


CARBONIFEROUS   LIFE  629 

advanced  and  diversified,  and  the  forests  were  of  the  same  gloomy, 
monotonous  character  as  before.  The  wide  distribution  and 
uniform  character  of  this  flora  are  very  remarkable;  we  find  the 
same  or  nearly  allied  species  of  plants  spread  over  North  America, 
Europe  (even  in  the  polar  lands,  like  Spitzbergen  and  Nova 
Zembla),  Siberia,  China,  the  Sinai  peninsula,  Brazil,  Australia, 
and  Tasmania. 

Foraminifera.  —  For  the  first  time  these  animals  assume  con- 
siderable importance  in  the  earth's  economy.  Many  genera 
which  are  still  living  had  representatives  in  the  Carboniferous 
seas,  but  the  most  conspicuous  and  abundant  is  the  extinct  Fusu- 
lina  (XII,  i,  i  a),  a  very  large  kind,  with  shells  resembling  grains 
of  wheat  in  size  and  shape.  This  genus  is  especially  developed 
in  the  Upper  Carboniferous,  while  Schwagerina  characterizes  the 
uppermost  part  of  the  system.  In  the  Salem  limestone  of  Indiana, 
a  well-known  building  stone,  of  the  Mississippian  series,  Endothyra 
(XI,  i)  is  abundant. 

Sponges  are  common,  though  rarely  found  in  good  preservation. 

Coelenterata.  —  Corals  were  abundant,  and  contributed  largely 
to  the  limestones;  the  genus  Lithostrotion  (XI,  2),  which  is  peculiar 
to  this  period,  plays  a  very  prominent  part.  Lophophyllum  is  found 
in  the  Upper  Carboniferous. 

Echinodermata  make  up  an  exceedingly  important  part  of  the 
Carboniferous  marine  fauna.  The  Cystoids  have  disappeared,  but 
the  Blastoids  have  developed  in  great  numbers,  and  are  highly 
characteristic  of  the  Carboniferous  limestones.  As  the  group  is 
entirely  extinct  and  does  not  pass  beyond  the  Carboniferous  sys- 
tem, its  structure  has  much  that  is  problematical  about  it.  The 
delicate,  symmetrical  body,  or  calyx,  which  is  carried  on  a  short 
stem,  is  composed  of  a  small,  definite  number  of  plates,  and  has 
five  "  pseudo-ambulacral  "  areas,  which  look  much  like  the  am- 
bulacra of  a  sea-urchin.  In  exceptionally  well-preserved  speci- 
mens numbers  of  delicate  pinnules  are  attached  to  these  areas. 
The  must  abundant  genera  are  Pentremites  (XI,  4-5)  and  Grana- 
tocrinus. 


PLATE  XL  — LOWER  CARBONIFEROUS  FOSSILS 

Figs,  i,  la,  Endothyra  baileyi  Hall,  x  9,  St.  Louis,  side  and  end  views.  2,  Lithostro- 
tion  canadense  Castelnau,  x  %,  St.  Louis.  3,  Eutrochocrinus  christyi  Shumard,  X  %, 
Burlington.  4,  Pentremites  elongatus  Shumard,  X  i,  Burlington.  5,  P.  conoideus  Hall> 
X  i,  Keokuk.  6,  Melonites  muhiporus  Norwood  and  Owen,  x  %,  St.  Louis.  7,  Archi* 


CARBONIFEROUS  LIFE  631 

All  other  Echinoderms  of  the  Carboniferous  seas  were  utterly 
insignificant  as  compared  with  the  Crinoids,  which  reach  their 
culmination  of  development  in  this  period :  more  than  600  species 
have  been  described  from  the  Carboniferous  limestones  of  North 
America  alone.  Certain  localities,  such  as  Burlington  (la.)  and 
Crawfordsville  (Ind.),  are  famous  for  the  vast  numbers  and  ex- 
quisite preservation  of  their  fossil  sea-lilies.  The  crinoid  remains 
occur  in  such  multitudes  that  in  many  places  the  limestones  are 
principally  composed  of  them;  in  such  places  they  must  have 
covered  the  sea-bottom  like  miniature  forests.  But  this  extraor- 
dinary abundance  is  not  general  over  North  America,  but 
characterizes  the  Central  States  only  and  Mississippian  time, 
especially  the  Osage.  All  the  Carboniferous  Crinoids,  like  those 
of  the  earlier  periods,  belong  to  the  extinct  order  Camerata,  none 
of  which  passed  over  into  the  Mesozoic  era.  Of  the  long  list  of 
Crinoids  found  in  the  rocks  of  this  system  may  be  mentioned 
Actinocrinus,  Platycrinus,  Rhodocrinus,  Eutrochocrinus  (XI,  3), 
Onychocrinus,  ^Esiocrinus,  and  Eupachycrinus  (XII,  2). 

The  Echinoids,  or  sea-urchins,  are  still  far  less  abundant  than 
the  Crinoids,  but  they  are  much  more  numerous  and  varied,  and 
of  larger  size  than  they  had  been  before;  some,  indeed,  are  as 
large  as  any  sea-urchins  that  are  known  from  any  period.  The 
Carboniferous  sea-urchins  are,  like  those  of  the  preceding  periods, 
members  of  the  anc'ent  and  now  extinct  subclass,  Palceechinoidea, 
and  the  commonest  genera  are  Melonites  (XI,  6),  Oligoporus,  and 
Arch&ocidaris .  In  addition  to  these  should  be  noted  the  pres- 
ence of  the  modern  subrlass,  Euechinoidea,  as  the  ancestor  of 
the  still  existing  genus  Cidaris  is  reported  from  the  Carboniferous. 

The  first  known  Holothuroidea,  or  sea-cucumbers,  date  from 
this  period. 

Arthropoda. — The  Trilobites  have  become  rare  and  are  soon 
to  die  out  altogether;  most  of  the  species  belong  to  the  peculiarly 


632  THE  CARBONIFEROUS   PERIOD 

Carboniferous  genera  Phillipsia  (XII,  21)  and  Gri/ithides,  but  the 
Devonian  Proetus  still  persists.  The  Eurypterids  continue,  even 
into  the  coal  measures,  where  they  liveoMn  the  fresh-water  swamps, 
but  they  cannot  compare  in  size  or  numbers  with  the  great  Devo- 
nian forms.  The  horse-shoe  crabs  are  represented  by  Prestwichia. 
Phyllopods  and  Ostracods  are  abundant,  and  in  the  coal  measures 
are  found  crustaceans  formerly  incorrectly  referred  to  the  Decapods, 
which  they  resemble;  of  these  Anthracopalczmon  is  the  best-known 
genus. 

Centipedes  and  Scorpions  are  much  commoner  than  in  the 
Devonian,  and -the  first  of  the  true  Spiders  are  found  here.  Insects 
likewise  show  a  great  increase  in  numbers,  though  the  Orthopters 
and  Neuropters  are  still  the  principal  orders  represented.  Many 
of  the  Carboniferous  insects  are  remarkable  for  their  great  size, 
some  of  them  measuring  30  inches  across  the  extended  wings, 
and  more  remarkable  is  the  fact  that  several  insects  of  this  period 
had  three  pairs  of  wings,  corresponding  to  the  number  of  legs. 
The  character  of  the  vegetation  has  a  very  direct  influence  upon 
insect  life,  and  the  monotonous,  flowerless  Carboniferous  forests 
could  not  have  supported  butterflies,  bees,  wasps,  ants,  or  flies. 
No  insects  of  these  groups  have  been  found  in  the  rocks  of  that 
system,  and  it  is  not  yet  certain  whether  even  beetles  were  then 
in  existence. 

The  land  life  of  the  Carboniferous  seems  to  be  very  much  more 
varied  and  luxuriant  than  that  of  the  Devonian,  and  it  probably 
was  so  in  reality.  It  must  be  remembered,  however,  that  the  im- 
mense development  of  fresh-water  and  marshy  deposits  in  the 
Carboniferous  was  much  more  favourable  to  the  preservation  of 
such  fossils  than  any  conditions  that  the  Devonian  had  to  offer. 
Part,  at  least,  of  the  striking  difference  in  the  terrestrial  fossils  of 
the  two  periods  is  to  be  accounted  for  in  this  way. 

subquadrata  Shumard,  x  %.  13,  Aviculopecten  occidentalis  Shumard,  X  %.  14,  Bake- 
vellia  parva  Meek  and  Hayden,  x  2,  PERMIAN.  15,  Pleurophorus  subcuneatus  M.  and 
H.,  x  i,  PERMIAN.  16,  Pseudomonotis  haiuni  M.  and  H.,  x  y2.  PERMIAN.  17,  Bellerophon 
percarinatus  Conrad,  x  i.  18,  Pleurotoma-ria  spheerulata  Conr.,  X  1/^.  19,  'iga,  Strapa- 
rollus  Pronodosus  M.  and  W,,  X  %.  20,  Waagenoceras  ciimtmnsi  White,  X  4/5.  zoa,  The 
same,  a  suture  line,  PERMIAN.  21,  Phillipsia  major  Shumard,  x  %. 


PLATE  XII.  — UPPER  CARBONIFEROUS  AND  PERMIAN  FOSSILS 

UPPER  CARBONIFEROUS.  Fig  i,  Fusulina  cylindrica  Fischer,  x  »A.  i«,  F.  secah'caSzy,  x 
6/9  longitudinal  section.  2,  Eupachycrinus  verrucosus  White  and  bt.  John,  X  y2.  ^,L>eroya 
btloba  Hall,  x  YO.  4,  Meekella  striatocostata  Cox,  x  y2.  5,  Chonetes  veneuihana 
Norwood  and  PraUen,  X  2.  6,  Productus  costatus  Sowerby,  x  %.  7,  Spirifer  cameratus 
Morton,  X  y2.  8,  Semin-ula  argentea  Shephard.  X  %.  9,  Dielasma  lovidens  Morton,  X 
V2.  10,  Pugnax  ufa  Marcou,  X  2.  n,  Monoptena  longispina  Cox,  X  %.  12,  Myahn.~, 


634  TH^  CARBONIFEROUS   PERIOD 

The  Bryozoa  become  much  more  important  than  they  had 
been  before,  and  contribute  materially  to  the  formation  of  the 
limestones.  A  characteristic  Carboniferous  genus  is  the  screw- 
shaped  Archimedes  (XI,  7),  while  Fenestella  continues  to  be  very 
abundant. 

The  Brachiopoda  have  undergone  a  marked  diminution,  as 
compared  with  those  of  the  Devonian,  though  they  are  still  very 
common.  Genera  of  long  standing,  like  A  try  pa  and  Pentamerus, 
have  died  out,  but  others,  like  Chonetes  (XII,  5),  Spirifer  (XI,  8; 
XII,  7),  and  Rhynchonella,  are  still  represented,  but  most  im- 
portant of  all  the  Carboniferous  genera  is  Productus  (XI,  u; 
XII,  6),  which  has  a  very  large  number  of  species,  among  them 
P.  giganteus,  the  largest  known  brachiopod.  Syringothyris  (XI, 
10)  and  Reticularia  (XI,  9)  are  allies  of  Spirifer;  while  Meekella 
(XII,  4)  and  Derbya  (XII,  3)  are  extreme  developments  of  the 
Strophomenoid  stock,  of  Ordovician  origin.  The  genus  Tere- 
bratula,  which  became  exceedingly  abundant  in  the  Mesozoic 
periods,  has  its  beginning  in  the  Carboniferous  genus  Dielasma 
(XII,  9),  though  we  have  already  found  the  family  represented 
in  the  Devonian  and  Silurian. 

Mollusca.  —  The  Bivalves  are  somewhat  more  abundant  than 
in  the  earlier  periods.  Examples  of  these  are  Amculopecten 
(XII,  13),  Monopteria  (XII,  n),  and  Myalina  (XII,  12).  Of  Gas- 
tropods, the  same  genera  that  occur  in  the  Silurian  and  Devo- 
nian are  continued  into  the  Carboniferous,  such  as  Bellerophon 
(XII,  17),  Euomphalus,  Pleurotomaria  (XII,  18),  Loxonema,  Platy- 
ceras,  with  the  interesting  addition  of  the  most  ancient  land- 
shells  yet  discovered.  The  genus  Conularia,  referred  to  the 
Pteropods,  is  common.  Among  the  Nautiloid  Cephalopods,  Ortho- 
ceras  still  persists,  but  this  group  reaches  its  acme  in  the  number 
and  variety  of  the  coiled  shells,  many  of  which  represent  new 
genera,  such  as  Cycloceras,  Trigonoceras,  etc.  These  Nautiloids 
have  shells  ornamented  with  prominent  ridges  or  tubercles.  The 
Ammonoids  continue  to  be  represented  by  Goniatites,  but  the 
Carboniferous  forms  of  this  group,  such  as  Brancoceras  (XI,  12) 


CARBONIFEROUS   LIFE  635 

and  Prodromites  (XI,  13),  display  an  advance  over  those  of  the 
Devonian  in  the  greater  complexity  of  their  sutures,  looking 
forward  to  the  remarkable  condition  attained  in  Mesozoic 
times. 

Vertebrata.  —  It  is  in  this  group  that  the  most  marked  advances 
of  Carboniferous  life  are  to '  be  observed,  and  the  incipient  stages 
of  Mesozoic  development  are  clearly  shown.  The  extraordinary 
and  bizarre  Ostracoderms  have  become  extinct,  though  the  Arthro- 
dirans  continue  into  the  coal  measures. 

The  Selachians  are  numerous  and  varied,  having  developed 
so  enormously  that  they  give  the  Carboniferous  fish-fauna  a  very 
different  aspect  from  that  of  the  Devonian.  Acanthodes  is  a  small 
shark  covered  with  a  dense  armour  of  exceedingly  minute  square 
scales,  and  the  fins  are  supported  by  heavy  spines  along  their  an- 
terior borders.  Another  remarkable  shark  is  Pleuracanthus  (a 
Permian  species  is  shown  in  Fig.  279),  which  has  many  features 
in  common  with  the  Dipnoi,  such  as  the  shape  of  the  tail,  the 
character  of  the  pectoral  fins,  and  the  bones  which  form  the  roof 
of  the  skull,  while  the  skin  is  naked.  Isolated  fin-spines  and 
teeth  show  that  many  other  kinds  of  sharks  existed  in  the  Car- 
boniferous, in  some  of  which  the  teeth  were  converted  into  a 
crushing  pavement,  adapted  for  a  diet  of  shell-fish.  (See  PI.  XI, 
Fig.  14.) 

The  Dipnoi  continue,  though  in  diminished  numbers,  and 
their  most  prominent  representative  is  the  genus  Ctenodus. 

The  Crossopterygians  are  much  less  abundant  than  in  the  De- 
vonian; the  commonest  American  genus  is  C&lacanthus,  which, 
though  unmistakably  a  member  of  this  group,  has  assumed  the 
form  of  a  bony  fish,  and  looks  much  like  a  chub. 

The  Actinopteri  are  still  represented  only  by  the  Ganoid  cohort; 
these  hold  their  own  and  even  increase  their  numbers,  many 
new  genera  replacing  those  of  Devonian  times.  Eurylepis,  Palczo- 
niscus,  Eurynotus,  and  Cheirodus  are  the  best-known  genera;  they 
are  all  of  moderate  size  and  in  appearance  are  not  strikingly  dif- 
ferent from  modern  fishes. 


636  THE  CARBONIFEROUS   PERIOD 

The  Amphibians,  which  we  have  seen  some  reason  to  believe 
existed  in  the  Devonian,  are  of  greatly  increased  importance 
in  the  Carboniferous.  At  the  present  time  the  Amphibia  are 
represented  by  the  dwarfed  and  specialized  frogs  and  toads, 
newts  and  salamanders,  which  give  but  an  imperfect  notion  of 
the  structure  of  the  extinct  members  of  the  class.  The  Carbo- 
niferous Amphibia  all  belong  to  the  extinct  order  Stegocephalia, 
in  which  the  skull  is  well  covered  with  a  roof  of  sculptured  bones, 
and  which  are  of  moderate  or  small  size,  not  exceeding  seven  or 
eight  feet  in  length  and  mostly  much  smaller.  The  backbone 
is  not  ossified,  the  limbs  are  weak,  the  tail  short  and  broad,  and 
in  many  forms  the  belly  is  protected  by  an  armour  of  bony 
scutes.  An  extraordinary  number  of  genera  of  Carboniferous 
Stegocephalia  are  known,  most  of  them  like  the  Salamanders 
in  shape,  but  some  are  elongate,  slender,  and  of  snake-like  form. 
Examples  are  Archegosaums,Branchiosaurus,  Dendrerpeton,  Ptyo- 
nius,  and  many  others. 


CHAPTER  XXXI 
THE   PERMIAN   PERIOD 

THE  name  Permian  was  given  by  Sir  Roderick  Murchison  in 
1841  to  a  series  of  rocks  which  is  very  extensively  developed  in 
the  province  of  Perm  in  Russia.  In  North  America  the  Permian 
followed  upon  the  Carboniferous  with  hardly  a  break,  so  that  the 
distinction  between  the  two  systems  must  be  made  entirely  upon 
the  fossils,  which  change  very  gradually,  by  drawing  a  somewhat 
arbitrary  line  of  demarcation.  In  various  countries  there  is  no 
general  agreement  regarding  the  upper  boundary  of  the  Car- 
boniferous, and  there  are  very  great  differences  of  opinion  as  to 
the  correlation  of  the  rocks  of  different  continents,  due  chiefly 
to  the  want  of  an  unbroken  succession  of  fossils  in  any  single 
area.  The  regions  where  the  Permian  beds  are  best  known, 
Germany,  England,  and  Russia,  are  not  those  which  yield  the 
normal  facies  of  marine  life,  and  hence  any  correlation  with  the 
latter  is  full  of  difficulties,  and  in  the  following  table  of  the  American 
Permian,  no  comparison  is  attempted  with  that  of  other  countries. 
In  consequence  of  these  uncertainties,  many  geologists,  especially 
in  this  country  and  in  France,  regard  the  Permian  as  a  mere 
subdivision  of  the  Carboniferous.  Its  relations  with  the  overlying 
Triassic  system  are,  however,  nearly  as  close,  and  by  some  au- 
thorities it  has  been  referred  to  the  latter.  The  Permian  is,  on 
the  whole,  distinctively  Palaeozoic,  but  it  has  several  features 
which  mark  it  out  as  transitional  to  the  Mesozoic. 

PERMIAN   SYSTEM 

W.  TEXAS  E.  TEXAS  PENNSYLVANIA 

fCapitan  Stage        Double  Mt.  Stage 
Guadalupian   ^^  ^         c}ear  ^  ^ 

I     Stage  Wichita  Stage  dtmkard  Stage 

637 


638  THE  PERMIAN   PERIOD 

DISTRIBUTION  OF  PERMIAN  ROCKS 

American.  —  Orogenic  movements  in  the  Appalachians  had 
probably  begun  in  the  middle  Carboniferous,  as  was  seen  in  the 
folding  which  inaugurated  the  Pottsville  trough,  and  toward 
the  end  of  the  Carboniferous  there  was  in  the  low-lying  Appala- 
chian coal-field  a  slowly  progressive  movement  of  elevation,  re- 
sulting in  the  draining  and  drying  up  of  most  of  the  region  over 
which  the  peat-bogs  had  been  extended.  The  movement  spread 
east,  north,  and  south,  leaving  in  the  middle  of  the  region  a  smaller 
area  in  which  the  conditions  of  .the  coal  measures  continued  very 
much  as  before.  In  the  northern  part  of  the  Acadian  province 
Permian  beds  overlie  the  coal  measures  in  Prince  Edward  Island, 
Nova  Scotia,  and  New  Brunswick.  These  beds  are  soft  red  shales 
and  sandstones,  which  were  laid  down  in  closed  basins,  not  in 
the  sea.  In  Pennsylvania,  Ohio,  West  Virginia,  and  Maryland, 
the  Permian  beds  follow  directly  and  without  any  break  upon 
the  Monongahela  stage  of  the  coal  measures:  they  were  for- 
merly called  the  Upper  Barren  Measures,  and  consist  of  1000 
feet  of  sandstones  and  shales  with  some  limestone  and  a  few 
seams  of  coal.  The  character  of  these  beds  is  entirely  like  that 
of  the  coal  measures,  to  which  they  were  once  referred,  and  their 
reference  to  the  Permian  is  due  to  the  marked  change  which  had 
come  over  the  vegetation.  South  of  West  Virginia  .10  Permian 
beds  have  been  found  in  the  Appalachian  area,  owing  to  the  ele- 
vation of  this  part  of  the  region  at  the  close  of  the  Carboniferous, 
but  the  Permian  occurs  in  Illinois,  in  what  appears  to  be  a  stream- 
channel  cut  in  the  coal  measures. 

As  we  proceed  westward  and  southward  through  Missouri 
into  Nebraska,  Kansas,  and  Texas,  we  find  the  Permian  assuming 
much  greater  importance,  and  becoming  more  and  more  promi- 
nently developed  in  extent  and  thickness.  A  study  of  this  region 
reveals  the  fact  that  only  a  part  —  the  lower  —  of  the  Permian 
is  developed  in  the  Acadian  and  Appalachian  areas.  At  the  end 
of  the  Lower  Permian  the'  entire  series  of  the  coal  measures  east 


DISTRIBUTION  OF  PERMIAN   ROCKS  639 

of  the  Mississippi  River  was  elevated  and  the  deposition  of  strata 
apparently  ended,  though  there  is  no  way  of  determining  exactly 
when  this  elevation  took  place,  nor  how  great  a  thickness  of  beds 
has  been  removed  by  denudation  since  the  upheaval.  In  the 
region  beyond  the  Mississippi  the  Permian  beds  thicken  south- 
ward, attaining  in  southern  Kansas  a  thickness  of  2000  feet,  and 
in  Texas  of  more  than  5000  feet.  The  mountains  of  Oklahoma, 
which  may  have  been  raised  late  in  the  Carboniferous  or  early 
in  the  Permian,  separate  the  Texas  and  Kansas  areas. 

During  the  greater  part  of  the  Permian  period  the  geographical 
state  of  North  America  was  somewhat  as  follows.  Except  for 
the  coastal  plain  on  the  Atlantic  and  Gulf  of  Mexico,  the  eastern 
portion  of  the  continent  had  very  much  its  present  limits,  though 
the  position  of  the  eastern  and  southern  coasts  cannot  be  deter- 
mined. The  coastal  plain  is  deeply  buried  under  deposits  which 
are  much  younger  than  the  Permian,  and  the  continent  may  have 
extended  farther  into  the  Atlantic  than  at  present,  or  the  ocean 
may  have  extended  more  over  the  land.  The  Interior  Sea  was 
greatly  changed  both  in  extent  and  character  from  what  it  had 
been  in  the  Upper  Carboniferous,  as  is  shown  by  the  nature  and 
distribution  of  its  sediments.  From  most  of  the  Mississippi 
valley  the  sea  had  withdrawn  entirely,  but  still  extended,  as  shal- 
low and  shifting  waters,  into  southeastern  Nebraska  and  Kansas, 
possibly  into  Iowa,  and  from  eastern  Kansas  the  line  ran  west- 
ward and  southwestward  across  Oklahoma  far  into  Texas.  In 
the  latter  part  of  the  period,  lagoons  were  cut  off  from  the  sea  and 
converted  into  salt  and  bitter  lakes  in  which  the  salt  and  gypsum 
of  Kansas  and  the  gypsum  of  Oklahoma  and  Texas  were  pre- 
cipitated. Occasionally  the  sea  broke  into  these  lakes,  bringing 
a  marine  fauna  with  it  for  a  short  time. 

The  Wichita  beds  of  Texas  have  two  very  distinct  facies;  in 
the  north  they  are  made  up  chiefly  of  fine  red  clays,  with  some 
beds  of  sandstone,  conglomerate  and  impure  limestone.  The 
clays  are  principally  river  deposits  made  in  a  delta,  or  along  a 
very  flat  coast,  but  with  marine  conditions  at  intervals.  Passing 


640  THE  PERMIAN   PERIOD 

southward,  these  beds  gradually  merge  into  marine  limestones 
which  were  originally  named  the  Albany  stage  and  placed  at  the 
top  of  the  Coal  Measures.  The  succeeding  Clear  Fork  beds, 
which  are  chiefly  clays  like  those  of  the  Wichita,  but  cut  by  many 
channels  filled  with  cross-bedded  sandstones,  extend  southward 
over  the  marine  limestones  of  the  Albany  facies,  but  even  in  the 
north  thin  layers  of  limestone  containing  marine  fossils  are  indica- 
tive of  transgressions  of  the  southern  sea.  The  Double  Moun- 
tain beds  are  largely  the  deposits  of  a  salt  lake  and  contain  much 
gypsum,  without  any  marine  fossils,  though  in  Oklahoma  beds  of 
a  corresponding  horizon  have  a  scanty  fauna  between  and  above 
the  gypsum  layers. 

Westward  from  Texas,  the  inland  Permian  sea  extended  over 
northern  Arizona  into  southern  Utah,  where  the  beds  are  sandy 
shales,  with  much  gypsum.  The  sea  continued  northward  through 
eastern  Utah,  western  Colorado,  and  probably  east  of  the  Rocky 
Mountains  also,  to  the  Black  Hills  and  central  Wyoming,  forming 
an  island  in  central  Colorado.  This  immense  body  of  water, 
or  perhaps  series  of  smaller  bodies,  was  land-locked  and  salt, 
and  in  it  were  formed  the  characteristic  "  Red  Beds  "  so  widely 
distributed  over  the  region  mentioned,  pointing  to  an  arid  climate. 
The  Red  Beds  are  not  all  Permian,  however,  and  the  rarity  of 
fossils  in  them  makes  it  often  impossible  to  decide  whether  a 
given  area  of  these  beds  should  be  referred  to  fhe  Permian, 
to  the  subsequent  Triassic,  or  to  both.  In  southern  Wyoming 
thin  bands  of  sandstone  and  limestone  in  the  Red  Beds  carry 
fossils  very  like  those  of  the  Kansas  and  Nebraska  Permian, 
but  the  course  of  this  marine  invasion  cannot  yet  be  made  out. 
Whether  the  Permian  has  been  removed  from  the  Great  Basin 
by  denudation,  or  never  deposited  over  the  greater  part  of  it, 
is  uncertain;  but  when  this  is  determined,  it  will  give  the  date 
of  the  upheaval  of  a  land  much  of  which  had  been  submerged 
throughout  the  Palaeozoic  era. 

Another  and  altogether  different  facies  of  the  North  American 
Permian  is  the  purely  marine  development  found  in  the  moun- 


DISTRIBUTION  OF  PERMIAN   ROCKS  641 

tains  of  western  Texas  and  on  the  Pacific  coast,  especially  in 
Alaska,  where  more  than  6000  feet  of  marine  Permian  have  been 
found  in  the  region  of  the  Copper  River.  The  Pacific  coast  fossils 
differ  strongly  from  those  of  the  more  eastern  regions,  and  the 
fauna  of  western  Texas  shows  affinity  with  the  Mediterranean 
and  Indian. 

Foreign.  —  In  Europe,  as  in  North  America,  the  Permian  is 
developed  in  two  very  distinct  facies.  Southern  Europe,  Sicily, 
and  the  Alps  have  almost  purely  marine  rocks  and  faunas,  which 
resemble  those  of  the  Texas  Guadalupian  series.  In  central  and 
western  Europe  the  disturbances  which,  in  many  places,  occurred 
at  the  end  of  the  Carboniferous  and,  in  those  areas,  produced  a 
marked  unconformity  between  the  Carboniferous  and  Permian, 
resulted  in  the  formation  of  a  great  inland  sea,  extending  from 
Ireland  to  central  Germany.  In  this  great  salt  lake  were  deposited 
masses  of  red  sandstones,  shales,  and  marls,  a  predominant  colour 
which  strongly  suggests  desert  conditions,  though  the  coal-beds 
of  France  and  Bohemia  and  central  Germany  might  seem  to 
contradict  this.  Occasionally  the  ocean  broke  into  this  closed 
basin,  but  the  invading  marine  faunas  soon  perished.  In  Ger- 
many the  Permian  is  in  two  very  strongly  marked  divisions,  the 
Rothliegendes,  or  Lower  Permian,  and  the  Zechstein,  or  Upper 
Permian,  whence  the  period  is,  in  that  country,  frequently  called 
the  Dyas.  The  very  interesting  discovery  has  quite  lately  been 
made  in  the  lower  Rothliegendes  of  Westphalia  in  Germany  of 
undoubted  and  characteristic  glacial  moraines,  resting  upon  a 
polished  and  striated  pavement  of  Upper  Carboniferous  rocks. 
(G.  Miiller,  1901.)  These  moraines  are  not  thick,  about  four 
feet,  and  suggest  local  rather  than  general  glaciation,  but  in  the 
Midlands  of  England  are  boulder  beds  which  apparently  show 
ice-action,  though  here  the  evidence  is  less  conclusive,  for  no 
striated  glacial  pavements  have  been  found,  and  the  glacial 
hypothesis  is  not  accepted  by  most  geologists. 

The  Lower  Permian  of  Europe  is  remarkable  for  the  great 
masses  of  volcanic  rocks,  lava  flows  and  tuffs,  which  it  contains, 

2T 


642  THE   PERMIAN    PERIOD 

and  which  occur  in  Great  Britain,  France,  Germany,  and  the 
Alps.  This  is  in  strong  contrast  to  the  corresponding  American 
series,  which  gives  no  evidence  of  vulcanism. 

Renewed  disturbances  at  the  end  of  the  Lower  Permian  shifted 
the  boundaries  of  the  inland  sea  and  changed  its  position,  so  that 
the  Zechstein  extends  beyond  the  Rothliegendes  and  overlaps 
upon  older  rocks,  and  at  the  same  time  brought  it  into  communi- 
cation with  the  ocean,  permitting  the  ingress  of  marine  animals, 
but  the  conditions  of  life  were  evidently  unfavourable,  for  the 
fauna  is  a  curiously  limited  one,  though  a  few  species  are  individ- 
ually abundant,  and  in  striking  contrast  to  the  varied  faunas  of 
the  truly  marine  facies.  Later,  the  sea  withdrew,  leaving  salt 
lakes,  in  which  enormous  bodies  of  rock-salt  were  formed  in  north 
Germany,  including  layers  of  the  salts  of  potassium  and  mag- 
nesium, already  referred  to  in  a  previous  shapter.  (See  p.  225.) 
Smaller  deposits  of  salt  extend  over  central  Germany  to  Russia. 
Upper  Permian  beds  with  gypsum  occur  in  England,  but  not  in 
France,  which  has  only  the  Lower. 

The  Permian  of  Russia  covers  a  very  large  area,  but  aside  from 
typical  Zechstein  limestones  on  the  Baltic  coast,  is  quite  different 
from  that  of  western  Europe;  the  principal  area  extends  along 
the  west  side  of  the  Ural  Mountains  to  the  Arctic  Sea,  and  into 
Nova  Zembla  and  Spitzbergen.  The  transition  from  the  under- 
lying Carboniferous  is  gradual,  and  the  lower  stage,  the  Artinsk, 
corresponds  to  the  Wichita  of  Texas.  Non-marine  beds  follow, 
which  are  again  succeeded  by  a  limestone  with  fossils  like  those 
of  the  German  Zechstein,  registering  another  invasion  of  the  sea. 
The  series  ends  with  the  Tataric  stage,  which  is  a  sequence  of 
red  marls,  passing  upward  without  any  apparent  break  into  the 
overlying  Triassic.  The  fossils  of  the  Tataric  stage  are  of  peculiar 
interest  and  will  be  referred  to  again.  Gypsum  and  salt  in  the 
non-marine  beds  testify  to  the  aridity  of  the  climate. 

In  Asia  the  two  facies  of  the  Permian  are  again  met  with.  The 
marine  facies  occurs  along  a  line  which  extends  the  course  of  the 
Mediterranean  eastward,  in  Armenia,  Persia,  northern  India, 


DISTRIBUTION   OF   PERMIAN   ROCKS  643 

Tibet,  China,  and  the  island  of  Timor.  In  the  central  Himalayas 
Upper  Permian  rests  upon  the  upturned  and  eroded  Lower 
Carboniferous,  and  is  conformably  overlaid  by  Triassic  beds.  The 
Salt  Range  of  northwestern  India  has  a  very  full  succession  of 
the  Middle  and  Upper  Permian,  the  Productus  limestone,  resting 
upon  continental  formations  of  the  Lower  Permian.  At  the 
base  of  the  latter  is  a  boulder  clay  of  glacial  or  iceberg  origin, 
which  is  an  outlier  of  a  great  ground  moraine  that  covers  large 
areas  in  central  India,  and  in  places  reaches  a  thickness  of  2000 
feet.  The  smoothed,  striated  and  characteristically  glacial  pave 
ment  upon  which  the  boulder  clay  rests,  has  also  been  observed. 
It  is  certainly  very  remarkable  to  find  glacial  deposits  formed 
on  such  a  scale  within  the  tropics  and  evidently  at  no  great  height 
above  the  sea-level.  The  boulder-clay  (Talchir)  forms  the  base 
of  the  Gondwdna  system,  a  succession  of  continental  deposits, 
with  much  coal,  laid  down  by  apparently  unbroken  sedimenta- 
tion, and  including  the  Permian,  Triassic,  and  Jurassic  systems. 
Northern  Asia  has  a  widespread  area  of  continental  deposits, 
which  are  presumably  Permian,  and  the  Artinsk  of  Russia  re- 
appears in  central  Asia,  but  the  continent  was  mostly  above  sea- 
level  and  the  great  seas  of  the  Carboniferous  had  withdrawn. 

South  Africa  has  a  Permian  development  so  closely  parallel 
to  that  of  India,  that  a  direct  land  connection  between  the  two 
regions  may  be  confidently  inferred.  Extending  almost  across 
the  continent  from  east  to  west  in  Cape  Colony  and  Natal  is  the 
thick  (1000  feet)  glacial  boulder  clay  of  the  Dwyka,  or  Lower 
Permian.  Part  of  the  Dwyka  is  shale  of  subaqueous  origin, 
but  most  of  it  is  a  mass  of  boulders,  striated  and  faceted,  em- 
bedded in  a  fine  unstratified  matrix.  (See  Figs,  in,  p.  231,  115, 
p.  233.)  The  formation  extends  northward,  growing  thinner  on 
the  way,  into  the  Transvaal  and  perhaps  into  Rhodesia,  and  in  the 
more  northerly  areas  the  underlying  ice-worn  pavement  of  older 
rocks  is  exposed  in  a  state  of  wonderful  freshness  (see  Figs.  70  and 
275)  entirely  comparable  to  the  recently  abandoned  beds  of  the 
shrinking  glaciers  of  the  Alps.  In  South  Africa,  therefore,  the 


644 


THE  PERMIAN   PERIOD 


remarkable  phenomenon  of  a  continental  glaciation  in  and  near 
the  tropics  presents  itself  as  well  as  in  India,  with  the  additional 
difficulty  that  in  the  former  region  the  movement  of  the  ice 
was  from  the  Equator  polewards.  Following  the  Dwyka  boulder 
clay  are  the  continental  strata  of  the  Karroo  system,  the  lower 
part  of  which  is  Permian  and  in  the  Transvaal  coal-bearing,  and 
which  corresponds  to  the  Indian  Gondwana  in  character,  in 
geological  date,  and  in  the  contained  fossils. 


FIG.  275. —  Roche  moutonnSe,  exposed  by  removal  of  Dwyka  boulder  clay. 
—  Riverton,  Cape  Colony.     (R.  B.  Young) 

The  eastern  portion  of  Australia  and  Tasmania,  which  had 
been  land  during  the  Upper  Carboniferous,  was  largely  sub- 
merged during  the  Permian,  but  Victoria  on  the  south  coast 
remained  above  sea-level  and  was  glaciated,  with  the  formation 
of  the  familiar  ice-pavements  and  thick  boulder  clays,  interstrati- 
fied  with  sandstones  and  shales.  The  Upper  Permian  is  coal- 
bearing,  as  it  is  also  in  New  South  Wales,  where  the  glacial  series 


DISTRIBUTION   OF  PERMIAN   ROCKS  64$' 

is  divided  into  two  distinct  parts  by  coal  measures.  The  glacial 
beds  occur  interstratified  with  marine  strata  and  some  of  the  ice- 
made  layers  themselves  contain  marine  fossils,  which  leads  to  the 
inference  that  the  great  blocks  were  deposited  by  icebergs  rather 
than  directly  by  glaciers.  In  Queensland  and  in  northwestern 
Australia  only  the  upper  boulder  succession  is  found,  and  the 
overlying  coal  measures  are  there  also.  The  Australian  ice  move- 
ment was  from  south  to  north,  as  would  naturally  be  expected 
in  the  southern  hemisphere,  but  this  makes  the  direction  of  move- 
ment in  South  Africa  only  the  more  inexplicable.  In  New  Zea- 
land the  Permian,  which  is  reported  to  be  7-10,000  feet  thick, 
contains  neither  coal  nor  evidence  of  ice-action,  but  includes 
lava-flows. 

In  South  America  Permian  beds  of  continental  origin  are  found 
in  Argentina  and  southern  Brazil.  In  the  latter  is  a  glacial  boulder 
clay,  followed  by  a  great  series  of  strata  which  resemble  those 
of  the  South  African  Karroo  system. 

The  distribution  of  the  Permian  rocks  and  fossils  leads  to  the 
inference  that  at  that  period  the  continents  were  so  grouped  as  to 
form  two  great  land  masses,  a  northern  including  North  America, 
most  of  Asia,  and  Europe,  and  a  southern  comprising  India, 
equatorial  and  southern  Africa,  Australia,  and  South  America. 
The  existing  southern  continents  were  probably  then  connected 
by  comparatively  narrow  land  bridges  across  the  site  of  the  present 
Atlantic  and  Indian  oceans.  '  Between  the  northern  and  southern 
land-masses  was  the  great  continuous  mediterranean,  a  sea  which 
has  been  named  "  Thetys  "  (Suess)  and  of  which  we  have  found 
indications  in  Texas,  Sicily,  the  eastern  Alps,  Asia  Minor,  northern 
India  and  southern  China  to  the  Pacific.  That  land  communica- 
tion was  occasionally,  at  least,  established  between  the  northern 
and  southern  lands  is  evident  from  the  distribution  of  the  land 
animals  and  plants  of  both  regions. 

Climate.  — The  plain  and  obvious  inferences  from  the  character 
of  the  Permian  rocks  are  so  remarkable  and  inexplicable  that 
they  were  long  received  with  incredulity  and  they  offer  a  series 


•646  THE   PERMIAN   PERIOD 

of  fascinating  problems  for  which  no  solution  can  yet  be  found. 
The  earliest  Permian  in  the  southern  hemisphere  was  a  time  of 
vast  glaciation  and  of  rigorous  climate,  as  is  convincingly  shown 
by  the  boulder  clays  and  ice  pavements  of  Australia,  South  Africa, 
and  South  America.  In  the  northern  hemisphere  the  glaciation 
was  extensive  in  peninsular  India,  apparently  local  in  Germany 
and  perhaps  in  England.  In  several  regions,  as  in  West  Virginia, 
France,  Germany,  and  Bohemia,  there  are  workable  coal  measures 
in  the  Lower  Permian,  but  arid  conditions  established  themselves 
over  all  parts  of  the  northern  hemisphere  where  Upper  and 
Middle  Permian  rocks  are  found,  western  North  America,  Texas, 
Kansas,  England,  Germany,  and  Russia.  The  extreme  conditions 
of  desert  climate  are  registered  in  the  great  bodies  of  gypsum  and 
rock-salt  which  characterize  so  much  of  the  Permian  areas. 

What  can  have  caused  these  climatic  vicissitudes  and  especially 
the  development  of  continental  ice-sheets  so  near  the  Equator 
and  so  little  above  sea-level,  is  a  problem  for  which  many  solu- 
tions have  been  propounded,  but  none  of  them  is  convincing. 

Close  of  the  Permian.  —  The  late  Palaeozoic  witnessed  mountain- 
making  disturbances  on  an  almost  world-wide  scale,  extending 
from  the  middle  Carboniferous  to  the  middle  of  the  Lower  Permian. 
In  central  Europe  and  Spain  vigorous  folding  took  place  at  the 
end  of  the  Lower  Carboniferous,  but  the  most  important  and 
widespread  disturbances  occurred  in  the  Permian.  In  North 
America  low  folds  were  formed  in  the  Appalachian  trough  from 
time  to  time  all  through  the  Palaeozoic,  the  evidence  of  which  is 
the  upheaval  of  the  barriers  described  in  the  preceding  chapters, 
which  separated  the  Interior  Sea  from  the  changing  bodies  of 
water,  such  as  the  Cumberland  Basin,  on  the  east.  A  more 
energetic  disturbance,  with  some  mountain  building,  inaugurated 
the  Pottsville  age  of  the  Upper  Carboniferous,  and  this  disturb- 
ance culminated  at  the  end  of  the  Permian  in  one  of  the  greatest 
geographical  revolutions  which  the  history  of  North  America  has 
recorded.  With  the  exception  of  the  mountain  making  at  the 
close  of  the  Ordovician,  the  Palaeozoic  era  in  North  America  had 


CLOSE  OF  THE  PERMIAN  647 

been  a  time  of  slow,  even  development,  with  many  oscillations  of 
level,  but  with  few  violent  disturbances,  and  with  singularly  few 
manifestations  of  volcanic  activity.  A  little  more  land  was  added 
to  the  northern  area  during  each  period,  but,  so  far  as  we  can  trace 
it,  the  geography  of  the  Ordovician  does  not  seem  to  have  been 
very  different  from  that  of  the  Carboniferous.  Throughout  this 
long  era  the  Appalachian  geosyncline  had  been  sinking,  though 
with  many  shifts  and  oscillations,  under  an  ever-increasing  load 
of  sediment,  until  the  great  trough  contained  a  thickness  of  25,000 
feet  or  more  of  strata.  Eventually  the  trough  began  to  yield  to 
lateral  compression,  and  its  contained  strata  were  thrown  into 
folds,  or  fractured  by  great  overthrusts.  .  Thus,  in  place  of 
a  sinking  sea-bottom  along  the  shore  of  the  great  Interior  Sea, 
rose  the  Appalachian  Mountains,  which  in  their  youth  may  have 
been  a  very  lofty  range,  rivalling  the  Alps  in  height.  This  range 
extends  from  the  Hudson  River  to  Alabama;  another  range  from 
Newfoundland  to  Rhode  Island,  and  a  third,  the  Ouachita  Moun- 
tains of  Arkansas,  are  attributed  to  the  same  set  of  disturbances, 
which  thus  made  themselves  felt  for  a  distance  of  2000  miles. 

Though  the  entire  continent  felt  the  effects  of  this  revolution, 
they  are  less  obvious  in  the  West.  On  the  western  side  of  the 
Rocky  Mountains  a  great  unconformity  is  found  between  the 
Permian  and  Triassic  members  of  the  Red  Beds.  "  There  are 
reasons  to  suppose  that  this  hitherto  unrecognized  break  is  wide- 
spread, and  explains  many  discordant  features  of  various  Red- 
bed  sections,  not  only  in  Colorado,  but  in  the  adjacent  Plateau 
province."  (Cross.)  The  Great  Basin  regiorl,  which  had  been 
submerged  through  nearly  the  whole  Palaeozojc  era,  became  land, 
and  at  the  present  time  the  surface  rocks  over  most  of  this  region 
are  Carboniferous.  It  is,  however,  probable  that  the  Permian  has 
been  stripped  away  by  denudation,  as  it  has  been  over  nearly  all 
of  the  northern  plateau  of  Arizona. 

Comparatively  soon  after  the  eastern  part  of  the  Great  Basin 
had  thus  been  converted  into  land,  the  ancient  land  area  of  its 
western  border  was  depressed  beneath  the  sea.  It  is  probable 


648  THE  PERMIAN   PERIOD 

that  these  two  movements  were  connected,  though  they  may 
have  been  separated  by  a  considerable  interval  of  time,  In  Ne- 
vada west  of  117°  30'  W.  long,  no  Palaeozoic  rocks  have  been 
found,  and  the  Trias  rests  directly  upon  the  Archaean. 

However  they  may  be  explained,  the  geographical  revolution 
which  closed  the  Palaeozoic  era  was  accompanied  by  the  most 
profound  and  far-reaching  changes  which  have  ever  occurred  in 
the  recorded  history  of  life,  after  which  we  find  ourselves  in  a  new 
world.  It  is  probable  that  the  change  was  a  relatively  rapid  one, 
but  there  are  sufficient  connections  between  the  faunas  and  floras 
of  the  two  eras  to  show  that  the  later  were  derived  from  the  earlier, 
and  that  the  gaps  are  due  to  the  imperfections  of  the  record. 

PERMIAN  LIFE 

We  have  to  note,  in  the  first  place,  that  the  animals  and  plants 
of  the  Permian  are  transitional  between  those  of  the  Palaeozoic 
and  those  of  the  Mesozoic  eras.  Here  we  find  the  last  of  many 
types  which  had  persisted  ever  since  Cambrian  times,  associated 
with  forms  which  represent  the  incipient  stages  of  characteristic 
Mesozoic  types,  together  with  others  peculiar  to  the  Permian. 

Plants.  —  The  flora  of  the  Lower  Permian  is  decidedly  Palaeozoic 
in  character,  and  that  of  the  Upper  Permian  as  decidedly  Mesozoic, 
so  that  if  the  line  dividing  these  two  great  eras  were  drawn  in 
accordance  with  the  vegetation,  it  would  pass  through  the  Per- 
mian. Even  in  the  Lower  Permian,  however,  the  change  from  the 
Carboniferous  flora  is  a  marked  one,  a  change  which  may  be 
largely  explained  by  the  increasing  aridity  of  the  climate.  The 
great  tree-like  Lycopods,  Lepidodendron  and  Sigillaria,  which 
were  so  abundant  in  the  Carboniferous  forests,  have  become 
very  rare;  none  of  the  former  genus  and  only  two  of  the  latter 
have  been  found  in  the  Upper  Barren  Measures  of  Pennsylvania 
and  West  Virginia.  The  Catamites  continue  in  hardly  diminished 
numbers  and  importance.  The  Ferns  are  exceedingly  abundant 
and  varied,  and  tree-ferns  seem  to  be  more  common  than  they 


PERMIAN  LIFE 


649 


FlG.  276.  —  Callipteris  conferta  Brngn. 
(Fontaine  and  White) 


had  been  before.     Especially  characteristic  genera  of  these  plants 

are   Pecopteris,  Callipteris    (Fig.  276),  Cynoglossa,  Neuropteris, 

Sphenopteris  (Fig.  277),  etc.    The  Gymnos perms  mark  a  notable 

advance;  in  addition  to  the 

ancient  Cordaites,  are  true 

Cycads  and  Conifers;  of  the 

latter    are    found    yew-like 

forms,    Walchia,    Saportcza 

with     leaves     nearly     four 

inches  wide,  and  the  Ging- 

koacece  are  probably  repre- 
sented by  Baiera. 

In   the  Upper   Permian, 

Lepidodendron,     Sigillaria, 

and  Calamites  are  quite  un- 
known, though  probably  a 

few  stragglers  still  existed, 

and  the  flora  is  made  up  of  Ferns,  Cycads,  Gingkos,  and  Conifers, 

the  Angiosperms  still  being  entirely  absent. 

The  Permian  flora  of  the  southern 
land  mass  differs  notably  from  that  of 
the  northern  continents  and  is  charac- 
terized especially  by  the  broad-leaved 
ferns,  Glossopteris  (Fig.  278)  and 
Gangamopteris,  whence  this  is  often 
called  the  "Glossopteris  Flora,"  to- 
gether with  the  cosmopolitan  Calli- 
pteris, the  Conifer  Voltzia  and  the 
Calamite  Schizoneura.  In  South  Amer- 
ica and  South  Africa,  but  not  in  India 
or  Australia,  these  plants  are  accom- 

FIG.  277.—  Sphenopteris  coriacea  panied  by   some    survivors  from   the 

F.  and  w.    (Fontaine  and  Carboniferous,  such  as  Lepidodendron. 

In  the  latter  part  of  the  Permian,  or 

possibly  not   till   the   earliest  Triassic,    the   Glossopteris    Flora 


650  THE   PERMIAN   PERIOD 

invaded  the  northern  continents  and  extended  its  range  to 
northern  Russia  (Tataric  stage). 

Foraminifera  are  almost  as  important  in  the  purely  marine 
limestones  as  they  had  been  in  the  Upper  Carboniferous. 

Coelenterata.  —  The  Corals  are  still  mostly  of  Palaeozoic  type 
and  belong  to  Carboniferous  genera,  but  some  of  the  modern 
Hexacoralla  have  appeared. 


FlG.  278. —  Glossopteris  browniana  Brngn.     Newcastle,  Australia 

Echinodermata. — This  group  has  dwindled  in  the  most  re- 
markable way,  and  instead  of  the  abundance  of  Crinoids  which 
flourished  in  the  Carboniferous  seas,  are  found  only  occasional 
specimens. 

Arthropoda.  — The  last  few  stragglers  of  the  genus  Phillipsia 
indicate  the  extinction  of  the  great  Palaeozoic  group  of  Crustacea, 
the  Trilobites,  which  henceforth  we  shall  meet  with  no  more ;  the 
Eurypterids  have  their  latest  known  representatives  in  the  little 


PERMIAN   LIFE  651 

coal  basin  of  Bussaco,  in  Portugal,  which  is  referred  to  the  Per- 
mian, though  with  some  doubt.  In  the  Kansas  Permian  have 
been  found  numerous  insects,  which,  though  resembling  those  of 
the  Carboniferous  in  a  general  way,  all  belong  to  species  different 
from  those  of  the  latter.  The  giant  insects  of  Commentry  in 
France,  which  have  been  mentioned  in  the  chapter  on  the  Carbo- 
niferous, are  now  referred  to  the  Permian  by  many  authorities. 

Bryozoa  are  prominent  in  all  marine  formations,  sometimes 
forming  reefs. 

The  Brachiopoda  are  still  very  abundant,  especially  in  the 
Lower  Permian;  they  are  closely  allied  to  those  of  the  Upper 
Carboniferous,  and,  as  in  that  period,  the  Productids  play  the 
most  important  role,  though  many  of  the  species  are  peculiar  to 
the  Permian.  The  curious  sessile,  irregularly  shaped  Richtho- 
fenia,  which  began  in  the  late  Carboniferous,  is  especially  char- 
acteristic of  the  Permian  limestone  accumulated  in  the  Mediter- 
ranean Thetys,  as  in  Asia,  Sicily,  and  western  Texas. 

Mollusca.  —  In  this  group  very  striking  changes  are  to  be 
noted.  The  Bivalves  increase  materially  in  variety,  and  in  addi- 
tion to  ancient  genera  like  Aviculopecten  (XII,  13)  and  Myalina 
(XII,  12)  the  typically  marine  Permian  has  many  new  forms, 
such  as  Area,  Lucina,  Lima,  Bakevellia  (XII,  14),  Pleurophorus 
(XII,  15),  Pseudomonotis  (XII,  16),  etc.  The  Gastropods  require 
no  particular  mention,  except  for  the  great  abundance  of  the 
genus  Bellerophon  (XII,  17).  It  is  among  the  Cephalopods  that 
the  great  advance  takes  place.  Orthoceras  and  Gyroceras  continue 
from  the  older  periods,  and  many  species  of  the  genus  Nautilus 
are  added,  but  the  chief  fact  consists  in  the  presence  of  Ammonoids 
with  highly  complex  sutures,  far  exceeding,  in  this  respect,  the 
Goniatites  of  the  Carboniferous,  some  of  which  continue  to  exist 
alongside  of  the  more  advanced  forms.  The  more  important 
new  genera  of  Ammonoids  are  Medlicottia,  Ptychites,  Popano- 
ceras,  Waagenoceras  (XII,  20),  which  have  been  found  in  Texas, 
Sicily,  Russia,  and  India.  The  presence  of  these  remarkable 
shells  gives  a  strong  Mesozoic  cast  to  the  Permian  fauna. 


652 


THE  PERMIAN   PERIOD 

Vertebrata.  —  The  Fishes  are  still 
of  Carboniferous  types,  and  many  of 
the  same  genera  occur,  while  new 
ones  are  brought  in.  To  the  Sharks 
are  added  the  curious  Menaspis, 
which  is  armed  with  numerous  long 
and  curved  spines.  Among  the 
Dipnoi  the  genus  Ceratodus,  very 
closely  allied  to  the  modern  lung- 
f  fish  of  Australia,  makes  its  first  ap- 
pearance. 

The    Amphibia  are  represented,  as 
in  the  Carboniferous,  by  the  Stego- 
cephalia,  and  several  of  the  older  genera 
'   persist,  but  many  new  forms  appear 
j   for  the   first  time,   several  of  which 
much  surpass  the  Carboniferous  genera 
in  size.     (See  Fig.  280.) 

The  most  important  character  that 
!    distinguishes  the  life  of  the  Permian 
|    from  that  of  all  preceding  periods  is 
the  appearance  in  large   numbers   of 
true  Reptiles.    There  is  no  reason  to 
suppose  that  such  a  variegated  reptil- 
?    ian  fauna  can  have  come  into  exist- 
!    ence  suddenly,  and  their  ancestors  will 
doubtless  be  discovered  in  the  Carbo- 
niferous; but  while  no  true  reptiles  are 
certainly  known  from  the  latter,  in  the 
Permian  they  are  the  most  conspicu- 
ous elements  of  vertebrate  life.    These 
reptiles  belong  to  several   orders,  one 
of  which,  the  Proganosauria,  is  rep- 
resented by  Mesosaurus  in  South  Af- 
rica and  by   Stereosternum   in    South 


PERMIAN  LIFE 


653 


America.    The  Proterosauria  are   a   very   central   group,   from 
which   many  other  reptilian  orders  appear  to  have  descended : 


FIG.  280.  —  Permian  Stegocephalian,  Eryops  megacephalus  Cope,  X 1/7.     Skull  seen 
from  side.     (Cope) 

Proterosaurus  and  Pal&ohatteria  are  the  most  important  Permian 
genera  of  this  group. 


FIG.  281.  —  Permian  Pelycosaurian,  Naosaurus  clavlger  Cope.     (Osborn) 

The  Pelycosauria  are  extremely  curious  animals  found  in  Texas 
and   Bohemia.,     These,    were   carnivorous    land   reptiles   which 


654  THE   PERMIAN   PERIOD 

had  short  tails  and  enormously  elongated,  sometimes  branching, 
spines  in  the  back  (Fig.  281).  The  Cotylosauria,  heavy  massive 
reptiles  of  exceedingly  primitive  character,  which  retained  several 
features  of  the  stegocephalous  Amphibia,  are  represented  in  Texas 
by  two  families,  including  several  genera,  of  which  Diadectes  may 
be  selected  as  typical.  In  South; Africa  is  found  the  extraordinary 
Pareiasaurus,  of  the  same  order.  Pareiasaurus  followed  the 
Glossopteris  Flora  in  its  northward  migration  and  appears  in 
the  uppermost  Permian  of  Russia  (Tataric  stage).  In  the  same 
stage  of  the  Russian  Permian  are  found  two  other  orders,  which 
likewise  seem  to  be  migrants  from  South  Africa,  for  they  are 
abundantly  represented  there,  the  Anomodontia,  with  turtle-like 
beaks  and  either  no  teeth  or  a  pair  of  large  tusks,  and  the  Therio- 
dontia,  the  latter  also  found  in  Bohemia. 


CHAPTER    XXXII 
THE   MESOZOIC    ERA— TRIASSIC    PERIOD 

THE  Mesozoic  era,  so  far  as  we  can  judge,  seems  to  have  been 
shorter  than  the  Palaeozoic;  in  North  America  Mesozoic  rocks 
are  very  much  more  important  and  widely  spread  in  the  western 
half  of  the  continent  than  in  the  eastern.  The  latter  region  was, 
in  a  measure,  completed  by  the  Appalachian  revolution,  and 
subsequent  growth  consisted  merely  in  the  successive  addition 
of  narrow  strips  to  the  coast-line,  but  in  the  West  many  great 
changes  were  required  to  bring  the  land  to  its  present  condi- 
tion. 

The  life  of  the  Mesozoic  constitutes  a  very  distinctly  marked 
assemblage  of  types,  differing  both  from  their  predecessors  of  the 
Palaeozoic  and  their  successors  of  the  Cenozoic.  In  the  course  of 
the  era  the  Plants  and  marine  Invertebrates  attained  substantially 
their  modern  condition,  though  the  Vertebrates  remain  through- 
out the  era  very  different  from  later  ones.  Even  in  the  Verte- 
brates, however,  the  beginnings  of  the  newer  order  of  things  may 
be  traced.  In  the  earlier  two  periods,  the  Triassic  and  Jurassic, 
vegetation  is  almost  confined  to  the  groups  of  Ferns,  Cycads,  and 
Conifers,  but  with  the  Cretaceous  come  in  the  Angios perms,  both 
Monocotyledons  and  Dicotyledons,  and  since  then  the  changes  have 
been  merely  in  matters  of  detail. 

With  few  exceptions,  the  ancient  Tetracoralla  had  all  disap- 
peared, and  the  modern  Hexacoralla  took  their  place.  The 
Echinoderms  were  all  markedly  different  from  the  Palaeozoic  types. 
The  Cystoids  and  Blastoids  had  died  out,  and  the  Crinoids  had 
been  revolutionized,  the  Camerata  being  replaced  by  the  Articu- 
lata.  Likewise  the  modern  sea-urchins,  Euechinoidea,  replaced  the 
ancient  Palceechinoidea,  and  many  Mesozoic  genera  of  the  former 
group  are  still  living  in  our  modern  seas.  The  Starfishes  also 

655 


656  MESOZOIC   ERA 

assumed  their  modern  condition.  Brachiopods  were  far  less  abun- 
dant and  diversified  than  they  had  been  in  the  Palaeozoic,  and  be- 
longed, for  the  most  part,  to  different  families,  while  the  Bivalve  and 
Gastropod  Mollusca  increased  to  a  wonderful  extent.  Especially 
characteristic  are  the  marvellous  wealth  and  variety  of  the  Am- 
monoid  Cephalopods,  which  disappear  at  the  close  of  the  era. 
The  Dibranchiate  Cephalopods,  with  internal  shells,  make  their 
first  appearance  in  the  Mesozoic,  and  one  group  of  them,  the 
Belemnites,  is  almost  exclusively  confined  to  the  era.  The  Arthro- 
pods showed  the  same  revolutionary  changes.  Among  the  Crus- 
tacea, the  Trilobites  and  Eurypterids  have  gone  out,  but  all  the 
modern  groups  were  well  represented,  though  many  of  the  Mesozoic 
genera  are  no  longer  to  be  found  in  the  seas  of  to-day.  Insects 
reached  nearly  their  modern  condition,  so  far  as  the  large  groups 
are  concerned,  butterflies,  bees,  wasps,  ants,  flies,  beetles,  etc., 
being  added  to  the  older  orthopters  and  neuropters. 

Fishes  became  modernized  before  the  close  of  the  era,  the 
Bony  Fishes  having  acquired  their  present  predominance.  The 
Amphibia  took  a  subordinate  place,  and  after  flourishing  for  a 
time,  the  great  Stegocephalia  died  out,  leaving  only  the  pygmy  sala- 
manders and  frogs  of  the  present.  Birds  and  Mammals  made 
their  first  appearance,  the  former  advancing  rapidly  to  nearly  their 
present  grade  of  organization,  though  not  reaching  their  present 
diversity,  while  the  mammals  remained  throughout  the  era  very 
small,  primitive,  and  inconspicuous.  The  most  significant  and 
characteristic  feature  of  Mesozoic  life  is  the  dominance  of  the 
Reptiles,  which,  in  size,  in  numbers,  and  in  diversified  adaptation 
to  various  conditions  of  life,  attained  an  extraordinary  height  of  de- 
velopment. The  Mesozoic  is  called  the  "  Era  of  Reptiles,"  be- 
cause these  were  the  dominant  forms  of  life.  They  filled  all  the 
roles  now  taken  by  birds  and  mammals;  they  covered  the  land 
with  gigantic  herbivorous  and  carnivorous  forms,  they  swarmed 
in  the  sea,  and,  as  literal  flying  dragons,  they  dominated  the  air. 
At  the  present  time  there  are  only  five  orders  of  reptiles  in  exist- 
ence, and  of  these  only  the  crocodiles  and  a  few  snakes  attain  really 


THE  TRIASSIC   PERIOD  657 

large  size.  In  the  Mesozoic  era  no  less  than  twenty-five  reptilian 
orders  flourished,  and  many  of  them  had  gigantic  members.  Some 
were  the  largest  land  animals  that  ever  existed,  and  the  sea-dragons 
rivalled  the  whales  in  size.  Nothing  so  clearly  shows  that  the 
Mesozoic  era  is  a  great  historical  fact,  as  the  dominance  of  its 
reptiles. 

The  Mesozoic  climates  offer  some  difficult  problems.  In  general, 
the  climate  was  mild,  as  is  shown  by  the  plants  found  in  the  Meso- 
zoic rocks  of  Arctic  lands,  for  in  Greenland,  Alaska,  and  Spitz- 
bergen  was  a  luxuriant  vegetation  of  warm  temperate  type.  On 
the  other  hand,  certain  geologists  have  maintained  the  existence 
of  distinct  climatic  belts  in  the  Mesozoic,  indicating  equatorial, 
northern,  and  southern  zones,  but  by  others  this  interpretation  is 
denied. 

The  Mesozoic  era  comprises  three  periods,  —  the  Triassic, 
Jurassic,  and  Cretaceous. 

THE    TRIASSIC    PERIOD 

The  Triassic  period  is  so  named  from  the  very  conspicuous 
threefold  subdivision  of  this  system  of  strata  in  Germany,  where 
its  rocks  were  first  studied  in  detail,  and  where  they  occupy  a 
greater  area  than  in  any  other  European  country.  The  German 
Trias  is,  however,  not  the  usual  facies  of  the  system,  but  a  very 
peculiar  one,  and  cannot  be  taken  as  the  standard  of  comparison 
for  most  other  countries. 

The  Trias  of  North  America  is  displayed  under  three  very 
different  facies,  —  that  of  the  Pacific  coast,  which  is  marine;  that 
of  the  interior,  which  is  lacustrine;  and  that  of  the  Atlantic  border, 
which  is  continental.  Owing  to  the  absence  of  fossils  common  to 
all,  it  is  not  yet  possible  accurately  to  correlate  the  three  facies, 
but  the  divisions  of  the  Pacific  and  Atlantic  borders  are  given  in 
the  following  table  together  with  those  of  Germany  and  the  general 
arrangement  of  the  Oceanic  Trias,  the  former  being  chiefly  of 
continental  facies:  — 


658 


THE  TRIASSIC  PERIOD 


TRIASSIC   SYSTEM 

GENERAL  OCEANIC                         GERMANY 

PACIFIC  COAST    ATLANTIC  COAST 

Bajuvaric  f  Rhaetic  Stage 

Bajuvaric 

Series   \Juvavian  Stage 

Series 

Newark 

fCarinthian  Stage 
Tirolic 

•     Keuper 

Series 

\  Norian  (or  La- 

Spripc 

Tirolic 

OCi  ICO           1      i  .       •           \     r, 

[dinian)  Stage 

Series 

Dinaric      f  Anisian  Stage 

c    •       i  TJ  j      •      I?,.          Muschelkalk 
Series     VHydaspian  Stage 

Dinaric 

Series 

Scythic      fjakutian  Stage           Bunter 

Scythic 

Series    \Brahmanian  Stage     Sandstein 

Series 

CHARACTER  AND  DISTRIBUTION  OF  TRIASSIC  ROCKS 

European.  —  The  Trias  of  Europe  has  been  so  thoroughly 
studied  and  throws  so  much  light  upon  American  problems, 
that  it  will  be  profitable  to  depart  from  the  usual  order  of 
treatment  and  take  up  first  the  development  in  that  conti- 
nent. As  in  the  Permian,  the  Triassic  rocks  of  Europe  are 
found  iri  two  contrasted  facies,  the  continental  and  the  oceanic; 
the  former  extending,  with  interruptions,  from  Ireland,  across 
England,  France,  central  and  southern  Germany,  to  Poland, 
and  consisting  chiefly  of  red  sandstones  and  red  marls  and  clays, 
with  conglomerates  and  some  limestones.  From  this  it  follows 
that  the  rocks  in  the  mountains  which  bordered  the  Triassic  basins 
and  plains  had  been  profoundly  decomposed,  as  in  the  southern 
Appalachians  of  to-day,  where  the  crystalline  rocks  are  changed 
into  a  red  clay  for  depths  of  100  feet  or  more,  and  the  quartz 
grains  are  covered  with  a  red  film.  As  we  have  learned  (Chapter 
IV)  the  red  laterites  of  warm  regions  may  be  derived  from  very 
many  different  kinds  of  parent-rock,  igneous  rocks,  crystalline 
schists,  limestones,  and  dolomites.  "  This  makes  intelligible 
the  close  agreement  of  the  continental  Triassic  rocks  over  great 
areas  of  the  earth's  surface.  It  is  not  at  all  necessary  that  the 
mountain  ranges  which  surrounded  the  Triassic  basins  and  plains 


CHARACTER  AND   DISTRIBUTION  OF  TRIASSIC   ROCKS     659 

should  have  been  built  up  of  the  same  rocks,  which,  as  a  matter 
of  fact,  was  probably  never  the  case.  ...  It  seems  to  be  un- 
doubted that  the  continental  Triassic  sediments  were  deposited 
in  basins,  or  on  low  plains,  and  that  their  material  was  derived 
from  plateaus  and  mountains.  It  may  likewise  be  inferred  from 
the  size  and  rounded  shape  of  the  conglomerate  pebbles  that 
running  water  transported  the  material  from  the  highlands  to  the 
basins."  (E.  Philippi.) 

In  Germany,  where  the  plainly  marked  threefold  division  of  the 
strata  has  given  its  name  to  the  system,  the  lower  series,  or  Bunter 
Sandstein,  varies  from  650  to  1800  feet  in  thickness  and  is  chiefly 
made  up  of  red  sandstones  and  sandy  shales.  In  northern  and 
central  Germany  it  is  so  intimately  connected  with  the  Upper 
Permian  that  any  line  of  separation  between  them  appears  to  be 
arbitrary,  but  in  the  west  and  southwest  there  is  an  overlap  of  the 
Bunter  upon  older  rocks.  Around  the  margins  of  the  Triassic 
basins  are  coarse  conglomerates,  with  finer  materials  toward  the 
centres.  Occasional  and  temporary  lakes,  or  playas,  were  formed 
in  the  basins,  and  floods  rushing  from  the  mountains  spread  sheets 
of  sand  and  gravel  far  out  over  the  plains,  while  cross-bedded 
sands  were  piled  up  by  the  winds  over  extensive  areas.  Locally, 
the  playas  formed  deposits  of  salt  and  gypsum,  and  clastic  beds 
marked  by  sun-cracks,  rain-prints,  and  tracks  of  animals.  All  of 
these  features  point  unmistakably  to  an  arid  climate,  though  one  that 
was  probably  less  extremely  dry  than  that  of  the  Upper  Permian. 
The  mountain  ranges  appear  to  have  been  sufficiently  high  to 
cause  abundant  precipitation  upon  them;  such  a  juxtaposition 
of  rainy  mountains  and  arid  plains  has  nothing  unusual  about  it. 

The  Middle  Trias,  or  Muschelkalk,  is  marked  by  successive  in- 
cursions of  the  sea,  the  first  of  which  came  at  the  end  of  the  Bun- 
ter epoch  and  eventually  extended  over  a  great  part  of  the  area 
occupied  by  the  Bunter  Sandstein,  leaving  deposits  with  a  maxi- 
mum thickness  but  little  exceeding  noo  feet.  The  fossils  show 
that  this  was  an  inland  sea,  connected  with  the  ocean,  but  having 
a  fauna  which  consists  of  comparatively  few  species,  though  these 


660  THE  TRIASSIC  PERIOD 

are  sometimes  individually  very  abundant.  The  relation  of  these 
fossils  to  those  of  the  contemporary  part  of  the  oceanic  Trias  is 
much  like  the  relation  between  the  modern  faunas  of  the  Black 
Sea  and  the  Mediterranean.  In  the  middle  of  the  Muschelkalk  the 
connection  with  the  ocean  was  shut  off  and  the  German  sea  con- 
verted into  a  salt  lake,  as  is  demonstrated  by  the  deposits  of  gyp- 
sum and  salt,  but  the  marine  conditions  were  soon  reestablished. 

An  elevation  of  the  land  caused  the  withdrawal  of  the  Middle 
Triassic  sea  and  the  resumption  of  the  conditions  of  continental 
sedimentation,  resulting  in  the  formation  of  the  Keuper  (maximum 
thickness  2000  feet).  The  lower  parts  of  the  Keuper  contain 
some  marine  beds  and,  locally,  thin  beds  of  coal,  but  most  of  it 
consists  of  sandy  and  clayey  beds,  which  change  rapidly  from  point 
to  point,  and  are,  on  the  whole,  of  finer  materials  than  those  of  the 
Bunter.  The  basins  had  been  largely  filled  with  sediment  and  the 
mountains  had  been  lowered  by  denudation,  so  that  the  coarser 
materials  could  no  longer  be  transported.  The  middle  Keuper 
was  a  time  of  extensive  salt  lakes,  in  which  large  bodies  of  gyp- 
sum and  some  salt  were  precipitated.  The  latest  stage  of  the 
Keuper,  the  Rhcetic,  witnessed  a  renewed  transgression  of  the  sea. 

Trias  of  continental  origin  occurs  in  other  European  countries. 
In  Great  Britain  the  Triassic  is  almost  all  continental,  the  Middle 
Triassic  marine  invasion  not  extending  so  far  to  the  northwest, 
but  the  Rhaetic  transgression  did,  and  beds  of  this  stage  form  a 
thin,  though  persistent  band  at  the  top  of  the  Keuper.  The  Tri- 
assic beds  cover  a  large  part  of  the  central  plains  of  England,  ex- 
tending to  northeastern  Ireland,  and  small  areas  occur  on  the  east 
coast  of  Scotland.  In  France  Trias  of  the  German  type,  including 
the  Muschelkalk,  extended  into  the  eastern  and  southern  parts  of 
the  country  to  the  Pyrenees  and  along  eastern  and  southern  Spain. 
Triassic  rocks  also  occur  around  the  margins  of  the  central  Plateau 
of  France.  In  the  south  of  Sweden  is  a  considerable  area  of 
Triassic  rocks:  the  Keuper  is  coal-bearing  and  is  overlaid  by 
marine  Rhastic  beds.  In  northeastern  Russia  is  a  great  extent 
of  beds  belonging  to  the  Tataric  stage,  which  has  been  mentioned 


CHARACTER  AND   DISTRIBUTION   OF  TRIASSIC   ROCKS      66 1 

in  connection  with  the  Permian,  but  which  may  be  in  part 
Triassic. 

In  the  Alps  are  two  well-distinguished  regions;  in  the  western 
part  conditions  were  not  unlike  those  of  Germany,  while  the  eastern 
part  displays  a  great  development  of  the  oceanic  limestones. 
The  western  Alpine  Trias  is  much  folded  and  metamorphosed  and 
becomes  very  thick  on  the  Italian  side  of  the  mountains,  and  is 
conspicuous  in  the  northern  Apennines.  On  the  east,  the  Palaeo- 
zoic rocks  of  the  Alps  extended  as  a  long  island,  or  chain  of  islands, 
from  the  Engadine  into  southern  Austria.  "  North  of  this  old 
insular  tract  the  Triassic  strata  are  on  the  whole  somewhat  sandy. 
...  On  the  south  side  the  deposition  of  limestone  and  dolo- 
mite went  on  more  continuously,  though  interfered  with  occa- 
sionally by  submarine  volcanic  eruptions."  (A.  Geikie.)  Almost 
the  whole  Triassic  succession,  except  the  lowest  members,  is  rep- 
resented here  by  great  limestones  and  dolomites,  many  of  the 
latter  probably  of  coral-reef  origin. 

Asiatic. — The  existence  of  the  great  Mediterranean  Thetys 
in  part  of  the  Triassic  period  is  indicated  by  the  oceanic  deposits 
which  occur  in  Asia  Minor,  Central  Asia,  Baluchistan,  Afghanistan, 
northern  India,  and  Burmah,  Tongking,  and  Southern  China, 
but  in  the  Lower  Trias  it  appears  that  India  was  not  in  connection 
with  Europe.  In  the  Salt  Range  of  northwestern  India  and  the 
Himalayas  is  a  remarkably  complete  succession  of  Triassic  rocks, 
which  overlie  the  Permian  in  a  conformity  that  is  at  least  apparent 
and  may  be  actual,  though  there  is  a  faunal  break  between  the 
two  systems.  The  Brahmanian  stage  of  India  is  not  represented 
in  the  Alps.  In  central  India  the  Gondwana  conditions  of  con- 
tinental sedimentation  continued  apparently  through  the  whole 
Triassic.  Rocks  of  this  period,  which  seem  to  belong  in  another 
faunal  province,  occur  in  Japan,  the  east  coast  of  Siberia,  and  the 
Arctic  islands,  Spitzbergen,  and  Bear  Island. 

American. —  In  the  early  part,  at  least,  of  the  period,  both  North 
and  South  America  extended  farther  east  than  at  present,  and  no 
marine  Triassic  rocks  are  known  on  the  Atlantic  slope  of  either 


662 


THE  TRIASSIC  PERIOD 


FIG.  282.  —  Map  of  North  America  in  the  Triassic  and  Jurassic  periods.     Black 

^  areas  =  known  exposures;    white   areas  =  land;    lined    areas  =  sea;    dotted 

areas  =  continental  deposits.     The  horizontal  lines  indicate  Triassic,  and  the 

vertical  lines  Jurassic,  seas. 


CHARACTER   AND   DISTRIBUTION  OF  TRIASSIC   ROCKS      663 

continent,  but  they  are  extensively  displayed  on  the  Pacific  side. 
The  land  barrier  which  during  the  Palaeozoic  era  had  bounded  the 
Great  Basin  sea  on  the  west  was  submerged  and  the  Pacific  ex- 
tended over  the  site  of  the  Sierras,  covering  western  Nevada  and 
sending  a  gulf  into  southeastern  Idaho,  and  in  British  Columbia 
it  transgressed  eastward  across  the  present  mountains,  and  it 
covered  part  of  the  coast  of  Alaska.  In  California  and  Nevada 
all  the  series  and  many  of  the  stages  of  the  oceanic  Trias  may  be 
identified  and  their  faunal  relations  change  in  a  very  interesting 
way.  The  lower  series  (Scythic)  "  shows  an  intimate  relationship 
to  that  of  Asia  and  none  with  that  of  the  Mediterranean  region  " 
(J.  P.  Smith),  and  this  relationship  is  both  with  India  and  northern 
Asia.  The  difference  from  Europe  is  no  doubt  to  be  explained  by 
the  fact,  above  referred  to,  that  in  the  Lower  Trias,  Thetys  was 
interrupted  somewhere  between  India  and  Europe.  In  the  Di- 
naric  series  an  invasion  from  the  Mediterranean  region  is  evident, 
though  the  track  followed  by  this  migration  is  not  clear.  In  the 
Upper  Trias  (Tirolic  series)  the  relations  were  first  with  India  and 
the  Mediterranean,  succeeded  by  another  migration  from  the  north 
of  Asia.  These  changes  in  faunal  relationships  have  been  variously 
explained  and  will  again  be  referred  to  in  considering  the  question  of 
theTriassic  climates.  Little  of  the  Bajuvaric  series  is  found  on  the 
Pacific  coast.  In  the  United  States  the  marine  Triassic  rocks  do 
not  exceed  4800  feet  in  maximum  thickness,  but  in  British  Columbia 
this  increases  to  13,000,  much  of  which  is  igneous  material,  and 
similar  material  is  widely  distributed  in  southeastern  Alaska. 

In  central  Mexico,  State  of  Zacatecas,  is  an  isolated  area  of 
marine  Trias,  belonging  to  the  Tirolic  series  of  the  Upper  Triassic 
and  with  a  fauna  allied  to  that  of  California.  This  is  probably 
only  a  remnant  of  a  formerly  widespread  area  of  such  rocks,  nearly 
all  of  which  were  eroded  away  during  the  Lower  and  Middle 
Jurassic,  when  most  of  Mexico  was  land. 

On  the  Atlantic  side  of  North  America  the  course  of  events  was 
entirely  different.  In  the  latter  half  of  the  period  was  formed 
a  series  of  long,  narrow  troughs,  running  closely  parallel  to  the 


664  THE  TRIASSIC  PERIOD 

trend  of  the  Appalachian  Mountains,  but  separated  from  them  by 
the  ridges  of  metamorphic  and  crystalline  rocks,  which  follow  those 
mountains  on  the  east,  and  which  then  probably  had  a  considerable 
altitude,  much  greater  than  at  present.  In  these  troughs  was  laid 
down  the  enormous  thickness  of  non-marine  rocks  which  constitute 
the  Newark  series  and  are  now  found  in  several  disconnected 
areas  from  Nova  Scotia  to  North  Carolina.  The  longest  continu- 
ous stretch  of  these  beds  is  from  the  Hudson  River  across  New 
Jersey,  southeastern  Pennsylvania  and  Maryland,  into  Virginia, 
while  another  extensive  area  occupies  the  Connecticut  valley, 
through  western  Connecticut  and  Massachusetts.  The  Newark 
areas  have  been  so  extensively  faulted  that  it  is  difficult  to  ascertain 
their  thickness,  and  the  figures  given  are  merely  an  approximation. 
The  series  reaches  its  maximum  in  southeastern  Pennsylvania, 
where  it  is  estimated  at  20,000  feet,  in  New  Jersey  12,000-15,000, 
and  in  the  Connecticut  valley,  13,000.  Southward  the  rocks  thin 
quite  rapidly,  and  about  Richmond,  Va.,  are  not  more  than  3000 
feet  thick,  and  farther  south,  still  less. 

The  Newark  rocks  are  prevailingly  red  sandstones  and  shales, 
especially  from  Pennsylvania- northward,  but  also  contain  some 
very  coarse  conglomerates  at  the  base,  and  higher  up  in  the  series 
along  the  western  border  of  the  area.  Thin  bands  of  limestone 
and  black,  fossiliferous  shales  are  intercalated,  and  in  New  Jersey 
is  a  thick  mass  of  very  hard,  slate-coloured  shales,  the  Lockatong 
stage.  In  the  northern  area,  Connecticut  valley,  New  Jersey, 
and  Pennsylvania,  many  of  the  beds  are  ripple-marked,  sun- 
cracked,  pitted  with  raindrops,  and  preserve  countless  footprints 
of  Amphibia  and  land  Reptiles.  In  Virginia  and  North  Carolina 
are  workable  coal-seams,  and  the  red  colour  of  the  other  rocks  is 
less  prevalent  than  in  the  North.  Except  in  the  black  shales,  fossils 
are  very  few,  and  the  plants  show  a  distinct  difference  between  the 
Virginia  and  North  Carolina  area,  where  ferns  predominate,  and 
the  New  Jersey-Connecticut  region,  where  ferns  are  less  abundant 
and  gymnosperms  more  so.  Whether  this  difference  is  climatic 
or  due  to  a  slight  difference  in  geological  date  it  is  difficult  to  say. 


CHARACTER  AND   DISTRIBUTION  OF  TRIASSIC   ROCKS      665 

It  is  sufficiently  evident  that  the  rocks  of  the  Newark  series  are 
not  marine,  but  just  how  they  were  accumulated  is  a  question  as 
to  which  there  is  much  disagreement.  It  has  been  usual  to  consider 
the  strata  as  of  estuarine  origin,  but  their  lithological  character,  the 
surface  markings  of  many  of  the  beds,  and  the  contained  fossils 
make  such  an  origin  improbable.  It  is  likely  that  fluviatile  and 
subaerial  agencies  have  had  more  to  do  with  the  accumulation 
of  the  materials  than  had  any  body  of  water  in  communication  with 
the  sea.  At  the  southern  end  of  the  trough,  the  coal-beds  point 
unmistakably  to  the  existence  of  fresh-water  swamps  and  bogs, 
while  the  red  colour  and  the  sun-cracks  prevalent  in  the  remainder 
suggest  conditions  resembling  those  of  the  German  Bunter  and 
Keuper,  though  the  absence  of  gypsum  and  salt  indicates  that  the 
climate  was  less  arid  than  in  Europe.  If  we  could  be  sure  that  the 
plant-bearing  beds  of  Virginia  were  contemporaneous  with  those 
of  New  Jersey  and  the  Connecticut  valley,  the  difference  in  the 
floras  would  confirm  the  inference  that  the  climate  was  sub-arid  in 
the  North,  growing  more  moist  southward,  but  the  difference  may 
be  geological  rather  than  geographical.  No  doubt  lakes  of  greater 
or  less  duration  were  formed  and  are  now  registered  in  the  fish- 
bearing  shales. 

Igneous  activity  was  a  conspicuous  feature  of  the  Newark 
epoch,  both  in  the  volcanic  and  the  intrusive  form.  Lava-flows 
were  poured  out  on  the  surface  and  were  subsequently  buried,  and 
beds  of  fragmental  products  have  been  found  in  the  Connecticut 
valley  and  in  New  Jersey,  while  dykes  and  sills  accompany  the 
strata  throughout  their  extent.  Now  that  they  are  exposed  by 
denudation,  these  plutonic  bodies  form  very  striking  features  of 
the  topography.  One  of  the  most  remarkable  of  the  intrusive 
masses  is  the  great  Palisades-Rocky  Hill  sill,  the  outcrops  of 
which  are  separate,  though  their  subterranean  connection  is  well 
ascertained.  An  account  of  the  Palisades  has  already  been  given 
in  Chapter  XV.  All  the  known  fossil-bearing  horizons,  with  per- 
haps one  exception,  correlate  the  Newark  rocks,  with  the  Keuper, 
possibly  extending  up  into  the  Rhaetic.  The  exception  noted  is  in 


666  THE  TRIASSIC  PERIOD 

southeastern  Pennsylvania,  where  the  beds  reach  their  maximum 
thickness  and  where  the  deeper  portions  are  said  to  have  yielded 
Permian  plants,  but  this  awaits  confirmation. 

In  the  Mexican  States  of  Sonora  and  Oaxaca  are  beds'  similar 
to  those  of  the  Newark,  which  contain  plants  that  show  the  forma- 
tions to  be  of  upper  Keuper  age,  passing  into  the  Rhaetic.  Still 
farther  south,  similar  beds  occur  in  Honduras. 

A  third  facies  of  the  North  American  Trias  is  that  of  the  western 
interior,  which  has  much  the  same  distribution  as  the  interior 
Permian,  extending  from  Texas,  Arizona,  and  New  Mexico,  on 
the  south,  around  the  Colorado  island,  north  to  the  Canadian 
provinces,  though  not  continuously.  In  many  areas  the  beds  have 
as  yet  yielded  no  fossils  and  are  referred  to  the  Triassic  upon  strati- 
graphical  grounds,  which  are  not  always  trustworthy.  However, 
the  presence  of  the  Trias  is  definitely  ascertained  at  many  points  in 
the  region  defined,  though  how  much  of  the  system,  or  what  part  of 
it,  is  present  at  any  particular  locality,  can  rarely  be  determined. 
As  in  the  underlying  Permian,  the  occurrence  of  gypsum  and  salt 
is  evidence  of  salt  lakes  and  an  arid  climate,  but,  as  was  probably 
true  in  the  Newark  region  of  the  East,  the  climate  appears  to  have 
become  less  arid  southward,  for  fresh-water  Trias  has  been  found 
in  Texas,  northeastern  New  Mexico,  and  southwestern  Colorado. 

South  America,  like  so  many  of  the  other  continents,  has  both 
the  continental  and  marine  facies  of  the  Trias;  ihe  former  is 
found  east  of  the  Andes,  in  the  Argentine  Republic,  and  is  coal- 
bearing,  with  plants  which  correlate  it  with  the  Rhastic.  On  the 
west  side  of  the  Andes  the  marine  Trias  is  upturned  in  the  moun- 
tains. 

African.  —  In  South  Africa  the  Karroo  system,  as  already 
pointed  out,  is  a  continental  formation,  extending  in  apparently 
unbroken  continuity  from  the  Permian  into  the  Jurassic.  The 
Triassic  portion  is  the  upper  part  of  the  Beaufort  series  and  has 
yielded  a  remarkable  array  of  fossil  Reptiles.  The  land  connection 
with  India,  "  Gondwana  Land,"  was  still  maintained.  In  north- 
ern Africa  Trias  of  the  German  type  covers  extensive  areas  in  the 


THE  LIFE  OF  THE  TRIASSIC  667 

province  of  Constantine,  Algeria,  corresponding  to  the  Muschel- 
kalk  and  Keuper,  the  latter  containing  gypsum. 

Australasian.  —  Oceanic  Trias,  especially  of  the  Upper  Triassic, 
is  found  in  many  of  the  Indo-Pacific  islands,  Borneo,  Sumatra, 
the  Moluccas,  New  Caledonia,  and  others.  In  Australia  conti- 
nental Trias  is  found  in  New  South  Wales,  where  the  lower  portion 
is  coal-bearing,  and  in  Queensland,  where  the  upper  division  carries 
coal-beds,  as  also  in  the  northwest.  In  New  Zealand,  there  is 
evidence  of  glaciation  in  this  period  and  the  glacial  beds  are  over- 
laid by  marine  deposits. 

Climate.  —  In  the  Triassic  of  the  northern  hemisphere  there  is 
evidence  of  very  widespread  aridity  of  climate,  accompanied 
by  general  warmth;  central  Europe,  north  Africa,  the  western 
interior  of  North  America  and,  in  lesser  degree,  the  northeastern 
part  of  the  same  continent.  From  the  distribution  of  the  fossils, 
there  is  reason  to  believe  that  in  the  Arctic  Sea  the  water  was  cooler 
than  in  lower  latitudes  and  that  the  remarkable  changes  in  the 
faunal  relations  of  our  Pacific  coast  during  the  period  are  to  be 
explained  rather  by  the  closing  and  reopening  of  Bering  Straits 
than  by  the  upheaval  and  depression  of  land  bridges  across  the 
wider  parts  of  the  Pacific.  This  problem  will  again  present  itself 
in  the  subsequent  periods. 

THE  LIFE  OF  THE  TRIASSIC 

Triassic  life  is  entirely  different  from  anything  that  had  pre- 
ceded it,  though  the  way  for  the  change  was  already  preparing  in 
the  Permian.  As  we  have  seen,  the  Upper  Permian,  if  classified 
by  its  plants  alone,  would  be  referred  to  the  Mesozoic  rather  than 
to  the  Palaeozoic,  and  we  are  therefore  prepared  to  learn  that  the 
Triassic  flora  is  very  similar  to  that  of  the  Upper  Permian,  though 
the  Upper  Trias,  especially  the  Rhaetic,  marks  a  decided  advance 
among  the  plants.  Among  the  animals  a  considerable  number 
of  surviving  Palaeozoic  types  persist  into  the  Trias  which  do  not  pass 
into  the  Jurassic. 


668 


THE  TRIASSIC   PERIOD 


Plants.  — Triassic  vegetation  is  composed  of  Ferns,  Horsetails, 
Cycads,  and  Conifers,  and  of  such  plants  were  the  Newark  coal 
of  Virginia  and  North  Carolina,  the  Keuper  coal  of  Germany  and 
Sweden,  and  the  Triassic  coal  of  South  Africa  and  Australia  ac- 
cumulated. The  Ferns  are  relatively  somewhat  less  abundant 

than  they  had  been  in  the  Car- 
boniferous, and  many  of  them 
belong  to  the  existing  tropical 
family  of  the  Marattiacea. 
T&niopteris,  Caulopteris,  Cla- 
thropteris,  are  among  the  most 
important  genera.  In  Vir- 
ginia a  magnificent  fern  with 
very  broad  leaves,  Macrotani- 
opteris  (Fig.  283),  is  the 
most  abundant  and  charac- 
teristic of  the  Triassic  plants 
there  found. 

The  Lycopods  have  under- 
gone a  great  reduction  since 
the  Carboniferous,  though  a 
few  straggling  specimens  of 
plants  related  to  Sigillaria, 
but  belonging  10  a  distinct 
family,  have  been  found  in 
the  Lower  Trias.  The  Cal- 
amites  are  no  longer  found, 
but  on  the  other  hand,  true  Horsetails  of  the  modern  genus  Equise- 
tum  now  make  their  first  appearance,  and  much  surpass  their  mod- 
ern representatives  in  size,  having  stems  of  4  inches  in  thickness. 
Rhizomes  and  stems  of  these  plants  are  very  common,  and  dense 
growths  of  them,  like  cane-brakes,  surrounded  the  inland  seas 
and  salt  lakes  of  the  period.  The  Cordaitea  have  disappeared, 
but  the  Cycadales  with  their  stiff  leaves  abounded,  growing, 
doubtless,  on  the  dryer  lowlands  above  the  swamps,  most  of  them 


FIG.  283.  —  A  Triassic  Fern,  Macrotceni- 
opteris  magnifolia  Rogers.  Restored 
(Russell) 


THE   LIFE  OF  THE  TRIASSIC 


669 


belonging  to  such  genera  as  Pterophyllum,  Zamites,  and  Otoza- 
mites  (Fig.  284).  This  group  of  plants  is  a  characteristic  Mesozoic 
one,  and  the  era  is  sometimes  called  the 
"  Age  of  Cycads."  The  term  Cycadales  is 
employed  to  indicate  "  a  group  enormously 
wider  than  our  recent  Cycadaceae."  (D.  H. 
Scott.)  The  Gingkoaceae  continue  to  be  rep- 
resented by  Baiera.  On  the  hills  and  up- 
lands grew  dense  forests  of  Conifers,  in 


FIG.  284.  — Leaf  of  a  Tri- 
assic  Cycad,  Otozamites 
latior  Saporta,  X  1/2. 
(Newberry) 

appearance  like  the 
Araucarians,  which 
are  found  to-day  in 

FIG.  285.  —  Triassic  Conifer,  Voltzia  heterophylla.          South  America,  Poly- 

(Fraas)  nesia,  and  Australia. 

Araucarites,  and  the  cypress-like    Voltzia  (Fig.  285),  the   latter 
much  resembling  the  Permian  Walchia,  are  common  genera. 
While  the  Triassic  flora  is  thus  different  from  that  of  the  Palaeo- 


6/0  THE  TRIASS1C   PERIOD 

zoic,  it  must  have  given  to  the  landscapes  of  the  period  much  the 
same  appearance  of  graceful  and  luxuriant,  but  somewhat  gloomy 
and  monotonous,  vegetation.  Probably  the  fern  forests  of  New 
Zealand  give  the  best  modern  picture  of  these  early  Mesozoic 
woodlands. 

Of  marine  plants,  the  Calcareous  Alga,  or  Coralline  Seaweeds, 
should  be  mentioned  as  very  abundant  about  the  coral  reefs,  to 
which  they  contributed  largely. 

Among  the  animals  the  change  from  Palaeozoic  times  is  much 
more  complete  than  among  the  plants. 

Coelenterata. — Corals  abounded  in  the  seas,  wherever  condi- 
tions were  favourable  to  their  growth,  but  the  Palaeozoic  Tetra- 
coralla  have  nearly  died  out,  though  a  few  of  the  Tetracoralla  and 
of  the  Tabulate  Hexacoralla  survived.  Their  place  is  taken  by 
the  modern  type  Hexacoralla,  though  the  two  groups  of  corals 
approach  each  other  so  closely  that  the  distinction  is  not  a  sharp 
one. 

Echinodermata.  —  In  this  type  a  more  marked  change  has  taken 
place.  The  Cystoids  and  Blastoids  have  disappeared,  and  the 
Crinoids  have  undergone  a  change  of  structure,  the  Camerata 
giving  way  to  the  Articulata,  but  the  latter  occur  only  in  small 
numbers  and  in  character  rather  transitional  from  the  older  forms 
than  typical  of  the  new.  Of  the  Triassic  Crinoids  much  the  com- 
monest is  Encrinus,  which  is  so  characteristic  of  the  German 
Muschelkalk.  Similarly,  the  ancient  type  of  the  sea-urchins, 
the  Palaechinoidea,  is  all  but  gone,  only  a  few  persisting  through 
the  Mesozoic,  while  the  Euechinoidej-,  which  began  in  a  small  way 
in  the  Carboniferous,  now  come  to  the  front.  The  Triassic 
Echinoids  are  all  of  regular  shape,  the  irregular  forms  not  appear- 
ing till  later. 

Arthropoda. — The  long-tailed  Decapod  Crustacea,  Macrura, 
are  found  in  the  Trias,  probably  the  most  ancient  representatives 
of  the  group.  The  Ostracoda  are  not  uncommon.  The  little 
genus  of  Phyllopoda,  Estheria,  is  very  common  in  the  German 
Keuper  and  the  American  Newark,  and  seems  to  be  indicative 


THE  LIFE  OF  THE  TRIASSIC  6/1 

of  brackish-water  conditions  where  it  occurs.  Among  the  Insects, 
the  Coleoptera  (Beetles)  are  added  to  the  two  orders  which  are 
definitely  known  to  occur  in  the  Palaeozoic. 

The  Bryozoa  undergo  a  marked  change  in  the  disappearance  of 
the  ancient  Fenestella-like  genera. 

Brachiopoda. — One  of  the  most  important  changes  from  the 
Palaeozoic  to  the  Mesozoic  consists  in  the  great  reduction  of  the 
Brachiopods  in  variety  and  numbers,  and  in  a  difference  of  char- 
acter, the  shells  with  long,  straight  hinge-line  giving  way  to  those 
with  short,  curved  hinge,  like  Terebratula  (PI.  XIII,  Fig.  7).  Even 
in  the  Trias  the  reduction  is  very  marked,  though  several  Palaeozoic 
genera  have  their  latest  representatives  in  the  rocks  of  this  system; 
as  examples,  may  be  mentioned  Productus,  Athyris,  and  Cyrtina. 
Koninckina  is  a  new  genus  of  the  Spirifer  family,  which  is  con- 
fined to  the  Trias.  The  still  existing  genera,  Terebratula  and 
Rhynchonella,  are  much  the  most  abundant  brachiopods  of  the 
period,  and  Thecidium,  which  later  becomes  important,  has  its 
beginning  here. 

Mollusca.  —  Almost  in  proportion  to  the  decline  of  the  brachio- 
pods is  the  rise  of  the  Pelecypoda,  or  Bivalves,  which  now  become 
far  more  varied  and  abundant  than  they  had  been  in  the  Palaeo- 
zoic. Pecten,  Pseudomonotis  (XIII,  8),  Myophoria,  Halobia,  Dao- 
nella  (XIII,  9),  and  Cardita,  may  be  selected  as  a  few  examples 
of  the  commoner  genera.  The  higher  forms  of  the  class  are, 
however,  still  rare.  The  Gastropoda  are  yet  in  a  transition  stage. 
Several  genera,  such  as  Murchisonia,  Loxonema,  etc.,  here  make 
their  last  appearance,  and  mingled  with  them  are  the  forerunners 
and  earliest  representatives  of  modern  types,  such  as  Cerithium 
and  other  genera,  in  which  the  mouth  of  the  shell  is  no  longer  a 
complete  ring,  but  is  drawn  out  into  a  grooved  siphon. 

The  Cephalopoda,  and  more  particularly  the  Ammonoids,  have 
already  acquired  a  wonderful  degree  of  abundance  and  variety. 
The  ancient  Nautiloid  genus  Orthoceras,  which  ranges  through 
almost  the  whole  Palaeozoic  group,  persists  into  theTriassic  system, 
but  not  later,  and  numerous  coiled  Nautiloids  with  angulated  and 


PLATE  XIII.— TRIASSIC  INVERTEBRATE  FOSSILS 

Figs,  i,  T.a,  Tropites  subbullatus  Hauer,  side  and  end  views,  X  %,  Up  Tr.  z,  -za,  Meek- 
oceras  gracilitatis  White,  side  &  front  views,  Low.  Tr.  3,  3*2,  Gymnotoceras  blakei 
Gabb,  X  %,  s;de  and  front  views,  Mid.  Tr.  •$&,  The  same,  a  suture  line,  x  i.  4, 
Sagenites  herbichi  Mojs.  x  %,  U.  Tr.  \a,  The  same,  a  suture  line,  X  i.  5,  $a,  Joan- 
nites  nevadanus  Hyatt  &  Smith,  X  y2,  side  and  front  views,  M.  Tr.  56,  The  same,  a  suture 
line,  x  i.  6,  6a,  Analcites  mef&z"M.o')s.  X  %,  side  and  back  views,  M.  Tr.  7,  "ja,  Terebrat- 
ula  semisimplex  White,  X  i,  dorsal  and  side  views,  L.  Tr.  8,  Psendomonotis  subcir- 
cularis  Gabb,  x  %,  M.  Tr.  9,  Daonella  lommeli  Wissmann,  x  %,  U.  Tr. 


THE  LIFE  OF  THE  TRIASSIC  6/3 

ornamented  shells  recall  those  of  the  Carboniferous.  Of  the 
Ammonoids  some  still  have  the  comparatively  simple  sutures  of 
the  Goniatites,  others,  like  Ceratites,  have  slightly  serrated  sutures, 
while  in  the  upper  Triassic  occur  some  shells  in  which  the  com- 
plexity of  the  sutures  is  carried  farther  than  in  any  other  known 
members  of  the  group.  Only  a  few  of  this  great  assemblage  of 
genera  can  be  mentioned;  especially  characteristic  of  the  Trias 
are:  Tirolites,  Trachyceras,  Meekoceras  (XIII,  2),  Arcestes,  Cera- 
tites, Tropites  (XIII,  i),  Joannites  (XIII,  5),  Gymnotoceras 
(XIII,  3),  Sagenites  (XIII,  4),  and  Analcites  (XIII,  6).  It  is 
very  interesting  to  observe  that  in  the  Trias  occur,  though  but 
rarely,  certain  unusual  forms  of  Ammonoid  shells,  which  do  not 
become  important  until  the  long  subsequent  time  of  the  Cretaceous 
period.  Rhabdoceras  has  a  straight  shell,  Cochloceras  one  that  is 
coiled  in  a  high  spiral  like  a  gastropod,  and  in  Choristoceras  the 
coils  are  open.  The  similar  Cretaceous  genera  were  not  derived 
from  these  Triassic  anticipations,  but  are  degenerate  members 
of  many  Ammonoid  families.  The  Dibranchiate  Cephalopods, 
and  especially  the  characteristic  Mesozoic  group  of  the  Belemnites, 
have  their  earliest  and  most  primitive  representatives  in  the 
Triassic  genera  Atractites  and  Aulacoceras : 

The  Vertebrata  are  of  extraordinary  interest,  and  if  the  Trias 
had  yielded  only  vertebrate  fossils,  it  would  still  be  apparent  that 
great  progress  had  been  made  since  the  time  of  the  latest  known 
Palaeozoic  beds.  The  Fishes  display  this  progress  least  of  all  the 
Vertebrates.  Shark  teeth  are  known,  but  not  skeletons.  The 
Dipnoan  Ceratodus  is  very  characteristic,  continuing  up  from  the 
Permian.  The  Crossopterygians  have  greatly  declined,  but  some 
very  large  and  curious  fishes  of  this  group,  like  Diplurus  (Fig. 
286),  still  linger.  The  Ganoids  continue  to  be  the  dominant  fish- 
type,  especially  of  the  inland  waters,  and  are  most  like  the  ex- 
isting garpikes.  Catopterus  and  Ischypterus  are  representative 
American  genera,  and  Semionotus,  Dictyopyge,  and  Lepidotus 
are  nearly  allied  European  fishes. 

The  A  mphibia  reach  their  culminating  importance  in  the  Trias, 

2X 


6/4  THE  TRIASSIC  PERIOD 

the  Stegocephalia  multiplying  and  diversifying  in  a  wonderful 
fashion,  and  far  surpassing  the  genera  of  the  Carboniferous  and 
Permian  in  size.  These  Amphibians  have  been  found  in  North 
America,  southern  Africa,  and  Europe;  but  those  of  the  last- 
named  continent  are  much  the  best  understood,  because  best 
preserved,  the  Bunter  sandstone  of  Germany  bemg  a  treasure- 
house  of  such  remains.  Mastodons  aurus,  Cydotosaurus,  and 
Labyrinthodon  are  common  European  genera,  but  there  are  many 
others.  Cheirotherium  (also  European)  is  known  only  from  its 
curious  footprints,  like  the  print  of  a  human  hand. 

Reptilia.  —  It  is  in  this  class  that  we  find  the  most  remarkable 
changes;   and  although  reptiles  are  common  in  the  Permian,  the 


FlG.  286.  —  Diplurus  longicaudatus  Newberry.     Newark  shales.     (Dean) 

abundance  and  diversity  of  the  Triassic  reptiles  are  incomparably 
greater.  Almost  all  the  orders  of  Mesozoic  reptiles  are  already 
represented  in  the  Trias,  though  often  by  comparatively  small  and 
rare  forms.  The  Triassic  reptiles  are  much  more  common  and 
better  preserved  in  Europe  than  in  America;  but  such  American 
genera  as  do  occur  show  that  there  was  no  essential  difference 
between  the  reptilian  faunas  of  the  two  continents. 

The  Gnathodontia,  which  are  very  near  to  the  Permian  Pro- 
ganosauria,  are  represented  by  Telerpeton  and  Hyperodapedon. 
Superficially  like  Crocodiles,  but  belonging  to  a  different  order, 
the  Parasuchia,  are  the  little  Aetosaurns  and  the  formidable 
Belodon  (Fig.  287),  the  latter  found  also  in  this  country.  The 
first  of  the  dolphin-like  Ichthyosaurs,  which  became  so  important 
in  the  Jurassic,  are  sparingly  found  in  the  Trias.  Another  group 


THE   LIFE  OF  THE  TRIASSIC  675 

of  sea-dragons,  the  Plesiosaurs,  which  attained  such  great  devel- 
opment in  Jurassic  times,  is  represented  in  the  Trias  by  small 
ancestral  forms,  Nothosaurus,  etc.  These  are  of  extraordinary  in- 
terest, as  showing  the  descent  of  the  purely  marine  Plesiosaurs, 
with  their  swimming  paddles,  from  terrestrial  reptiles  which  had 
feet  adapted  for  walking.  Another  order  of  Triassic  reptiles, 
the  Thalattosauria,  were  already  well  adapted  to  a  marine,  pre- 
daceous  life;  as  yet  they  are  known  only  in  Nevada. 

One  of  the  most  characteristic  of  the  Mesozoic  groups  of  reptiles 
is  the  super-order  Dinosauria,  of  which  the  Trias  has  many  rep- 
resentatives; but  clearly  there  were  very  many  more  than  have 
yet  been  found,  for  the  Newark  sandstones  of  the  eastern  United 


FIG.  287.  —  Skull  of  Belodon  kapffi  v.  Meyer,  about  T»B  natural  size.     (Zittel) 

States  have  preserved  a  great  variety  of  Dinosaurian  footprints, 
but  very  few  bones  have  been  found  in  these  rocks.  The  Di- 
nosauria were  much  diversified,  adapted  for  very  different  habits 
of  life:  some  were  herbivorous,  others  carnivorous;  some  walked 
on  all  fours;  others  were  occasionally  or  habitually  bipedal,  and 
walked  upright  after  the  manner  of  birds,  with  which  they  have 
many  structural  features  in  common.  The  gigantic  size  attained 
by  some  of  these  creatures,  even  in  the  Trias,  is  shown  by  the  foot- 
prints, some  of  which  are  14  to  18  inches  in  length.  Of  the  few 
American  forms  of  which  the  bones  have  been  found,  the  best 
known  is  Anchisaurus  from  the  Connecticut  valley,  and  of  the 
European  genera,  the  much  larger  Zanclodon. 

The  earliest  Turtles  are  found  in  the  Triassic  of  Europe,  and 
these  first-known  members  of  the  order  are  as  typically  differen- 
tiated as  any  of  the  later  members.  No  doubt  the  Turtles  origi- 


676  THE  TRIASSIC  PERIOD 

nated  in  the  Permian,  in  some  region  as  yet  unknown,  and  mi- 
grated to  Europe  in  the  Trias.  The  Theriodontia,  which  we  found 
beginning  in  the  Permian,  culminated  in  the  Trias,  especially 
in  southern  Africa.  Of  this  group  there  are  two  Triassic  orders, 
the  Anomodontia  and  the  Therocephalia.  The  former  have  cut- 
ting jaws,  like  Turtles,  and  may  or  may  not  possess  a  pair  or 
great  tusks  in  the  upper  jaw.  Dicynodon,  a  genus  of  this  sub- 
order, has  been  found  in  South  Africa,  India,  Russia,  and  Scot- 
land. The  Therocephalia  present  extraordinary  approximations 
to  the  mammals,  and  have  left  a  great  wealth  of  remains  —  some 
of  them  very  large  —  in  the  Karroo  beds  of  South  Africa,  and  less 
abundantly  in  India.  The  earliest  known  members  of  the  flying 
reptiles,  or  Pterosauria,  occur  in  the  Rhaetic  of  Europe. 

The  Trias  has,  as  yet,  yielded  no  Lizards  or  Snakes,  which  be- 
came very  important  at  a  later  date.  No  birds  are  known  from 
this  period,  nor  any  reptiles  which  can  be  regarded  as  the  ancestors 
of  birds. 

Mammalia.  —  Still  another  great  advance  in  the  progress  of 
life  is  registered  in  the  first  appearance  of  the  Mammals,  which 
occurred  in  the  Trias.  Mammals  are  the  most  highly  organized 
forms  of  animals;  but  these,  their  earliest  known  representatives, 
were  very  small  and  very  primitive,  giving  little  promise  of  being 
the  future  conquerors  of  the  world,  as  they  were  tiny  creatures 
which,  in  a  measure,  represent  the  transition  from  lower  verte- 
brates upward.  Two  American  genera,  Dromatherium  and 
Microconodon,  and  one  European  genus,  Microlestes,  have  been 
recovered. 


CHAPTER    XXXIII 


THE    JURASSIC    PERIOD 

WILLIAM  SMITH,  the  father  of  historical  geology,  was  the  first 
to  work  out  the  divisions  of  the  Jurassic,  which  he  did  early  in 
the  last  century.  The  terms  which  he  employed  are  local  English 
names,  and  these,  somewhat  Latinized  in  form  by  the  French 
geologists,  are  now  very  generally  used  as  an  international  scale. 
These  are  given  in  the  table.  Smith's  name  for  the  system, 
"Oolitic,"  has  been  abandoned  in  favour  of  the  term  Jurassic 
which  was  first  used  by  Brongniart  and  Humboldt.  It  was  taken 
from  the  Jura  Mountains  of  Switzerland,  where  the  rocks  of  this 
system  are  admirably  displayed.  In  Europe  the  Jurassic  has  long 
been  a  favourite  subject  of  study,  because  of  the  marvellous  wealth 
of  beautifully  preserved  fossils  which  it  contains.  For  this  reason, 
the  Jurassic  is  known  with  a  fulness  of  detail,  such  as  has  been 
acquired  regarding  very  few  of  the  other  systems;  and  no  less  than 
thirty  well-defined  subdivisions  have  been  traced  through  many 
countries  of  the  Old  World.  In  this  country  the  Jurassic  is  ill 
represented  and  its  divisions  are  not  clear. 


JURASSIC   SYSTEM 


ENGLAND 


GENERAL 


Oolite  < 


fPurbeck   1 
Upperj  Pprtland  I 

iKimmeridge  Clay 
'Coral  Rag 
Oxford  Clay 
.Kellaway  Rock 
'Great  Oolite 
Inferior  Oolite 
Lias 

677 


Middle 


Lower 


Portlandian 

Kimmeridgian 

Corallian 

Oxfordian 

Callovian 

Bathonian 

Bajocian 

Lias 


Upper 

Middle 
Lower 


678  THE  JURASSIC  PERIOD 

DISTRIBUTION  OF  JURASSIC  ROCKS 

American.  —  No  undoubted  Jurassic  strata  occur  in  the  Atlantic 
border  of  the  United  States,  though  by  some  authorities  the  upper- 
most part  of  the  Trias  (Newark  Series)  is  referred  to  this 
system,  and  by  others  the  Potomac  Series  of  the  Cretaceous  is 
regarded  as  Jurassic,  at  least  the  lowest  portion  of  it.  Whether 
or  not  these  references  be  correct,  no  marine  Jura  is  known  on  the 
Atlantic  slope  of  North  America  except  in  Mexico.  In  eastern 
North  America  the  Jura  was  a  time  of  great  denudation,  when  the 
high  ranges  of  the  Appalachian  Mountains  were  much  denuded 
and  the  newly  upheaved,  tilted,  and  faulted  beds  of  the  Trias  were 
wasted  and  probably  worn  down  to  a  peneplain. 

At  the  beginning  of  the  period  Mexico  was  generally  elevated, 
causing  the  sea  to  withdraw  from  those  areas,  as  yet  of  unknown 
extent,  which  it  had  covered  with  shallow  water  in  the  Upper 
Triassic.  At  the  same  time  the  southeastern  portion  of  the  country 
was  invaded  by  the  sea  and  would  seem  to  have  remained  sub- 
merged during  the  Lower  and  Middle  Jurassic,  but  at  this  time 
most  of  Mexico  was  land,  and  denudation  swept  away  most  of 
the  marine  Triassic,  leaving  but  a  single  known  area. 

In  the  western  Interior  region,  what  is  believed  to  be  Jurassic  is, 
for  the  most  part,  placed  there  on  stratigraphical  grounds  only, 
for  few  of  the  rocks  have  yielded  fossils.  The  beds  in  question  are 
largely  sandstones  which  in  many  places  rest  upon  Triassic  strata 
and  extend  from  northern  Utah  southward  into  Arizona  and  New 
Mexico,  and  westward  from  the  Rocky  Mountains  to  the  Great 
Basin  land  which  bordered  the  Pacific.  Whether  these  doubtful 
beds  represent  the  whole  Jura  or  only  a  part  and,  if  so,  what  part, 
are  questions  that  cannot  yet  be  answered.  Some  of  the  sand- 
stones contain  gypsum,  testifying  to  salt-lake  conditions. 

On  the  Pacific  coast  the  Lias  occurs  in  California,  Nevada,  and 
Oregon,  but  not  in  British  Columbia,  and  recurs  in  Alaska  and 
some  of  the  Arctic  islands.  The  migration  of  marine  animals 
from  the  north,  which  had  been  so  conspicuous  in  the  Upper 


DISTRIBUTION  OF  JURASSIC   ROCKS  679 

Triassic,  was  interrupted,  and  an  influx  of  European  forms 
probably  by  way  of  the  South  American  coast,  took  its 
place,  and  the  Middle  Jura  appears  to  have  retained  the  same 
connections. 

The  line  between  Middle  and  Upper  Jurassic  is  not  always 
drawn  in  the  same  place;  by  some  writers  the  Calloman  is  placed 
in  the  older  and  by  others  in  the  younger  division.  In  accordance 
with  the  arrangement  in  the  table,  the  Upper  Jurassic  is  here  con- 
sidered as  beginning  with  the  Callovian  and  is  marked  by  extensive 
geographical  changes  in  the  southern  and  western  regions  of  the 
continent.  Mexico  was  very  largely  depressed  beneath  the  sea, 
except,  probably,  a  belt  along  the  western  coast.  The  small  area 
of  marine  Jurassic  which  has  quite  lately  been  found  in  western 
Texas,  is  doubtless  an  outlier  of  this  Mexican  transgression.  In 
the  northwestern  United  States  was  formed  an  extensive  gulf  or 
shallow  bay,  which  covered  most  of  Montana,  Wyoming  as  far 
east  as  the  Black  Hills,  and  Utah.  The  beds  reach  their  maxi- 
mum thickness,  1800  feet,  in  the  Wasatch  Mountains,  which  is 
reduced  to  800  in  the  Black  Hills.  The  sediments  laid  down  in 
this  bay  are  limestones,  shales,  and  marls,  while  the  presence  of 
gypsum  shows  that  salt  lagoons  were  formed  by  isolation  of  parts  of 
the  bay.  The  fossils  are  of  Boreal  types,  like  those  of  Alaska  and 
Siberia,  and  point  to  an  incursion  from  the  north  or  northwest, 
though  the  oceanic  connections  of  this  Jurassic  bay  have  not  yet 
been  determined,  but  the  difference  between  this  Boreal  fauna 
and  that  of  California,  which  is  of  central  European  character, 
makes  any  direct  communication  between  the  two  areas  unlikely. 
In  the  Upper  Jurassic*  of  California,  of  somewhat  later  date  than 
the  interior  bay,  the  land  connection  between  Alaska  and  Asia 
appears  to  have  been  again  interrupted,  opening  the  way  for  a 
current  of  cooler  water  to  flow  from  the  Arctic  down  the  American 
coast.  At  all  events,  a  Boreal  fauna,  with  animals  which  were 
then  abundant  in  northern  Europe  and  Siberia,  extended  its  range 
through  California  to  southern  Mexico,  though  the  difference  in 
the  fossils  shows  that  some  barrier,  however  low  and  shifting, 


680  THE  JURASSIC  PERIOD 

prevented  the  waters  of  the  Atlantic  from  meeting  those  of  the 
Pacific.  It  is  remarkable  that  these  northern  animals  should  have 
extended  their  range  so  much  farther  south  on  the  American 
than  on  the  Asiatic  coast,  but  the  explanation  is  probably  to  be 
found  in  the  cool  current  from  the  north  mentioned  above. 
Similar  cases  are  known  at  the  present  time,  in  which  the  distri- 
bution of  marine  animals  is  controlled  by  warmer  and  colder 
currents  in  the  sea. 

The  western  interior  gulf  did  not  persist  throughout  the  Upper 
Jurassic,  but  was  drained  by  an  elevation  and  was  succeeded  by 
a  continental  formation,  partly  fluviatile,  partly,  it  may  be,  lacus- 
trine, the  Morrison.1  The  beds  have  quite  a  different  distribution 
from  those  of  the  marine  Jurassic  and  extend  down  the  eastern 
front  of  the  Rocky  Mountains  from  Wyoming  to  Texas  and  New 
Mexico,  with  disconnected  areas,  perhaps  outliers,  in  the  Black 
Hills  and  western  Colorado.  The  Morrison  has  yielded  a  re- 
markable fauna  of  Dinosaurs  and  Mammals,  which  have  made 
it  the  object  of  great  interest  to  the  palaeontologist.  Its  exact 
geological  position  has  been  vigorously  discussed  and  it  has  been 
referred  now  to  the  uppermost  Jurassic,  again  to  the  lowest 
Cretaceous.  It  has  been  suggested  by  Professor  Williston  that 
different  areas  of  the  Morrison  are  of  different  dates,  just  as  we 
saw  that  the  Millstone  Grit  (Upper  Carboniferous)  of  the  Mis- 
sissippi valley  is  not  a  single  uniform  bed,  but  different  beds  of 
similar  character,  formed  successively  and  corresponding  to  several 
horizons  in  the  great  mass  of  the  Appalachian  Pottsville.  On 
this  view,  which  is  probably  the  solution  of  the  problem,  the  Mor- 
rison includes  several  distinct  horizons,  extending  from  the  Upper 

1  This  is  the  formation  called  in  the  first  edition  of  this  book  the  Como  stage 
and  referred  to  the  Cretaceous.  The  term  Como  must  be  abandoned  in  favour  of 
Morrison,  which  was  first  proposed.  "  The  question  whether  the  Morrison  forma- 
tion is  Jurassic  or  Cretaceous  is  still  to  be  answered,  and  if  a  satisfactory  answer 
is  ever  received  it  will  doubtless  be  from  vertebrate  paleontology."  (Stanton.) 
The  stratigraphic  evidence  leaves  it  open  whether  these  beds  should  be  called 
Jurassic  or  Cretaceous  and,  on  the  evidence  of  the  mammals,  they  are  here  included 
clrefly  in  the  Jurassic. 


DISTRIBUTION   OF  JURASSIC  ROCKS  68 1 

Jurassic  into  the  Lower  Cretaceous,  but  the  discrimination  of 
these  horizons  is  yet  to  be  made. 

In  British  Columbia,  where  the  Lower  and  Middle  Jurassic  are 
not  known,  the  Upper  Jura  occurs  and  shows  a  transgression  of 
the  sea  to  the  eastward  of  the  Cascade  Mountains.  Here  and 
in  California  the  rocks  are  prevailingly  slates,  with  included 
diabase  tuffs,  demonstrating  volcanic  activity. 

The  close  of  the  Jurassic  in  North  America  was  a  time  of  ex- 
tensive orogenic  movements  on  the  western  side  of  the  continent, 
comparable  to  the  Appalachian  revolution  which  closed  the  Pa- 
laeozoic era  on  the  eastern  side,  though  not  of  such  far-reaching 
extent  and  consequences.  The  Sierra  Nevada  had  long  been  a 
slowly  subsiding  geosynclinal  trough,  in  which  great  thicknesses 
of  sediment  had  accumulated.  At  the  end  of  the  Jura  it  yielded 
to  the  forces  of  lateral  compression  and  rose  into  a  folded  moun- 
tain chain,  transferring  the  coast-line  from  the  eastern  to  the 
western  side  of  the  mountains.  Farther  north,  the  Klamath 
and  Cascade  ranges  probably  participated  in  the  movement,  as 
did  the  Coast  Range,  which  seems  to  have  become  a  chain  of 
islands.  Little  is  known  regarding  this  primary  condition  of  the 
Sierra  Nevada,  which  may  then  have  been  of  no  great  height, 
and  which  was  not  yet  separated  from  the  Great  Basin  by  faults, 
the  present  mountains  being  due  to  long  subsequent  events. 
The  Humboldt  Range  in  Nevada  appears  to  have  had  its  begin- 
ning in  the  same  disturbances,  while  a  widespread  movement 
of  elevation  affected  the  interior,  though  this  movement  began  at 
a  somewhat  earlier  date. 

Foreign. — The  greater  part  of  South  America  was  above  the 
sea  during  the  Jurassic  period,  as  it  had  been  in  the  Trias.  Marine 
deposits  of  the  former  period  are  found  only  along  the  western 
border  of  the  continent,  where  they  extend  from  5°  to  35°  S.  lat. 
Throughout  the  Jurassic  the  sea  which  covered  this  western  coast 
retained  its  faunal  connection  with  the  central  European  region, 
and  even  the  minuter  divisions,  the  substages  and  zones  of  the 
European  Jura,  are  applicable  to  the  classification  of  the  South 


682  THE  JURASSIC   PERIOD 

American  beds.  The  Boreal  invasion,  so  conspicuous  in  Cali- 
fornia and  Mexico,  did  not  extend  to  South  America,  doubtless 
because  of  the  warmer  water  on  that  coast. 

In  Europe  the  Jurassic  rocks  are  magnificently  displayed,  but 
they  differ  much  both  in  thickness  and  in  character  as  they  are 
traced  from  one  country  to  another,  which  results  from  more 
frequent  and  more  localized  changes  of  level  than  had  occurred 
during  the  Palaeozoic. 

The  Lias  has  a  much  more  restricted  extension  than  the  later 
Jurassic  stages.  At  the  end  of  the  Triassic  had  begun  a  trans- 
gression of  the  sea  (the  Rhaetic)  which  flooded  many  of  the  inland 
basins,  and  the  same  transgression  continued  into  the  Lias,  pro- 
ceeding northward  from  the  Mediterranean,  and  covering  large 
areas  in  central  and  southern  Europe,  as  well  as  a  belt  across 
England,  but  not  extending  to  Russia.  By  far  the  greater  part  of 
the  Eurasian  land  mass  was  above  the  sea,  and  fresh-water  lakes 
extended  across  Siberia,  while  in  China  widespread  deposits  of 
Liassic  coal  were  accumulated. 

Very  early  in  the  Upper  Jura  the  transgression  of  the  ocean 
was  renewed,  and  this  time  on  a  vastly  larger  scale,  cutting  the 
continents  by  seas  and  straits,  invading  great  areas  that  had  long 
been  land,  and  covering  the  larger  part  of  Europe  and  Asia. 
This  is  one  of  the  greatest  transgressions  of  the  sea  in  all  recorded 
geological  history,  but  it  did  not  greatly  affect  Nortn  America. 
Central  and  northern  Russia  were  submerged  by  an  extension  of  the 
sea  from  the  north,  and  in  this  Russian  sea  was  developed  a  highly 
characteristic  fauna.  Strata  distinguished  by  the  Russian  fauna 
extend  into  the  northeast  of  England  and  through  Siberia,  Spitz- 
bergen,  Nova  Zembla,  Alaska,  the  Black  Hills  of  South  Dakota, 
and  the  uppermost  Jurassic  of  California  and  Mexico,  and  even 
penetrated  to  the  Himalayan  sea.  In  peninsular  India  the  Jura 
is  represented  by  the  upper  division  of  the  Gondwana  system, 
which,  as  before,  was  laid  down  as  continental  deposits,  con- 
taining much  coal.  The  continental  mass  to  which  India  then 
belonged  was  cut  off  from  Asia  by  a  strait  or  sea  which  covered 


DISTRIBUTION  OF  JURASSIC  ROCKS  683 

the  site  of  the  Himalayas,  and  which  was  connected  with  the  Medi- 
terranean or  Thetys  by  way  of  Persia  and  Asia  Minor.  The  great 
Jurassic  transgression  submerged  considerable  areas  of  northern 
India,  as  it  also  covered  narrow  areas  along  the  eastern  and  west- 
ern coasts  of  Australia.  Much  of  Madagascar  was  under  water, 
but  the  southern  portion  is  believed  to  have  formed  part  of  the 
narrow  land  which  extended  from  South  Africa  to  India.  Some 
of  eastern  Africa  was  covered  by  a  bay  of  the  Indian  Ocean,  but 
no  marine  Jurassic  has  been  found  in  the  southern  or  western 
portions  of  that  continent.  In  South  Africa,  the  uppermost  part 
of  the  Karroo  system,  like  the  corresponding  portion  of  the  Indian 
Gondwana,  is  Jurassic. 

Climate. — The  very  striking  faunal  differences  which  obtain 
between  different  regions  have  led  certain  observers,  especially 
the  late  Professor  Neumayr  of  Vienna,  to  the  conclusion  that 
climatic  zones  had  already  been  established  in  Jurassic  times, — 
Boreal,  central  European,  and  Alpine  or  Equatorial  zones,  with 
corresponding  ones  in  the  southern  hemisphere.  This  conclu- 
sion as  to  climatic  belts  is,  however,  a  very  doubtful  one,  and  is  in 
conflict  with  the  distribution  of  the  several  geographical  faunas, 
for  the  central  European  fauna  is  found  in  equatorial  Africa,  and 
the  supposed  equatorial  fauna  occurs  in  the  Andes  20°  of  latitude 
south  of  its  proper  boundary.  It  is  much  more  likely  that  some 
of  the  marked  faunal  differences  are  due  to  varying  facies,  depth 
of  water,  character  of  bottom,  etc.,  and  even  more  to  the  partly 
isolated  sea-basins  and  the  changing  connections  which  were  es- 
tablished between  them.  On  the  other  hand,  the  Boreal  fauna, 
both  in  its  restrictions  and  in  its  migrations,  seems  definitely 
to  lead  to  the  conclusion  that  the  Arctic  Sea  was  distinctly  colder 
than  the  other  oceans,  though  there  is  no  likelihood  that  the  dif- 
ference of  temperature  was  nearly  so  great  as  at  present,  or  that 
the  Arctic  contained  ice,  even  in  winter.  "The  suggestion  of 
climatic  influence  on  the  dispersion  of  marine  animals  in  the 
Upper  Jurassic  is  very  strong."  (J.  P.  Smith.) 


684  THE  JURASSIC  PERIOD 

JURASSIC  LIFE 

The  life  of  the  Jurassic  has  been  preserved  in  wonderful  fulness 
and  variety;  but  with  comparatively  few  exceptions,  our  know- 
ledge of  it  has  been  principally  derived  from  Europe,  where  a  host 
of  eminent  geologists  have  long  studied  the  great  wealth  of  mate- 
rial. The  contrast  between  North  America  and  Europe  in  regard 
to  the  relative  abundance  of  Jurassic  marine  fossils  is  seen  from 
the  fact  that  while  in  Great  Britain  alone  more  than  4000  spe- 
cies have  been  described,  in  America  hardly  one-tenth  of  that 
number  has  so  far  been  found. 

Plants.  —  The  flora  of  the  Jurassic  differs  little,  on  the  whole, 
from  that  of  the  Trias,  and  is  made  up  of  Ferns,  Horsetails, 
Cycads,  Conifers,  and  Gingkos.  Tree  ferns  flourished  in  northern 
Europe  in  great  variety.  The  Cycads  attain  their  culmination  of 
abundance  and  diversity  in  this  period,  no  less  than  forty  species 
occurring  in  a  single  horizon  of  the  English  Upper  Jura  and  an 
extinct  order  of  the  Cycadales,  the  Bennettitea,  flourished  re- 
markably. The  Conifers  are  of  somewhat  more  modern  aspect 
than  those  of  the  Trias,  and,  from  their  resemblance  to  genera 
which  are  still  extant,  have  received  such  names  as  Thujites,  Tax- 
ites,  Cupres sites,  Pinites,  etc.  The  Araucarian  pines  abounded  in 
Europe.  The  Gingkos,  or  Maidenhair  Trees,  continued  to  be  repre- 
sented by  Baiera.  Monocotyledons  have  been  reported  from  the 
Jurassic,  but  the  evidence  for  their  existence  is  very  doubtful. 

Foraminifera  are  found  in  great  numbers  and  variety  in  the 
soft  Jurassic  clays,  many  of  them  belonging  to  genera  which  still 
abound  in  the  modern  seas.  It  must  not  be  supposed  that  these 
organisms  first  became  so  abundant  in  Jurassic  times;  it  is  merely 
that  the  conditions  for  the  preservation  of  these  microscopic  and 
exquisite  shells  had  not  been  so  favourable  before. 

Radiolaria. — The  beautiful  siliceous  tests  of  the  Radiolarians 
are  also  found  in  multitudes.  In  the  Alps  occur  whole  strata  of 
red  flints  and  jaspery  slates,  which  are  composed  almost  entirely 
of  these  tests. 


PLATE  XIV.  —  JURASSIC  INVERTEBRATE  FOSSILS 

Fig.  i,  Pentacrinus  asteriscus  M.  and  H.,  section  of  stem,  X  2,  U.  J.  2,  Cidaris 
coronata  Goldf.,  x  l/z,  Kimmeridgean,  Europe.  3,  Antinomia  ntftt//**  Pictet,  x  %,  Tithon., 
Alps.  4,  Eumicrotis  curia  Hall,  X  3/2,  U.  J.  5,  Tancredia  corbuliformis  Whitf.  , 
6,  Gervillia  montanaensis  Meek,  X  %,  IL  J.  7,  Volsella  subimbricata  Meek, 
8,  Trigonia.  ante  vie  ana  Meek,  X  %,  U.  J.  9,  Pholadomya  kingi  Meek, 
10,  Camptonectes  bellistriata  Meek,  X  %.  U.  J.  n,  Pleuromya  inconstans  Aguilera  and 
Costello,  x  %,  U.  J.  12,  Myacites  subcompressus  Meek,  X  %,  U.  J.  13,  Gryphaa 
arcuata  Lam.,  x  2/5>  Lias,  France.  14,  Ostrea  marshi  Sowerby,  X  %,  Bajocian,  Germany. 
15,  Lyosoma  powelli  White,  X  %,  U.  J.  16.  Pleurotomaria  conoidea  Deshayes,  X1^, 
France.  17,  Nerinea  dilatata  d'Orb,  x  l/2,  France.  18,  Peltoceras  cf.  athleta  Phillips, 
X  14,  Callovian,  Europe.  19,  Lytoceras  fintbriatum  Sowerby,  X  %>  Lias,  England. 
20,  Crioceras  bifurcatum  OupnstpHt  x  J/2»  Bathonian,  Germany, 


x  ,  ton., 
itf.  ,  x  i,  U.  J. 
k,  X  %,  U.  J. 
,  X  %,  U.  J. 


686  THE  JURASSIC  PERIOD 

Spongida.  —  Sponges  are  found  in  wonderful  profusion  and 
diversity  and  in  such  perfect  preservation  that  every  detail  of  their 
beautiful  structure  may  be  made  out  with  the  microscope.  In 
some  localities  these  sponges  are  heaped  up  in  such  masses  that 
they  fill  the  strata,  while  other  localities  of  the  same  horizon  are 
entirely  free  from  them. 

Ccelenterata.  —  Corals  abound,  especially  in  the  Upper  Jurassic 
of  central  Europe.  The  Anthozoan  Corals  all  belong  to  the  mod- 
ern Hexacoralla,  in  decided  contrast  to  the  Tetracoralla  of  the 
Palaeozoic  seas.  Isastma,  Montlivaultia,  and  Thecosmilia  are  the 
dominant  genera. 

The  Echinodermata,  especially  the  Crinoids  and  Sea-urchins, 
are  of  great  importance.  The  Crinoids  are  vastly  more  abundant 
than  they  had  been  in  the  Trias,  and  although  the  number  of 
genera  and  species  is  not  at  all  comparable  to  the  great  assem- 
blage of  Carboniferous  times,  yet  for  profusion  and  size  of  indi- 
viduals the  Jurassic  has  never  been  surpassed.  Especially  char- 
acteristic are  the  superb  species  of  Pentacrinus,  a  close  relative  of 
which  still  exists  in  the  West  Indian  seas.  Other  common  genera 
are  Apiocrinus  and  Eugeniacrinus.  These  genera  all  belong  to 
the  Articulata,  which  have  a  very  different  type  of  structure  from 
the  Palaeozoic  forms,  but,  like  nearly  all  the  latter,  they  were  at- 
tached to  the  sea-bottom  by  their  long  stems.  In  the  Jurassic 
appear  the  first  of  the  Articulate  free-swimming  Crinoids,  like  Co- 
matula,  the  commonest  of  modern  genera.  These  animals  possess 
a  stem  only  in  their  early  stages  of  development;  subsequently 
they  become  detached  and  free.  Star-fishes  and  Brittle  Stars  are 
not  very  common,  but  have  attained  a  completely  modern  structure. 

The  Echinoids  have  undergone  a  wonderful  expansion  and 
diversification  by  the  time  of  the  Middle  Jurassic.  In  the  Lias, 
as  in  the  Trias,  we  find  only  the  regular,  radially  symmetrical 
sea-urchins,  with  mouth  and  anus  at  the  opposite  poles  of  the  shell, 
such  as  Cidaris  (PL  XIV,  Fig.  2),  but  in  the  Middle  and  Upper 
Jura  appear  the  irregular  Spatangoids  and  Clypeastroids.  In 
these  the  shell  is  bilaterally  symmetrical,  rather  than  radially 


JURASSIC  LIFE  687 

so,  the  anus,  and  even  the  mouth,  losing  their  polar  positions,  and 
the  shape  of  the  ambulacral  areas  being  greatly  changed.  This 
is  another  instance  of  the  attainment  of  modern  structure  which 
so  many  of  the  Mesozoic  Invertebrates  display. 

Arthropoda.  — The  Crustacea  are  not  found  in  very  many  locali- 
ties, but  places  like  the  famous  lithographic  limestones  of  Solen- 
hofen  in  Bavaria,  where  the  conditions  of  preservation  were 
favourable,  show  that  this  group  was  very  abundant  and  far  ad- 
vanced in  the  Jurassic  seas.  The  long-tailed  (macrurous)  Deca- 
pods (of  which  the  lobster  is  a  familiar  example)  are  in  the  as- 
cendant and  are  represented  by  many  genera,  several  of  which 
still  exist.  The  Crabs,  or  short-tailed  Decapods,  which  are  now  so 
very  common,  make  their  first  known  appearance  in  the  Jurassic, 
but  they  were  still  rare,  and  connecting  links  between  the  long- 
tailed  and  short-tailed  series  were  more  abundant.  Isopods  and 
Stomatopods  also  abounded. 

The  Xiphosura  are  reduced  to  the  single  genus  Limulus,  which 
then  occurred  in  the  European  seas,  while  the  living  horseshoe 
crabs  of  that  genus  are  found  only  on  the  east  coast  of  the  United 
States  and  in  the  Molucca  Islands. 

Insects  are  found  in  multitudes  in  certain  localities,  and  display 
a  great  advance  in  the  number  of  types  over  any  of  the  Palaeozoic 
periods.  The  Orthopters  and  Neuropters  which  we  found  in  the 
Palaeozoic  are  enriched  by  many  new  forms,  such  as  grasshop- 
pers, while  beetles  (Coleoptera)  become  very  abundant.  The  Hy- 
menopters  (ants,  bees,  wasps,  etc.)  and  the  Dipters  (flies)  date 
from  the  Jurassic,  and  Lepidopters  (butterflies)  have  also  been 
reported,  though  doubtfully.  As  the  latter  insects  are  dependent 
upon  a  flowering  vegetation,  definite  proof  of  their  presence  in  the 
Jura  will  establish  the  existence  of  the  Angiosperms  at  that  time. 

Brachiopoda.  —  These  shells  are  still  common  in  the  Jura,  but 
they  are  simply  a  profusion  of  individuals  belonging  to  a  few 
genera,  most  of  which  persist  in  our  recent  seas;  compared  even 
with  the  Trias,  Jurassic  brachiopods  are  much  diminished.  Tere- 
bratuloids  like  Antinomia  (XIV,  3),  Waldheimia,  and  Rliyn- 


688  THE  JURASSIC  PERIOD 

chonella  are  much  the  most  important  genera,  and  the  last  strag- 
glers of  the  long-lived  Palaeozoic  Spiriferina  are  here  found. 

Mollusca.  — The  Bivalves,  which  had  already  become  such  im- 
portant elements  of  the  Triassic  fauna,  greatly  increase  in  the  Jura, 
their  shells  forming  great  banks  and  strata.  Many  of  the  genera 
are  still  living,  and  only  a  few  of  the  more  abundant  ones  can  be 
mentioned  here.  Oysters  like  Gryph&a  (XIV,  13),  Exogyra,  and 
Ostrea  itself  (XIV,  14)  are  common,  and  the  Scallop  shell,  Camp- 
tonectes  (XIV,  10),  is  important.  Trigonia  (Fig.  288  and  PL  XIV, 


FIG.  288.  —  Slab  of  Trigonia  clavellata,  from  the  English  Oxfordian 

Fig.  8)  is  especially  characteristic  of  the  Jura,  but  a  few  repre- 
sentatives of  that  genus  have  persisted  to  the  present  time  and 
are  found  in  the  Australian  seas.  Diceras  and  Pholadomya 
(XIV,  9)  are  likewise  common  genera,  and  there  are  very  many 
others.  Among  the  Gastropoda  the  most  significant  change  lies 
in  the  importance  which  the  siphon-mouthed  shells  now  for  the 
first  time  assume;  examples  of  this  group  are  Nerinea  (XIV,  17), 
Alaria,  Purpurina,  etc.  Of  the  shells  with  entire  mouths  the 


JURASSIC   LIFE 


689 


ancient  Palaeozoic  genus  Pleurotomaria  (XIV,  16)  is  as  abundant 
as  ever,  not  beginning  to  decline  until  the  Cretaceous  period. 

The  Cephalopods  are  at  the  very  height  of  their  culmination, 
and  are  present  in  an  astonishing  profusion  and  diversity,  filling 
whole  strata  with  their  heaped-up  shells.  The  Nautiloids  differ 
from  those  of  the  Trias  in  their  smoother  and  more  involute  shells. 
The  Ammonoids  do  not  display  so  many  types  of  shell  structure 
as  we  have  found  in  the  Trias,  and  the  genera  are  mostly  different 


FlG,  289.  —  Slab  of  Belemnites  compressus  Blainv,  from  the  Lias  of  England 

from  those  of  the  latter  period;  but  in  number  of  distinct  species 
the  Jura  much  surpasses  the  other  Mesozoic  periods.  Phylloceras 
and  Lytoceras  (XIV,  19)  continue  on  from  the  Trias,  but  the 
most  abundant,  characteristic,  and  widely  spread  genera  are  new. 
Of  these  may  be  mentioned:  Peltoceras  (XIV,  18)  Arietites, 
jEgoceras,  Harpoceras,  Stephanoceras,  Peris phinctites,  and  many 
others,  each  with  large  numbers  of  species.  Crioceras  (XIV,  20)  is 
an  uncoiled  ammonoid.  The  Belemnites  (Fig.  289),  which  were  in- 

2  Y 


690  THE  JURASSIC   PERIOD 

troduced  in  a  small  way  in  the  Trias,  in  the  Jurassic  blossom  out 
into  an  incredible  number  of  forms,  exceeding  even  the  Ammonites 
in  abundance  of  individuals,  if  not  of  species.  These  extinct  Ce- 
phalopods  belonged  to  the  Dibranchiata,  as  do  all  the  living  forms 
except  the  Pearly  Nautilus;  they  in  some  measure  serve  to  connect 
the  extinct  genera  having  external  shells  with  the  existing  naked 
squids  and  cuttle-fishes,  which  have  only  rudimentary  internal 
shells,  the  pen  or  cuttle-bone.  The  Belemnites  have  a  straight, 


FIG.  290.  —  Dapedius politus.     (Smith  Woodward) 

conical,  chambered  shell,  called  the  phragmocone,  which  ends  above 
in  a  broad,  thin  plate.  The  phragmocone  was  partly  external 
to  the  animal,  and  its  lower,  pointed  end  was  inserted  into  a  dart- 
6r  club-shaped  body  called  the  guard  or  rostrum  which  is  composed 
of  dense,  fibrous,  crystalline  calcite.  Usually  only  the  guard 
is  preserved  in  the  fossil  state,  and  specimens  are  so  common  that 
they  have  attracted  popular  interest  and  bear  the  folk-name  of 
"  thunderbolts."  In  a  few  instances  the  animal  has  been  pre- 
served almost  entire,  so  that  the  structure  is  well  understood. 

Vertebrata.  — The  Fishes  have  advanced  much  beyond  those 
of  the  Trias.  The  Sharks  have  attained  practically  their  modern 
condition,  and  the  broad,  flattened  Rays  are  a  new  type  of  the 
order.  The  Chimaroids  were  much  more  numerous  and  rela- 


JURASSIC  LIFE 


691 


tively  important  than  they  are  at  present,  when  only  a  few  are 
left.  The  Dipnoans  had  become  very  scarce  and  are  hardly  rep- 
resented in  the  northern  hemisphere,  save  for  the  persistence  of 
Ceratodus.  The  Crossopterygians  were  greatly  reduced,  though  a 
few  exceedingly  curious  forms,  like  Undina,  still  linger.  Of  the 


FlG.  291.  —  Aspidorhynchus  acutirostris  Ag.     (Smith  Woodward) 

Teleostome  fishes  the  Ganoids  were  still  the  dominant  type,  as  they 
had  been  since  the  Devonian.  Some  of  these  Jurassic  forms  are 
evidently  the  forerunners  of  the  Sturgeons,  but  most  of  them  re- 
semble the  Gar-pike  of  our  Western  rivers  (Lepidosteus),  and  are 
covered  with  a  heavy  armour  of  thick,  shining,  rhomboidal  scales. 


FlG.  292. — Hypsocormui insignis  Wagner.     (Smith  Woodward) 

Many  of  these  Ganoids  are  of  small  or  moderate  size,  such  as 
Dapedius  (Fig.  290)  and  Aspidorhynchus  (Fig.  291),  while  others, 
like  the  superb  Lepidotus,  were  very  large,  evidently  the  kings  of 
their  race.  Some  of  the  Jurassic  fishes  approximate  the  Teleosts 
so  closely  that  it  seems  arbitrary  to  call  them  Ganoids.  Caturus, 
Leptolepis,  Hypsocormus  (Fig.  292),  and  Megalurus  are  much, 
like  what  the  ancestral  Teteosts  must  have  been. 


6Q2  THE  JURASSIC  PERIOD 

No  Amphibia  are  certainly  known  from  the  Jurassic. 

The  Reptiles  have  attained  a  higher  and  more  diversified  plane 
of  existence  than  in  the  Trias.  Most  of  the  Triassic  genera  and 
several  entire  orders  have  become  extinct,  but  new  and  more  ad- 
vanced forms  come  in  to  take  their  places.  The  Rhynchocepha- 
lians  abound  and  give  rise  to  many  diversified  types  of  terrestrial, 
semi-aquatic  and  fully  aquatic  reptiles,  and  the  first  of  the  true 
Lizards  (Lacertilia)  appear.  Turtles  have  grown  much  more  nu- 
merous than  in  the  Trias  and  have  distributed  themselves  over 
the  world.  The  Ichthyosauria  are  a  highly  characteristic  Jurassic 
group;  for  though  they  are  found  in  both  the  Trias  and  the  Cre- 
taceous, the  Jura,  and  especially  the  Lias,  is  the  time  of  their 
principal  expansion.  Certain  localities  in  the  Lias  of  England 
and  Germany  have  yielded  an  incredible  number  of  skeletons, 
and  some  of  the  specimens  have  preserved  the  impressions  of 
the  outline  of  the  body  and  limbs,  showing  recognizably  the  nature 
of  the  skin.  These  reptiles  were  entirely  marine  in  their  habits 
and  preyed  upon  fishes,  and  their  limbs  were  converted  into 
swimming  paddles;  there  is  a  dorsal  fin  and  a  large  tail-fin,  the 
principal  organ  of  propulsion  (see  Fig.  .293).  The  muzzle  is 
drawn  out  into  an  elongate  slender  snout,  armed  with  numerous 
sharp  teeth,  which  were  set  in  a  continuous  groove,  not  in  sepa- 
rate sockets.  The  eye  is  very  large  and  protected  by  a  number  of 
bony  plates,  which  are  often  preserved  in  the  fossil  state.  The 
neck  is  very  short  and  hardly  distinguished  from  the  porpoise- 
like  body.  The  skin  was  smooth,  having  neither  horny  scales 
nor  bony  scutes,  which  was  of  advantage  in  lessening  the  friction 
of  the  water.  In  length,  these  reptiles  sometimes  exceeded  25 
feet,  and  in  appearance  they  must  have  been  very  like  the  modern 
porpoises  and  dolphins,  but  the  resemblance  is  entirely  superficial, 
for  porpoises  and  dolphins  are  warm-blooded  Mammals.  Bap- 
tanodon,  found  in  Wyoming,  is  an  Ichthyosaur  without  teeth  and 
must  have  fed  upon  small  and  soft  marine  invertebrates,  as  do 
the  toothless  whales. 
Another  group  of  carnivorous  marine  reptiles  is  that  of  the 


FlG.  293.  —  Ichthyosaurus  quadriscissus  Quenst.,  Lias. 
Knight  under  the  direction  of  Prof.  H.  F.  Osborn. 
Museum  of  Natural  History,  N.Y.) 


Restoration  by  C.    R. 
(Copyright,  American 


694 


THE  JURASSIC   PERIOD 


Plesiosauria,  which  began  in  the  Trias  and  culminated  in  the  Jura, 
and  which  forms  a  curious  contrast  to  the  Ichthyosaurs.  In  the 
typical  genus  Plesiosaurus  (Fig.  294)  the  head  is  relatively  very 
small,  and  the  jaws  are  provided  with  large,  sharp  teeth,  set  in 

distinct  sockets.  The  neck  is  ex- 
ceedingly long,  slender,  and  serpent- 
like,  and  marked  off  distinctly  from 
the  small  body.  The  swimming 
paddles  are  much  larger  than  in 
the  Ichthyosaurs  and  probably  had 
more  to  do  with  locomotion;  the 
skeleton  of  the  paddle  departs 
much  less  widely  from  the  structure 
of  a  terrestrial  reptile's  foot  than 
does  that  of  an  Ichthyosaur.  With 
their  long  necks,  the  Plesiosaurs 
could  lie  motionless  far  below  the 
surface,  occasionally  raising  their 
heads  above  the  water  to  breathe, 
or  darting  them  to  the  bottom 
after  their  prey,  which  consisted 
chiefly  of  fish.  The  Jurassic  species 
of  Plesiosaurus  do  not  much  exceed 
a  length  of  20  feet,  but  Pliosaurus 
of  the  same  group  was  gigantic,  a 
single  paddle  sometimes  measur- 
ing 6  feet  in  length;  the  reptiles 
of  the  latter  genus  had,  however, 
proportionately  larger  heads  and 
shorter  necks. 

The  seas  and  rivers  of  Jurassic  times  were  swarming  with  Croco- 
diles, the  most  ancient  yet  known,  Teleosaurus  being  the  common- 
est genus  of  the  period.  In  appearance  these  reptiles  much  re- 
sembled the  modern  Gavial  of  India  and  had  a  similar  elongate 
and  slender  snout.  The  fore  legs  were  much  smaller  than  the 


JURASSIC  LIFE 


695 


hind,  and  these  animals  were  doubtless  of  more  exclusively  aquatic 
habits  than  the  crocodiles  and  alligators  of  the  present  day.  One 
suborder  of  the  Crocodiles,  the  Thalattosuchia,  was  almost  entirely 
marine  in  habits,  the  skin  being  smooth  and  without  scales  and  the 
fore  limbs  converted  into  paddles,  while  the  very  long  tail  ended 
in  a  large  fin.  In  the  Jurassic  of  South  Africa  has  been  found  the 
other  extreme  of  crocodilian  development,  a  little  reptile  which 
was  terrestrial  and  had  long,  running  legs. 


FlG.  295.  —  Allosaurus  agilis  Marsh,  a  carnivorous  Dinosaur  from  the  Morrison. 
Restoration  by  C.  R.  Knight  under  the  direction  of  Prof.  H.  F.  Osborn. 
(Copyright,  American  Museum  of  Natural  History,  N.Y.) 

The  Dinosauria  became  much  larger,  more  numerous  and 
diversified  than  they  had  been  in  the  Trias,  though,  as  the  foot- 
prints in  the  Newark  sandstones  teach  us,  only  a  small  fraction  of 
the  Triassic  Dinosaurs  has  yet  been  recovered.  Making  all  due 
allowance  for  this,  it  seems,  nevertheless,  to  be  true  that  the  group 
had  made  notable  progress  in  the  Jurassic.  The  known  American 
Jurassic  Dinosaurs  are  from  the  Morrison,  and  so  some  of  those 
mentioned  below  may  be  Cretaceous.  The  group  of  Dinosauria 
is  a  greatly  varied  one,  comprising  reptiles  of  very  different 


696 


THE  JURASSIC  PERIOD 


size,  appearance,  structure,  and  habits  of  life.  Some  were  heavy, 
slow-moving  quadrupeds,  having  fore  and  hind  legs  of  not  very 
unequal  length,  with  hoof-like  toes,  and  usually  with  very  small 
heads.  Dinosaurs  of  this  type  were  mostly  plant-feeders  and  had 
rows  of  grinding  teeth  adapted  for  such  a  diet.  Brontosaums, 
from  the  Morrison,  is  an  example  of  this  kind  of  Dinosaur,  which 
attained  a  length  of  60  feet,  and  Diplodocus  was  a  not  very  dissimi- 
lar and  even  larger  reptile.  Stegosaurus  was  another  herbivorous 


reptile,  but  with  such  short  fore  legs 
that  the  gait  must  have  been  bipedal, 
or  else  the  back  must  have  been  arched 
upward  very  strongly  to  the  hind  quarters. 
This  animal,  and  its  European  allies,  Sceli- 
dosaurus  and  Omosaurus,  were  provided 
with  an  armour  of  bony  plates  and  spines 
covering  the  back  and  tail.  Ceratosaurus, 
Allosaurus  (Fig.  295),  and  the  very  similar 
European  genus,  Megalosaurus,  on  the 
other  hand,  were  gigantic  carnivorous  Dino- 
saurs, having  terrible,  sharp  pointed  teeth, 
while  the  toes  were  armed  with  sharp, 
curved  claws.  These  creatures  walked 
upon  their  elongated  hind  legs  and  were  the 
most  formidable  beasts  of  prey  that 
scourged  the  Jurassic  lands.  Not  all  of  the  Jurassic  Dinosaurs 
were  gigantic;  very  small  ones  also  ranged  through  the  forests, 
or  may  even  have  been  arboreal  in  their  habits.  Compsogna- 
thus,  for  example,  was  a  bipedal,  carnivorous  Dinosaur  hardly 
larger  than  a  house  cat. 

Another  very  remarkable  order  of  reptiles,  the  Pterosauria,  the 
earliest  known  appearance  of  which  is  in  the  Rhaetic,  became 
important  and  characteristic  in  the  Jurassic  (Fig.  296).  These 


FIG.  296.— Restoration 
of  Pterosaurian,  Rham- 
phorhynchus.  (Zittel) 


JURASSIC  LIFE  697 

animals  were  provided  with  wings,  and  were  true  fliers,  thus 
realizing  the  old  myth  of  flying  dragons.  The  head  is  relatively 
large,  but  very  lightly  constructed,  and  set  at  right  angles  with  the 
neck,  as  in  birds.  In  the  Jurassic  species,  the  jaws  are  more  or 
less  completely  armed  with  teeth,  which  by  their  form  show  the 
carnivorous  propensities  of  the  animal.  The  joints  of  the  external 
or  little  finger  of  the  hand  are  much  thickened  and  elongated,  this 
finger  being  longer  than  the  body  and  legs  together.  A  membrane, 
or  patagium,  was  stretched  between  the  elongate  finger  on  one 
side,  and  the  body  and  leg  on  the  other,  thus  forming  the  wing, 
which  rather  resembled  the  wing  of  the  bat  than  that  of  a  bird, 
though  differing  from  the  former  in  being  supported  by  one  finger 
instead  of  four.  A  few  exceptionally  well-preserved  specimens 
found  in  the  Solenhofen  limestones  have  retained  the  clearly- 
marked  impressions  of  these  wing  membranes.  The  legs,  like 
those  of  bats,  were  small  and  weak,  and  the  tail  was  very  short 
in  some  species,  very  long  in  others.  Some,  at  least,  of  the  latter 
had  a  membranous,  oar-like  expansion  at  the  tip  of  the  tail.  That 
the  Pterosaurs  had  the  power  of  true  flight,  and  did  not  merely 
take  great  leaps  like  the  flying  squirrels,  is  shown  by  the  hollow, 
pneumatic  bones  (like  those  of  birds),  and  by  the  keel  on  the  breast- 
bone for  the  attachment  of  the  great  muscles  of  flight.  This  keel 
is  found  in  both  birds  and  bats.  The  skin  was  naked,  having 
neither  scales  nor  feathers.  The  Jurassic  Pterosaurs  were  small, 
the  spread  of  wings  not  exceeding  three  feet. 

Birds.  —  One  of  the  most  remarkable  advances  which  Juras- 
sic life  has  to  show  consists  in  the  first  appearance  of  the  birds. 
As  yet,  only  a  single  kind  of  Jurassic  bird  has  been  found,  and  that 
in  the  Solenhofen  limestones.  This,  the  most  ancient  known  bird, 
is  called  Archaopteryx  (Fig.  297),  and  has  many  points  of  resem- 
blance to  the  reptiles,  and  many  characters  which  recur  only  in 
the  embryos  of  modern  birds.  The  peculiarities  which  strike  one 
at  the  first  glance  are  the  head  and  tail;  there  was  no  horny  beak, 
but  the  jaws  are  set  with  a  row  of  small  teeth,  while  the  tail  is  very 
long,  composed  of  separate  vertebrae,  and  with  a  pair  of  quill- 


698 


THE  JURASSIC   PERIOD 


feathers  attached  to  each  joint.  The  wing  is  constructed  on  the 
same  plan  as  that  of  a  modern  bird,  but  is  decidedly  more  primitive. 
The  four  fingers  are  all  free  (in  recent  birds  two  of  the  three  fingers 
are  fused  together) ;  they  have  the  same  number  of  joints  as  in  the 
lizards,  and  are  all  provided  with.claws.  The  plumage  is  thoroughly 
bird-like  in  character,  but  is  peculiar  in  the  presence  of  quill-feathers 
on  the  legs,  and  apparently  also  in  the  absence  of  contour  feathers 
from  the  head,  neck,  and  much  of  the  body,  leaving  those  parts 
naked.  This  very  extraordinary  creature  was,  then,  a  true  bird, 
but  had  retained  many  features  of  its  reptilian  ancestry,  and  shows 

us  that  those  ancestors  have 
still  to  be  sought  in  the  Trias 
or  even  the  Permian. 

Mammalia.  —  The  mam- 
mals of  the  Jurassic  are  still 
very  rare  and  imperfectly 
known,  and  have  been  found 
in  only  a  few  places.  How 
many  mammalian  genera 
should  be  referred  to  the 
Jurassic  will  depend  upon 
where  the  somewhat  arbi- 
trary line  is  drawn,  which 
separates  that  system  from 
the  Cretaceous. 

The  Multituberculata  are 
regarded  as  belonging  to  the 
most  primitive  kind  of  Mam- 
mals, the  Monotremata,  at  present  represented  only  by  the  duck- 
billed Mole  (Ornithorhynchus)  and  Spiny  Ant-Eater  (Echidna)  of 
Australia,  animals  which,  though  warm-blooded  and  suckling 
their  young,  reproduce  by  laying  an  egg.  Of  the  Multituber- 
culates  the  most  prominent  Jurassic  representatives  are  the  Eng- 
lish genus  Plagiaulax,  from  the  Purbeck,  and  the  American 
genera  Ctenacodon  and  Allodon,  from  the  Morrison  of  Wyo- 


FlG.   297.  —  Restoration  of  Arch&opteryx 
lithographica  v.  Meyer.     (Andreae) 


JURASSIC   LIFE  699 

ming.  In  another  group,  which  may  be  related  to  the  Marsu- 
pials, the  teeth  are  simpler  and  more  numerous;  examples  of  this 
group  are  the  Purbeckian  genera,  Stylodon,  and  Triconodon,  and 
the  Morrison  genera,  Dryolestes  and  Dicrocynodon.  Thus,  at  the 
very  end  of  the  Jurassic,  the  mammals  are  still  tiny,  insignificant 
creatures,  which  play  but  a  very  subordinate  role  in  the  luxuriant 
terrestrial  life  of  the  period. 


CHAPTER    XXXIV 
THE   CRETACEOUS   PERIOD 

.  THE  name  Cretaceous  is  derived  from  the  Latin  word  for  chalk 
(Creta),  because  in  England,  where  the  name  was  early  used,  the 
thick  masses  of  chalk  are  the  most  conspicuous  members  of  the 
system.  Though  first  made  known  in  England,  the  main  sub- 
divisions of  the  Cretaceous,  as  employed  in  geological  literature, 
bear  French  names,  which  have  proved  themselves  better  adapted 
to  general  use. 

In  very  marked  contrast  to  the  scanty  development  of  the  Jura, 
the  Cretaceous  strata  of  North  America  are  displayed  on  a  vast 
scale,  and  cover  enormous  areas  of  the  continent,  eloquent  wit- 
nesses of  the  great  geographical  changes  in  that  long  period. 
Continental,  estuarine,  and  marine  rocks  are  all  well  represented, 
and,  in  consequence,  our  information  regarding  the  life  of  North 
America  and  its  seas  during  Cretaceous  times  is  incomparably 
more  complete  than  it  is  for  the  Triassic  and  Jurassic. 

The  Cretaceous  rocks  of  North  America  are  of  very  different 
character  in  the  different  parts  of  the  continent,  and  require  sep- 
arate classification. 

DISTRIBUTION  OF  CRETACEOUS  ROCKS 

American.  —  At  the  opening  of  the  Cretaceous,  the  Atlantic 
coast  of  North  America  appears  to  have  been  farther  to  the  east- 
ward than  it  is  at  present;  but  just  as  had  happened  in  the  Triassic 
period,  a  long,  narrow  depression  was  formed,  running  roughly 
parallel  with  the  coast,  and  in  this  depression  for  a  long  period  of 
time,  sediments  in  the  form  of  gravels,  sands,  and  clays  were  de- 

700 


DISTRIBUTION  OF  CRETACEOUS   ROCKS  701 


FlG.  298.  —  Map  of  North  America  in  the  Cretaceous  period.  Black  areas  = 
known  exposures;  white  =  land;  dotted  areas  =  continental  formations; 
lined  areas  =  sea.  Vertical  lines  indicate  Lower,  and  horizontal  lines  Upper 
Cretaceous  seas 


THE  CRETACEOUS   PERIOD 


posited.  This  is  the  Potomac  series,  which  is  divisible  into  several 
stages.  There  are  unconformities  within  the  series,  which  con- 
tains driftwood,  some  lignite,  and  iron  ore.  The  beds  are  of  con- 
tinental origin  and  probably  differed  locally  in  their  circumstances 
of  deposition,  flood-plain,  delta,  and  marsh,  being  apparently  all 
represented.  The  Potomac  has  been  traced  through  the  islands  of 
Martha's  Vineyard,  Nantucket,  Block  Island,  Long  Island,  Staten 
Island,  across  New  Jersey,  and  thence  southward  to  Georgia, 

CRETACEOUS   FORMATIONS  OF  THE   UNITED   STATES 


European 

Atlantic 
Border 

Gulf 
Border 

Southern 
Interior 

Northern 
Interior 

Pacific  Border 

Danian 

Senonian 

Manasquan 
Ran  cocas 

Wanting 

Wanting 

Livingstone 
and  Denver 

Wanting 

Laramie 

Laramie 

Matawan 

Ripley 
Selma 

Montana 
2.  Glauconitic 

Montana 
2.  Fox  Hills 

Monmouth 

Eutaw 

i.  Ponderosa  Marls 

i.  Fort 
Pierre 

(Belly 
River) 

CHICO 

Colorado 

Colorado 

Turonian 

2.  Austin 

2.  Niobrara 

i.  Eagle  Ford 

i.  Benton 

Cenomanian 
Aibian 

Wanting 

Wanting 

Dakota 

Dakota 

Aptian 
Neocomian 
and 
Wealden 

u  (\,  Raritan 
3)3.  Patapsco 
b  1  2.  Arundel 
PH  1  1.  Patuxent 

Tuscaloosa 

«  (3.  Washita 
g  1  2.  Fredericks 
burg 
(j   1  1.   Trinity 

Washita 
Fuson 
Lakota 
(Kootanie) 

H  (  Horsetown 
|  (  Knoxville 

w 

where  it  turns  northwestward,  following  the  Mississippi  embay- 
ment  into  Tennessee,  and  from  there  turning  southwestward 
through  Arkansas.  In  the  northern  part  of  this  region,  from  Nan- 
tucket  to  the  Delaware  River,  only  the  upper  part  of  the  Potomac 
has  been  found,  and  the  same  appears  to  be  true  of  the  Tuscaloosa, 


DISTRIBUTION   OF  CRETACEOUS   ROCKS  703 

as  the  extension  around  the  Mississippi  embayment  is  called.  The 
Potomac  is  nowhere  marine,  and  everywhere  rests  unconformably 
upon  the  underlying  Triassic  and  older  rocks.  As  the  thickness 
of  sediment  is  not  great  (not  exceeding  600  feet),  the  process  of 
deposition  must  have  been  very  slow,  or  broken  by  long  interrup- 
tions with  intervals  of  erosion. 

While  along  the  Atlantic  border  the  land  was  more  extended 
than  at  present,  in  the  southern  part  of  the  continent  a  different 
order  of  events  was  brought  about.  Nearly  the  whole  of  Mexico 
had  been  submerged  by  the  great  Upper  Jurassic  transgression, 
and  in  the  Lower  Cretaceous  the  sea  extended  over  Texas  and  New 
Mexico  into  Arizona,  and  gradually  expanded  northward  in  the 
successive  stages  of  the  Comanche  epoch,  or  Lower  Cretaceous. 
At  the  base  of  the  Lower  Cretaceous  strata  in  Texas  is  found  a 
deposit  of  continental  sands,  the  Trinity  stage,  which  is  the  recog- 
nized equivalent  of  the  basal  Potomac.  The  advancing  sea  cov- 
ered these  sands,  and  the  continued  depression  soon  established  a 
clear  and  quite  deep  sea,  in  which  were  formed  the  great  masses  of 
the  Comanche  limestones,  that  are  the  surface  rocks  of  large  areas 
in  Mexico,  and  cover  much  of  Texas.  The  Ouachita  Mountains 
of  Arkansas  stood  out  as  a  promontory  in  the  Lower  Cretaceous 
sea,  and  the  ancient  shore-line  has  been  traced  around  their  foot. 
Over  a  great  part  of  Texas  the  Comanche  limestones  are  soft,  and 
beds  of  chalk  occur  among  them;  while  in  Mexico,  where  they  have 
been  folded  into  mountain  ranges,  they  have  become  much  harder 
and  more  compact.  The  thickness  of  the  limestones  increases 
southward;  from  1000  feet  in  northern  central  Texas,  it  rises  to 
5000  feet  on  the  Rio  Grande,  and  on  the  Mexican  plateau  to  an 
even  greater  amount.  No  less  than  six  distinct,  successive  marine 
faunas  are  found  in  the  Comanche  limestones  of  Texas,  and  the 
faunal  relationships  of  this  region  are  closest  with  the  Mediterra- 
nean province  of  Europe,  and  especially  with  the  Lower  Cretaceous 
of  Portugal.  The  greatest  expansion  of  the  Comanche  sea  north- 
ward took  place  in  the  last  of  its  stages,  the  Washita,  when  Okla- 
homa, southern  Kansas,  and  eastern  Colorado  were  covered  by  it. 


704  THE  CRETACEOUS   PERIOD 

Just  how  far  north  this  sea  reached  has  not  yet  been  determined, 
but  there  is  reason  to  think  that  it  extended  into  central  Wyoming. 
This  Lower  Cretaceous  marine  invasion  of  the  northern  interior 
lasted  but  a  relatively  short  time,  and,  until  quite  recently,  its  thin 
deposits  had  escaped  detection. 

In  the  northern  interior  region  the  Lower  Cretaceous  beds,  ex- 
cept those  of  the  Washita,  are  of  continental  origin,  and  it  is  not 
practicable  to  correlate  those  of  different  areas  where  the  strati- 
graphic  connections  cannot  be  traced.  Part  of  the  Morrison  is 
probably  Lower  Cretaceous,  though  we  cannot  yet  say  how  much 
of  it,  nor  what  particular  areas.  *  Another  non-marine  formation 
found  east  of  the  Gold  Range  of  British  Columbia,  extending 
southward  into  Montana,  is  the  Kootanie  stage,  the  plant  remains 
of  which  correlate  it  with  the  lower  Potomac,  and  it  certainly  is 
not  the  oldest  Cretaceous,  for  in  British  Columbia  it  has  been  found 
lying  unconformably  upon  marine  Lower  Cretaceous.  In  part, 
the  Kootanie  was  formed  in  tracts  of  low-lying,  swampy  lands,  on 
which  a  luxuriant  vegetation  produced  valuable  deposits  of  coal. 
Lower  Cretaceous  beds  have  been  found  surrounding  the  Black 
Hills  where  they  have  been  divided  into  the  Lakota  and  Fuson 
stages,  of  continental  origin,  with  abundant  remains  of  land  plants. 

Along  the  Pacific  coast  Lower  Cretaceous  rocks  are  displayed 
on  a  great  scale.  The  Great  Basin  land  then  extended  from  south- 
ern Nevada  to  54°  N.  lat.  in  British  Columbia,  with  the  Sierra 
Nevada  rising  along  part  of  its  western  border,  to  which  the  Pacific 
extended.  North  of  the  Gold  Range  in  British  Columbia,  the 
ocean  spread  eastward,  though  no  doubt  broken  by  many  islands, 
to  the  eastern  base  of  the  Rocky  Mountains.  The  Coast  Range  of 
California  formed  a  chain  of  islands  and  reefs.  In  the  Sierra 
Nevada  occurs  an  unconformity  between  the  Lower  Cretaceous  and 
the  uppermost  Jurassic,  but  it  does  not  imply  the  lapse  of  a  very 
long  period  of  time. 

The  older  division  of  the  Californian  Lower  Cretaceous  is  called 
the  Knoocville,  and  has  an  estimated  maximum  thickness  of  20,000 
feet,  laid  down  upon  a  slowly  subsiding  sea-bottom.  This  enor- 


DISTRIBUTION   OF  CRETACEOUS   ROCKS  705 

mous  thickness  is  no  doubt  due  to  an  extremely  rapid  deposition 
of  the  debris  abundantly  supplied  from  the  waste  of  the  newly 
upheaved  Sierras.  At  the  end  of  the  Knoxville  age,  the  subsidence 
became  more  rapid  and  the  sea  began  to  encroach  upon  the  land, 
for  the  Horsetown  beds,  which  have  a  thickness  of  6000  feet,  over- 
lap the  Knoxville  shoreward  and  extend  over  upon  the  underlying 
Jurassic  and  other  pre-Cretaceous  systems.  Although  the  two 
stages  of  the  Calif ornian  Lower  Cretaceous  are  entirely  conformable 
throughout,  and  appear  to  have  been  formed  by  a  continuous  pro- 
cess of  sedimentation,  yet  there  is  a  very  marked  faunal  change  be- 
tween them.  The  Knoxville  beds  have  a  northern  fauna,  allied  to 
that  of  Russia,  showing  that  the  connection  with  Russian  seas,  which 
had  been  established  in  late  Jurassic  times,  was  still  kept  up.  With 
the  beginning  of  the  Horsetown  age,  however,  this  northern  com- 
munication was  interrupted  doubtless  by  the  closing  of  Bering  Sea, 
and  a  connection  was  formed  with  the  waters  of  southern  Asia, 
and  in  that  way  with  central  Europe.  The  decided  contrast  which 
we  find  between  the  Lower  Cretaceous  faunas  of  California  and 
those  of  Texas  points  to  the  existence  of  a  land  barrier  between 
the  seas  of  the  two  regions. 

In  the  southern  region  the  Lower  Cretaceous  was  terminated  by 
an  upheaval,  which  caused  the  Comanchean  Sea  to  withdraw 
from  Texas  and  the  area  to  the  west  and  north  of  it.  This  mid- 
Cretaceous  land  epoch  must  have  continued  for  a  considerable 
time,  permitting  extensive  denudation  and  a  complete  change  in 
the  fauna.  Wherever  the  marine  Upper  Cretaceous  is  in  contact 
with  the  Comanche  limestones  north  of  Mexico,  the  two  are  un- 
conformable,  and  no  species  of  animal  is  known  to  pass  from  one 
to  the  other.  In  Mexico  the  Lower  Cretaceous  passes  into  the 
Upper  without  a  break,  the  disturbances  there  taking  place  at  a 
later  date. 

The  Upper  Cretaceous  rocks  have  a  far  wider  distribution  over 
North  America  than  have  those  of  the  lower  division,  which  is  due 
to  an  enormous  transgression  of  the  sea  over  the  land,  one  of  the 
greatest  in  all  recorded  geological  history.  Over  the  region  of  the 


706  THE  CRETACEOUS   PERIOD 

Great  Plains  the  Upper  Cretaceous  was  inaugurated  by  the  forma- 
tion of  a  non-marine  stage,  the  Dakota.  These  strata  cover  much 
of  Texas,  lying  unconformably  upon  the  Comanche  series,  and  ex- 
tend northward  into  Canada.  In  Kansas,  however,  the  connec- 
tion of  the  Dakota  with  the  Washita  is  very  close,  bands  of  sand- 
stone carrying  Dakota  species  of  plants  being  interstratified  with 
the  marine  beds.  On  the  western  side  of  the  Colorado  uplift,  the 
Dakota  is  less  distinctly  a  sandstone  formation,  and  is  characterized 
by  beds  of  shale  and  even  coal  seams  of  workable  thickness.  In 
most  parts  of  the  Rocky  Mountain  region  the  Dakota  rests  in 
apparent  conformity  upon  the  lowest  continental  Cretaceous,  and 
even  upon  the  Jurassic.  In  the  Uinta  and  Wasatch  ranges  there 
is  no  apparent  break  in  sedimentation  from  the  Palaeozoic  to  the 
end  of  the  Mesozoic,  though  the  whole  Lower  Cretaceous  is  there 
wanting.  From  this  we  may  infer  that  during  the  long  Lower 
Cretaceous  time  all  these  regions  had  been  low-lying  lands,  nearly 
or  quite  at  base-level,  and  therefore  not  subject  to  profound  denu- 
dation. 

It  was  at  the  end  of  the  Dakota  age  that  the  great  subsidence 
took  place  which  affected  nearly  all  parts  of  the  continent,  and 
brought  the  sea  in  over  vast  areas  where  for  ages  had  been  dry 
land.  South  of  New  England  the  Atlantic  coastal  plain  was  sub- 
merged, and  in  New  Jersey,  at  least,  the  waters  covered  even  the 
nearly  base-levelled  Triassic  belt,  bringing  the  sea  up  to  the  foot  of 
the  crystalline  highlands.  The  lowlands  of  Maryland,  Virginia, 
and  the  Carolinas,  and  all  of  Florida  were  under  the  ocean,  and 
the  Gulf  of  Mexico  was  extended  northward  in  a  great  bay  (the 
Mississippi  embayment),  covering  western  Tennessee  and  Ken- 
tucky and  extending  into  southern  Illinois.  In  Mexico  important 
changes  occurred  during  the  Upper  Cretaceous.  Early  in  this 
epoch  a  general  elevation  restricted  the  sea  to  the  northeastern 
part  of  the  country,  but  in  the  south  was  an  opposite  movement, 
the  sea  transgressing  upon  the  ancient  land  which  lay  to  the  west 
of  the  Isthmus  of  Tehuantepec,  finally  covering  it  late  in  the  period. 
In  this  region  the  Upper  Cretaceous  overlaps  upon  the  Archaean. 


DISTRIBUTION   OF  CRETACEOUS    ROCKS  707 

Elsewhere,  the  disturbance  referred  to  was  erogenic,  resulting  in  the 
formation  of  most  of  the  Mexican  mountain  ranges  and  continuing 
nearly  or  quite  to  the  end  of  the  period.  Texas  was  again  exten- 
sively submerged,  and  a  wide  sea  connected  the  Gulf  of  Mexico  with 
the  Arctic  Ocean.  The  eastern  coast  of  this  interior  sea  began  in 
northwestern  Texas,  running  through  Kansas  and  Iowa  nearly  to 
the  present  line  of  the  Mississippi  River.  Westward  the  coast- 
line was  the  uplift  which  ran  from  the  west  coast  of  Mexico  into 
British  Columbia.  The  Colorado  region  was  again  converted  into 
islands.  North  of  the  Great  Basin  land  the  interior  sea  was  con- 
nected with  the  Pacific  and  Arctic  Oceans,  which  united  over  the 
northwestern  part  of  the  continent. 

On  the  Pacific  side,  the  Sierras,  which  had  suffered  greatly  from 
denudation,  were  again  folded.  A  moderate  transgression  of  the 
sea  caused  the  Upper  Cretaceous  to  extend  farther  east  than  the 
Lower.  Volcanic  activity  continued  and  immense  bathyliths  were 
formed  deep  within  the  mountains.  The  sea  extended  from 
Lower  California  northward  along  the  Sierra  into  eastern  Oregon 
at  the  foot  of  the  Blue  Mountains. 

The  North  American  continent  was  thus  divided  into  two  prin- 
cipal land  masses,  the  larger  one  to  the  east  and  comprising  the 
pre-Cambrian  and  Palaeozoic  areas.  In  the  limits  of  the  United 
States  this  land  lay  almost  entirely  east  of  the  Mississippi,  except 
for  a  southwestern  peninsula,  including  Missouri,  Arkansas,  Okla- 
homa, and  part  of  Texas.  The  western  area  was  much  smaller, 
extending  from  Mexico  into  British  Columbia,  and  having  its 
greatest  width  between  the  fortieth  and  forty-fifth  parallels  of  lati- 
tude. Between  the  two  lands  lay  the  Colorado  Islands,  and  doubt- 
less many  smaller  ones  as  well. 

The  character  of  sedimentation  differed  so  much  in  the  various 
regions  of  the  continent  that  the  subdivisions  of  the  Upper  Creta- 
ceous have  received  different  names  in  the  separate  provinces,  and 
only  approximately  correspond  in  time.  The  greater  number  of 
these  subdivisions,  which  are  founded  chiefly  upon  physical 
changes,  gives  to  the  Upper  Cretaceous  the  appearance  of  being 


708  THE  CRETACEOUS   PERIOD 

longer  and  more  important  than  the  Lower,  but  this  is  only  an  ap- 
pearance. In  Europe  the  Lower  Cretaceous  has,  in  recent  times, 
been  divided  into  six  series,  an  arrangement  which  proves  to  be 
of  general  validity. 

Along  the  Atlantic  border  the  Upper  Cretaceous  strata  -are  a 
series  of  marine  sands  and  clays,  which  are  still  almost  horizontal 
in  position  and  of  loose,  incoherent  texture.  In  New  Jersey  there 
are  extensive  developments  of  green  sands  locally  called  marl.  The 
Appalachian  Mountains,  which  had  been  subjected  to  the  long- 
continued  denudation  of  Triassic,  Jurassic,  and  Lower  Cretaceous 
times,  were  now  reduced  nearly  to  base-level,  the  Kittatinny  plain 
of  geographers  (see  p.  512).  This  peneplain  was  low  and  flat, 
covering  the  whole  Appalachian  region,  and  the  only  high  hills  upon 
it  were  the  mountains  of  western  North  Carolina,  then  much  lower 
than  now.  Across  this  low  plain  the  Delaware,  Susquehanna,  and 
Potomac  must  have  held  very  much  their  present  courses,  meander- 
ing through  alluvial  flats. 

On  the  Gulf  border  the  Upper  Cretaceous  beds  of  Alabama  and 
Mississippi,  which  were  laid  down  in  the  Mississippi  embayment, 
are  in  3  stages.  Below  are  the  sands  and  clays  of  the  Eutaw 
(300  feet)  which  is  correlated  with  the  Matawan  of  New  Jersey; 
next  follows  the  soft  limestone,  or  chalk,  of  theSelma  (500-1200 
feet),  and  at  the  top  are  200  to  500  feet  of  the  Ripley  sands.  East- 
ward the  water  shallowed,  and  in  Georgia  we  find  abjut  1400  feet 
of  clays  and  sands.  Northward  along  the  Mississippi  embayment 
the  beds  thin  greatly  and  are  mostly  clays  and  sands. 

In  the  interior  region  lying  upon  the  Dakota  are  the  marine 
beds  of  the  Colorado,  of  which  the  lower  division  is  the  Benton, 
a  mass  of  shales  and  limestones  with  a  maximum  thickness  of 
1000  feet,  though  varying  much  from  point  to  point.  The  depres- 
sion still  continuing,  the  sea  became  quite  deep,  making  favourable 
conditions  for  the  formation  of  the  chalk  and  harder  limestones 
of  the  Niobrara.  This  chalk  is  best  seen  in  Kansas,  but  extends 
into  South  Dakota;  elsewhere  are  sandstones  and  limestones  with 
a  maximum  thickness  of  2000  feet.  A  movement  of  reelevation 


DISTRIBUTION  OF  CRETACEOUS   ROCKS 

of  the  sea-bottom  began  even  in  Colorado  times,  and  in  the  north- 
ern part  of  the  interior  region  oscillations  of  level  produced  alter- 
nating fresh-water,  or  estuarine,  and  marine  conditions.  In 
Montana  and  the  Canadian  province  of  Alberta  is  a  thick  body 
of  estuarine  or  fresh-water  strata  with  seams  of  coal  (the  Belly 
River  formation)  interposed  between  the  marine  deposits  of  the 
Colorado  below  and  the  Montana  above.  In  Utah  is  another 
fresh- water  deposit  of  coal-bearing  rocks  of  Colorado  age. 

In  the  Montana  epoch  marine  conditions  still  prevailed,  but  the 
waters  of  the  northern  sea  had  generally  become  much  shallower, 
and  a  marked  change  of  fauna  was  produced.  In  Alberta  are  coal 
measures  of  this  date.  Two  divisions  of  the  Montana  are  dis- 
tinguished, although  not  everywhere  separable;  the  Fort  Pierre, 
which  is  composed  of  shales  and  sandstones  with  a  maximum 
thickness  of  8000  feet,  and  the  Fox  Hills,  sandstones  and  some 
shales,  which  do  not  exceed  1000  feet.  This  movement  of  up- 
heaval in  the  interior  was  accompanied  or  followed  by  an  uplift 
on  the  Atlantic  and  Gulf  coasts,  for  along  these  borders  the  upper- 
most Cretaceous  beds  are  either  wanting  or  represented  by  ex- 
ceedingly thin  deposits.  In  the  interior  the  continued  upheaval 
caused  fresh-water  and  swampy  conditions  to  prevail  over  very 
wide  areas,  though  not  so  widely  extended  as  had  been  the  Upper 
Cretaceous  sea.  This  great  continental  formation  is  the  Laramie, 
which  has  no  such  eastward  extension  as  the  marine  Cretaceous, 
but  is  restricted  to  the  western  side  of  the  basin  and  is,  in  part, 
probably  equivalent  to  the  latest  marine  Cretaceous,  the  difference 
being  in  facies  rather  than  in  time.  However,  this  applies  only 
to  the  older  part  of  the  Laramie,  which  as  a  formation  continues 
much  later  than  any  of  the  marine  stages  and  may  even  have 
persisted  into  the  Eocene.  The  northwestern  part  of  the  continent 
had  been  converted  into  dry  land,  but  a  broad  area  of  non-marine 
deposition  extended  up  the  course  of  the  present  Mackenzie  River 
to62°N.lat.  Another  and  vastly  larger  area  began  about  57°N.lat., 
and  reached,  though  perhaps  with  interruptions,  to  northeastern 
Mexico,  surrounding  the  Colorado  island.  This  area  was  2000 


/IO  THE  CRETACEOUS   PERIOD 

miles  long  and  500  miles  wide,  and  reproduced  the  conditions 
which  obtained  around  the  Mississippi  valley  in  the  Upper  Car- 
boniferous, immense  swamps  and  peat-bogs  in  which  gathered 
the  quantities  of  vegetable  matter  now  converted  into  coal  seams. 
The  clastic  rocks  interstratified  with  the  coal  are  probably  fluviatile 
and  lacustrine  deposits,  and  occasional  brackish-water  conditions 
are  reported  from  some  areas.  Workable  coal  is  found  in  all  the 
stages  of  the  western  Cretaceous,  but  none  of  these  stages  is  com- 
parable to  the  Laramie  for  the  extent  and  thickness  of  its  coal 
measures. 

The  Laramie  was  a  time  of  tranquillity,  with  only  slow  and  gentle 
changes  of  level,  but  towards  its  close  some  important  disturb- 
ances took  place,  especially  along  the  Rocky  Mountains.  The 
first  of  these  movements  affected  only  the  Colorado  island,  and 
its  effects  are  especially  well  shown  in  the  Denver  basin,  where 
some  800  feet  of  conglomerates  (the  Arapahoe)  rest  upon  the 
Laramie  unconformably.  The  second  series  of  movements  was 
much  more  extensively  felt,  producing  marked  unconformities 
both  in  Colorado  and  Montana.  In  Colorado  there  was  a  great  vol- 
canic outburst,  and  the  Denver  stage,  which  overlies  the  Arapahoe 
unconformably,  is  principally  composed  of  andesitic  tuffs.  In 
Montana  the  equivalent  stage  (Livingstone},  which  also  contains 
considerable  volcanic  material,  is  7000  feet  thick  and  unconform- 
able  with  the  Laramie.  It  is  probable  that  the  Arapahoe,  Denver, 
and  Livingstone,  all  of  which  occur  along  the  Rocky  Mountains, 
correspond  to  beds  which  elsewhere  are  a  part  of  the  Laramie. 
The  latter  in  eastern  Wyoming  passes  into  undoubted  Eocene 
above,  by  what  appears  to  be  an  unbroken  continuity  of  sedimen- 
tation. 

The  Upper  Cretaceous  of  the  Pacific  coast  comprises  the  Chico 
series,  with  a  maximum  thickness  of  4000  feet.  In  Vancouver's 
Island  the  Chico  is  coal-bearing.  The  faunal  connections  of  the 
Chico  are  with  southern  Asia,  that  series  having  very  little  in  com- 
mon with  the  fossils  of  the  interior  region.  The  uppermost  Cre- 
taceous is  wanting  along  the  Pacific  coast,  except  for  certain  coal- 


DISTRIBUTION   OF  CRETACEOUS   ROCKS 

bearing  beds  in  Washington,  which  appear  to  represent  the 
Laramie. 

The  resemblance  of  the  Chico  fossils  to  those  of  southern  Asia 
indicates  the  closing  of  Bering  Sea  and  thus  the  possibility  of  a 
migration  of  shoal-water  animals  all  around  the  shores  of  the 
North  Pacific,  at  the  same  time  providing  a  way  for  the  interchange 
of  land  animals  and  plants  between  North  America  and  the  Old 
World.  The  Upper  Cretaceous  faunas  of  Mexico  are  surprisingly 
different  from  those  of  the  United  States,  and  so  like  those  of  the 
Mediterranean  region  of  Europe  as  to  suggest  an  east  and  west 
shore-line  across  the  Atlantic  in  the  latitude  of  Brazil,  while  the 
northern  connection  of  America  with  Europe  probably  continued 
also,  for  the  shallow-water  fossils  of  New  Jersey  resemble  those 
of  central  Europe. 

The  Mesozoic  era  was  closed  in  the  West,  as  the  Palaeozoic  had 
been  in  the  East,  by  a  time  of  great  mountain  making,  and  to  this 
movement  is  attributed  the  formation  of  most  of  the  great  Western 
mountain  chains.  From  the  Arctic  Ocean  to  Mexico  the  effects  of 
the  disturbance  were  apparent.  The  Rocky  Mountains,  the  Wa- 
satch  and  Uinta  ranges,  the  high  plateaus  of  Utah  and  Arizona, 
and  the  mountains  of  western  Texas  date  from  this  time,  though 
subsequent  movements  have  greatly  modified  them.  Vast  volcanic 
outbreaks  accompanied  the  upheaval,  which  was  on  a  far  grander 
scale  than  the  Appalachian  revolution  had  been. 

Foreign.  —  In  South  America  the  Cretaceous  history  is  much 
like  that  of  the  northern  continent.  The  subsidence  which  inau- 
gurated the  Lower  Cretaceous  extended  the  sea  over  the  north- 
ern part  of  South  America  and  covered  northeastern  Brazil,  with 
fresh-water  deposits  in  central  Brazil.  All  along  the  Cordillera, 
from  Venezuela  to  Patagonia,  marine  Cretaceous  is  found,  but  east 
of  the  mountains,  with  the  exceptions  already  noted,  the  system  is 
represented  chiefly  by  non-marine  sandstones.  In  Patagonia, 
however,  is  an  area  of  marine  Lower  Cretaceous  east  of  the  Andes, 
though  its  extent  is  not  known.  Thick  continental  sandstones 
represent  most  of  the  period,  but  toward  the  close,  the  entire  Pata- 


712  THE  CRETACEOUS   PERIOD 

gonian  plain  appears  to  have  been  submerged  for  a  short  time  by  a 
transgression  of  the  Upper  Cretaceous  sea.  The  faunal  relations 
of  the  South  American  Lower  Cretaceous  are  very  intimate  with 
northern  and  western  Africa.  Gigantic  volcanic  activity  went 
on  along  the  Cordillera  in  Mesozoic  times;  in  Chili  and  Peru  the 
marine  Cretaceous  is  principally  made  up  of  stratified  igneous  ma- 
terial, and  the  Andes  contain  the  largest  known  area  of  Mesozoic 
eruptives.  The  mountain-making  upheaval  probably  came  at  the 
close  of  the  Cretaceous. 

In  Europe,  toward  the  end  of  the  Jura,  the  sea  retired  from 
nearly  all  of  the  central  region,  which  in  part  became  dry  land 
and  in  part  was  covered  with  lakes  and  inland  seas.  One  of  the 
largest  of  these  covered  much  of  southern  England,  extending  far 
into  Germany,  and  in  it  was  deposited  a  great  thickness  of  sand 
and  clay,  with  some  shell  limestone,  the  Wealden.  The  Alpine  re- 
gion remained  submerged  under  a  clear  and  deep  sea,  and  the  tran- 
sition from  the  Jurassic  is  very  gradual.  In  the  oldest  Cretaceous 
epoch  (Neocomiari)  a  renewed  transgression  submerged  large 
parts  of  central  Europe,  though  the  sea  was  far  less  extensive  than 
that  of  the  Middle  and  Upper  Jurassic.  In  consequence,  a  great 
gulf  was  established  over  southern  England,  northern  France,  and 
north  Germany  to  Poland,  a  gulf  bounded  on  the  north  by  the 
highlands  of  Britain,  Scandinavia,  and  northwestern  Russia,  and 
on  the  south  by  a  land  stretching  from  Ireland  to  Bohemia;  Bel- 
gium was  mostly  an  island.  The  expanded  Mediterranean  covered 
southeastern  Asia  Minor  and  northern  Africa.  In  the  Upper  Cre- 
taceous the  northern  gulf  was  greatly  extended,  covering  many 
areas  that  had  been  land  since  Palaeozoic  or  pre-Cambrian  times. 
Parts  of  this  basin  became  very  deep,  and  its  most  characteristic 
deposit,  especially  over  southern  England  and  northern  France, 
was  chalk,  which  the  microscope  shows  to  be  made  up  of  the  shells 
of  Foraminifera  and  to  resemble  the  modern  foraminiferal  oozes. 
Over  the  Alpine  region  upheavals  in  the  Upper  Cretaceous  had 
established  land  areas,  indicated  by  extensive  fresh-water  deposits 
recurring  at  intervals  from  Spain  to  Hungary,  in  the  latter  country 


DISTRIBUTION  OF  CRETACEOUS   ROCKS  713 

containing  coal.  The  Cretaceous  was  closed  in  Europe  by  a 
gradual  upheaval  which  excluded  the  sea  from  wide  areas  that  it 
had  occupied. 

In  Africa  the  only  extensive  Cretaceous  areas  are  those  of  the 
north,  where  the  Atlas  Mountains,  and  much  of  the  surface  of 
the  Libyan  desert  are  made  up  of  these  rocks.  A  limited  trans- 
gression of  the  sea  also  took  place  along  the  western  coast.  In 
South  Africa  are  traces  of  two  Cretaceous  invasions  of  the  sea,  both 
of  which  merely  occupied  old  valleys  for  quite  a  short  distance  from 
the  coast  of  Cape  Colony  and  Natal.  The  first  invasion  is  of 
Neocomian  date  ( Uitenhage  beds)  and  its  fossils  have  a  distinct 
likeness  to  those  of  Patagonia.  The  second  incursion  took  place 
later  in  the  Cretaceous,  at  a  horizon  not  yet  determined. 

Southern  and  eastern  Asia  display  many  areas  of  Cretaceous 
rocks,  as,  for  example,  in  southern  India  and  Japan.  Australia 
also  has  extensive  areas  of  this  system,  which  are  best  known 
in  Queensland,  where  they  are  chiefly  Lower  Cretaceous  and 
contain  coal.  The  New  Zealand  Cretaceous  is  also  coal- 
bearing. 

Climate.  — The  evidence  for  the  existence  of  climatic  zones  is 
more  distinct  in  the  Cretaceous  than  in  the  Jurassic,  though  the 
difference  between  the  zones  must  have  been  slight,  for  the  Upper 
Cretaceous  flora  extends  to  Greenland  with  hardly  any  change. 
On  the  other  hand,  the  marine  animals  show  a  decided  difference 
according  to  latitude.  In  the  Mediterranean  region  of  Europe  and 
Asia,  the  West  Indies,  Mexico,  and  the  north  coast  of  South  America 
the  seas  abounded  in  reef-building  corals,  in  the  extraordinary 
groups  of  bivalve  molluscs,  or  Pelecypoda,  called  the  Rudistes 
and  Caprotince,  and  in  certain  genera  of  Ammonites,  such  as 
Lytoceras,  Haploceras,  and  Phylloceras.  In  northern  and  central 
Europe  and  on  the  Atlantic  coast  of  the  United  States  these  forms 
are  rare  or  absent  and  other  groups  take  their  place.  The  probable 
explanation  of  the  seeming  contradiction  in  the  testimony  of  land 
plants  and  marine  animals  is  in  the  existence  of  a  cool  polar  sea 
and  southward  currents  from  it. 


THE   CRETACEOUS   PERIOD 


CRETACEOUS  LIFE 

The  life  of  the  Cretaceous  displays  so  great  an  advance  over  that 
of  the  Jurassic  that  the  change  may  fairly  be  called  a  revolution. 

Plants.  —  If  the  separation  between  the  Mesozoic  and  Cenozoic 
eras  were  made  entirely  with  reference  to  the  plants,  it  would  pass 
between  the  Lower  and  the  Upper  Cretaceous,  just  as  a  similar 
criterion  would  remove  the  Upper  Permian  to  the  Mesozoic.  The 
vegetation  of  the  Lower  Cretaceous,  especially  of  the  lowest,  is  still 
much  like  that  of  the  Jura.  Ferns,  Horsetails,  Cycads,  and  Conifers 


FlG.  299.  — Cretaceous  leaves,  Dakota  stage,  i.  Dammarites  emarginatus  Lesq., 
1/2.  2.  Betulites  Westi  Lesq.,  3/4.  3.  Liriodendron  giganteum  Lesq.,  1/2. 
(Lesquereux) 

continue  to  make  up  most  of  the  flora.  The  Cycadales,  in  particular, 
abound  in  the  Lower  Cretaceous,  many  of  them  belonging  to  the 
Bennettiteae.  On  the  other  hand,  the  impending  revolution  is  an- 
nounced by  the  appearance  of  Dicotyledons  of  archaic  and  primi- 
tive type.  In  the  higher  parts  of  the  Potomac  the  Cycads  become 
less  abundant  and  the  Dicotyledons  very  much  more  so.  Here 
we  find  many  leaves  which  belong  to  genera  that  cannot  be  distin- 
guished from  those  of  modern  forest  trees,  such  as  Sassafras,  Popu- 


PLATE  XV.  —  CRETACEOUS  INVERTEBRATE  FOSSILS 


Fig.  i,  Frondicularia  major  Bornernann,  X  3,  Up.  Cret  ,  N.J.  2,  Haplophragmium 
concavitm  Bagg,  X  12,  U.  C.  3,  Cardiaster  cintus  Morton,  X  •%,  U.  C.  4,  Terebratella 
plicata  Say,  X 


U.  C. 


Terelratula  harlani  Whitf. 


U.  C.     6,  Inocercuitus 

.....«-.  /,  Aucella  pioche  Gabb,  X  i,  Knoxville. 
5,  Idonearca  nebrascensis  Owen,  x  %,  Fox  Hills.  9,  Exogyra  costata  Say,  x  %,  U.  C. 
10,  Veniella  conradi  Morton,  X  %,  U.  C.  n,  Ostrea  larva  Lam.,  x  %,  U.  -C 
12.  Pyropsis  baz'rdiM..  and  H  ,  x  %,  Fox  Hills.  13,  AncJinra  americana  x  i,  Evans  and 
Shumard,  Fox  Hills  14,  Baculites  compressus  S  y,  X  %,  fragment  of  adult  with  suture- 
lines,  Ft  Pierre.  15,  The  same,  a  very  young  shell,  x  5,  showing  apical  coil.  i6,Scaphite3 
nodosus  Owen,  x  %,  Ft.  Pierre  17,  Heteroceras  stevensoni  Whitf.,  x  2/5,  Ft.  Pierre. 
18,  Belemnitella  americana  Morton,  x  %,  U.  C. 


716  THE  CRETACEOUS   PERIOD 

lus,  Liriodendron,  etc.  No  Dicotyledons  have  been  found  in  the 
Kootanie  of  the  Northwest,  or  in  the  Wealden  of  northern  Europe, 
but  they  occur  in  the  Lower  Cretaceous  of  Portugal,  Greenland, 
and  Spitzbergen.  In  the  latter  part  of  the  Lower  and  in  all  the 
Upper  Cretaceous  of  North  America  the  flora  assumes  an  almost 
completely  modern  character,  and  nearly  all  of  our  common  kinds 
of  forest  trees  are  represented:  Sassafras,  Poplars,  Willows,  Oaks, 
Maples,  Elms,  Beeches,  Chestnuts,  and  very  many  others.  A  new 
element  is  the  Monocotyledonous  group  of  Palms,  which  speedily 
assumes  great  importance.  Each  successive  plant-bearing  horizon 
of  the  Cretaceous  is  characterized  by  its  own  special  assemblage 
of  plants,  but  in  its  general  features  the  Upper  Cretaceous  flora 
is  essentially  modern,  and  this  is  true  of  the  world  at  large,  while 
in  the  Lower  division  it  was  only  in  North  America  and  a  few 
other  scattered  areas  that  the  Angiosperms  had  gained  a  foothold. 
Cretaceous  animals  are  sufficiently  different  from  those  of  the  Jura, 
but  the  change  is  not  so  revolutionary  as  we  have  found  among 
the  plants. 

Foraminifera  play  an  important  part  in  the  construction  of 
Cretaceous  rocks,  especially  of  the  great  masses  of  chalk  (PI.  XV, 
Figs,  i,  2),  while  the  green  sands  are  casts  of  foraminiferal  shells  in 
glauconite.  The  most  abundant  genus,  as  in  the  recent  Atlantic 
oozes,  is  Globigerina. 

Spongida.  —  In  the  Cretaceous  of  Europe  Sponges  are  more 
numerous  and  varied  than  at  any  other  time,  but  in  North  America 
they  are  far  less  common. 

Ccelenterata.  — The  Corals  were  very  much  as  they  are  to-day 
and  require  no  special  description. 

The  Echinodermata  undergo  some  very  marked  changes.  The 
Crinoids  are  much  reduced  since  the  Jurassic,  and  were  never 
again  to  assume  their  ancient  importance;  characteristic  Cre- 
taceous genera  are  the  stemless,  free-swimming  Uintacrinus  and 
Marsupites.  The  Sea-urchins  are  incomparably  more  numerous 
in  Europe  than  in  North  America;  of  the  Regular  forms  may  be 
mentioned  Pseudodiadema,  Cidaris,  and  Salenia,  and  of  the 


CRETACEOUS   LIFE 

Irregular  forms,  Toxaster,  Holaster,  Cassidulus,  Cordiaster  (XV, 
3),  etc. 

Arthropoda.  —  Among  the  Crustacea  we  need  only  note  the 
great  increase  in  the  Brachyuran  Decapods,  or  Crabs,  in  the  beds 
of  the  Gulf  border  and  of  Europe. 

Brachiopoda  are  very  much  as  in  the  Jurassic;  the  common 
genera  are  Terebratula  (XV,  5),  Terebratella  (XV,  4),  and  Rhyn- 
chonella. 

Mollusca.  — This  group  is  very  richly  developed,  and  many 
genera  are  peculiar  to  the  period.  The  large,  curious  oysters  be- 
longing to  the  genera  Ostrea  (XV,  n),  Gryphcea,  and  especially 
Exogyra  (XV,  9),  are  common,  and  the  many  species  of  Inocera- 
mus  (XV,  6)  are  very  characteristic,  especially  of  the  northern 
facies.  More  modern  types  are  Idonearca  (XV,  8)  and  Veniella 
(XV,  10).  Confined  to  the  Cretaceous  are  the  extraordinary  shells 
classed  as  Rudistes,  in  which  one  valve  is  long  and  horn-shaped, 
and  the  other  a  mere  cover  for  it.  These  shells  of  the  genera 
Hippurites,  Radiolites,  and  Coralliochama  are  much  commoner 
in  Europe  than  in  America  and  are  preeminently  southern  in 
distribution.  Other  peculiar  Cretaceous  Bivalves  are  Requienia 
and  Caprotina.  Aucella  (XV,  7)  is  of  interest  both  in  the  Upper 
Jurassic  and  the  Cretaceous  as  a  typically  Boreal  group  of  shells. 
The  Gastropods  (XV,  12,  13),  are  very  much  as  in  the  Jura,  but  in 
the  latter  part  of  the  period  come  in  many  genera  which  reach 
their  fullest  development  in  Tertiary  and  recent  times,  such  as 
Fusus,  Murex,  Voluta,  Cyprcea,  and  many  others.  . 

The  Cephalopods  are  very  peculiar;  in  addition  to  numerous 
Ammonoid  genera  with  closely  coiled  shells  of  normal  type,  such 
as  Hoplites,  Schlcenbachia,  Placenticeras,  we  find  very  many  shells 
entirely  or  partially  uncoiled,  or  rolled  up  in  peculiar  ways,  which 
give  to  the  Cretaceous  Cephalopod  fauna  a  character  all  its  own. 
In  Crioceras  (XIV,  20)  the  shell  is  coiled  in  an  open,  flat  spiral,  the 
whorls  of  which  are  not  in  contact.  Ancyloceras  has  a  similar 
open  coil,  followed  by  a  long,  straight  portion,  and  recurved  ter- 
minal chamber.  Scaphitqs  (XV,  16)  is  like  a  shortened  Ancylo- 


/1 8  THE  CRETACEOUS   PERIOD 

ceras.  In  Ptychoceras  the  shell  consists  of  two  parallel  parts, 
connected  by  a  single  sharp  bend.  Turrilites  is  coiled  into  a  high 
spiral,  like  a  Gastropod,  and  Baculites  (XV,  14,  15)  has  a  perfectly 
straight  shell  except  for  a  minute  coil  at  one  end.  Heteroceras 
(XV,  17)  displays  the  extreme  of  irregularity  of  growth.  Nautilus 
is  represented  by  many  specie^,  some  of  them  very  large.  Belem- 
nites  are  very  abundant,  but  in  the  Upper  Cretaceous  the  genus 
Belemnitella  (XV,  18)  replaces  the  true  Belemnites. 

The  Vertebrata  form  the  most  characteristic  element  of  the 
Cretaceous  fauna.  Among  the  Fishes  a  revolution  has  occurred. 
Sharks  of  modern  type  abound,  and  their  teeth  are  found  in  count- 
less numbers;  but  the  principal  change  consists  in  the  immense 
expansion  of  the  Teleosts,  or  Bony  Fishes,  which  now  take  the  domi- 
nant place,  while  Ganoids  become  rare.  Most  of  the  Cretaceous 
Teleosts  belong  to  modern  families  and  even  genera,  such  as  the 
Herrings,  Cod,  Salmon, Mullets,  Catfishes,  etc.;  but  a  characteristic 
Cretaceous  type,  now  extinct,  is  that  of  the  Saurodonts,  fierce, 
carnivorous  fishes  of  great  size  and  power.  The  genus  Portheus, 
common  in  the  Kansas  chalk,  was  12  to  15  feet  long,  and  was 
provided  with  great  reptile-like  teeth. 

The  Reptiles  continued  to  be  the  dominant  types  of  the  land, 
the  sea,  and  the  air,  and  it  may  fairly  be  questioned  whether  the 
Jura  or  the  Cretaceous  should  be  regarded  as  the  culminating 
period  of  Reptilian  history.  Ichthyosaurs  and  Plesiosaurs  are  per- 
haps less  abundant  than  in  the  Jura,  but  are  of  greatly  increased 
size.  Elasmosaurus,  a  Plesiosaur  from  the  Kansas  chalk,  had  a 
length  of  40  to  50  feet,  of  which  22  feet  belonged  to  the  slender 
neck.  Confined  to  the  Cretaceous  are  the  remarkable  marine 
reptiles  of  the  group  Mosasauria,  which  swarmed  on  the  Atlantic 
and  Gulf  coasts,  and  especially  in  the  interior  sea.  These  were 
gigantic,  carnivorous  marine  lizards,  with  the  limbs  converted 
into  swimming  paddles  (see  Fig.  300).  Turtles,  both  fresh-water 
and  marine,  abound,  and  some  were  very  large.  Lizards  and 
Snakes  are  but  scantily  represented,  not  displaying  the  manifold 
variety  of  structure  which  they  afterwards  acquired,  An  order  of 


CRETACEOUS   LIFE  719 

aquatic  reptiles,  the  Choristodera,  nearly  allied  to  the  Rhyncho- 
cephalia,  appeared  in  the  latter  part  of  the  period.  Crocodiles, 
like  those  of  modern  days,  were  ubiquitous  in  both  fresh  and 
salt  waters,  and  in  North  America,  at  least,  some  of  the  long- 
snouted  Jurassic  type  of  crocodiles,  Teleosaurus,  continued  into 
the  Upper  Cretaceous. 


FlG.  300.  —  Tylosaurus  dyspelor  Marsh.  A  Mosasaurian  in  pursuit  of  Saurodont 
Fishes  (Portheus).  Restoration  by  C.  R.  Knight,  under  the  direction  of  Pro- 
fessor H.  F.  Osborn.  (Copyright  by  Amer.  Mus.  Nat.  Hist.,  N.Y.) 

The  Pterosaurs  of  the  Cretaceous  are  remarkable  for  their 
great  size,  far  exceeding  that  of  the  Jurassic  species.  The  closely 
allied  genera,  Ornithostoma  of  Europe  and  Pteranodon  of  Kansas, 
had  a  head  of  nearly  3  feet  in  length,  with  a  long,  pointed,  tooth- 
less bill,  like  that  of  a  bird;  the  spread  of  wings  exceeded  20  feet. 

The  Dinosaurs  continue  in  even  greater  profusion  than  in  the 
Jurassic;  they  are,  of  course,  much  commoner  and  better  pre- 
served in  continental  deposits  than  in  marine,  and  hence  are  best 


720 


THE   CRETACEOUS   PERIOD 


FIG.  301.  —  Skull  of   Triceratops  flabellatus  Marsh, 
from  the  side,  1/30.     (Marsh) 


known  from  the  base  and  the  summit  of  the  system.  Many  of 
the  genera  were  the  largest  land  animals  that  ever  lived,  and  the 
size  of  the  bones  is  astonishing.  The  Wealden  of  Europe  has 
yielded  some  magnificent  Dinosaurs;  especially  the  genus  I  guano- 
don,  of  which  many 
complete  skeletons 
have  been  found  in 
Belgium.  Dinosaurs 
are  much  less  com- 
mon in  the  marine 
Upper  Cretaceous, 
but  the  green  sands 
of  New  Jersey  have 
yielded  Hadrosaurus, 
a  herbivorous  Dino- 
saur much  like  Igua- 
nodon,  and  some  car- 
nivorous types  also.  The  Laramie  and  Denver  beds  have 
preserved  many  fine  specimens,  which  show  that  the  Dinosaurs 
flourished  in  almost  undiminished  variety  till  the  end  of  the  Cre- 
taceous. The  erect,  herbivorous  type  is  represented  in  these  beds 
by  Monodonius  and  Diclonius  (Fig.  302),  which  are  nearly  re- 
lated to  Hadrosau- 
rus. Triceratops  (Fig. 
301)  and  Torosaurus 
are  huge,  quadru- 
pedal reptiles,  with 
three  large  horns  on 
the  head  and  an  ex- 
traordinary frill-like 
extension  of  the  skull  over  the  neck.  Carnivorous  Dinosaurs  like- 
wise continued,  such  as  L&laps,  Tyrannosaurus  and  Ornithomi- 
mus,  the  latter  with  hind  limbs  which  are  especially  birdlike 
in  structure. 
The  Birds  of  the  Cretaceous  are  much  more  abundant  and 


FlG.  302.  —  Skull  of  Diclonius  mirabihs  Cope,  from 
above,  1/19.     (Cope) 


CRETACEOUS   LIFE  72 1 

advanced  than  the  known  Jurassic  birds.  In  the  Upper  Creta- 
ceous of  Kansas,  and  probably  of  England  also,  are  found  two 
remarkable  birds,  Hesperornis  and  Ichthyornis.  In-  the  former, 
which  was  nearly  6  feet  high,  the  wings  were  rudimentary,  while 
Ichthyornis,  a  much  smaller  bird,  had  powerful  wings.  Both  of 
these  genera  possessed  teeth,  like  Arch&optcryx,  but  except  in 
that  feature  and  in  certain  minor  details  of  structure,  they  are 
entirely  like  modern  birds.  Bird  bones  like  the  corresponding 
parts  of  the  Cormorants  and  Waders  have  been  found  in  the  green 
sands  of  New  Jersey,  but  it  is  not  known  whether  they  had  teeth. 

Mammalia. — Cretaceous  Mammals  are  more  numerous  and 
varied  than  those  of  the  Jurassic,  but  they  continue  to  play  a  very 
modest  role,  and  are  nearly  all  of  minute  size.  In  America  they 
have  been  found  only  in  the  uppermost  Cretaceous,  and  in  Europe 
they  are  not  known  as  yet,  though  doubtless  they  existed  in  that 
part  of  the  world.  The  mammals  of  the  Laramie  already  begin 
to  show  affinities  with  the  forms  which  are  to  succeed  them 
in  the  Tertiary.  The  Multituberculata  are  represented  by  two 
genera,  Meniscoessus  and  Ptilodus,  while  other  mammals  of 
doubtful  affinities  are  Didelphops,  Pediomys,  and  Cimolestes. 
Many  others  are  known,  but  they  are  too  imperfect  for  reference. 
With  one  exception,  Thlaodon,  which  is  of  moderate  size,  all  these 
mammals  are  exceedingly  small. 

In  brief,  Cretaceous  life  is  still*  typically  Mesozoic,  but  a  change 
toward  Cenozoic  conditions  is  already  manifest,  especially  in  the 
Plants,  the  Gastropods,  the  Teleostean  Fishes,  and  the  Birds. 
There  is  still  a  gap  between  the  life  systems  of  the  two  eras,  but  it 
is  not  so  wide  as  it  was  once  believed  to  be,  and  it  may  be  hoped 
that  future  discoveries  will  bridge  it  entirely. 


CHAPTER    XXXV 
CENOZOIC    ERA— TERTIARY    PERIOD 

THE  history  of  the  Cenozoic  era  brings  us  by  gradual  steps  to 
the  present  order  of  things.  Of  no  part  of  geological  history  have 
such  full  and  diversified  records  been  preserved  as  of  the  Ceno- 
zoic, and  yet  this  very  fulness  is  a  source  of  difficulty  and  embar- 
rassment when  we  attempt  to  arrange  the  various  phenomena  in 
their  chronological  order. 

The  sedimentary  rocks  of  the  Cenozoic  era  are,  for  the  most 
part,  quite  loose  and  uncompacted;  it  is  relatively  rare  to  find 
hard  rocks,  such  as  so  generally  characterize  the  older  formations. 
They  are  also  most  frequently  undisturbed,  retaining  nearly  their 
original  horizontal  positions,  except  when  they  have  been  upturned 
in  the  formation  of  great  mountain  chains.  Another  characteristic 
feature  of  Cenozoic  strata  is  their  locally  restricted  range;  only 
in  the  oldest  parts  of  the  group  do  we  find  such  widely  extended 
formations  as  are  common  in  the  Palaeozoic  and  Mesozoic  groups, 
and  the  later  Cenozoic  strata  become  more  and  more  local  in  their 
character.  This  implies  the  restriction  of  the  changes  of  level, 
the  great  transgressions  and  withdrawals  of  the  sea  no  longer  tak- 
ing place  as  they  had  in  the  preceding  eras.  On  the  other  hand, 
mountain  making  was  effected  on  a  very  grand  scale  in  the  Ceno- 
zoic, and  vuicanism  was  prevalent  to  an  extent  that  seems  never 
to  have  been  reached  before. 

•  The  climate  of  the  era  underwent  some  very  remarkable  and 
inexplicable  changes.  •  At  the  beginning  it  resembled  that  of  the 
Cretaceous  in  its  generally  mild  and  equable  character,  a  luxuriant 
vegetation  flourishing  far  within  the  Arctic  Circle;  but  by  very 

722 


THE  CENOZOIC   ERA  723 

slow  degrees  and  with  many  fluctuations,  the  climate  grew  colder, 
culminating  in  the  Glacial  Age,  when  much  of  the  land  in  the 
northern  hemisphere  was  covered  with  sheets  of  ice  and  snow 
and  reduced  to  the  condition  of  modern  Greenland. 

The  life  of  the  Cenozoic  era  is  very  clearly  demarcated  from 
that  of  the  Mesozoic,  though  many  modern  characteristics  began 
in  the  Cretaceous  or  even  earlier.  The  peculiar  Mesozoic  Am- 
monoids,  Belemnites,  and  many  curious  Bivalves  disappeared 
almost  entirely  at  the  end  of  the  Cretaceous,  leaving  only  a  few 
stragglers  here  and  there  to  persist  into  the  older  Tertiary.  Even 
more  striking  is  the  dwindling  of  the  Reptiles;  the  Ichthyosaurs, 
Plesiosaurs,  Mosasaurs,  Dinosaurs,  and  Pterosaurs,  which  had 
given  such  a  marked  individuality  to  the  Mesozoic  fauna,  have 
become  totally  extinct,  leaving  only  Lizards  and  Snakes,  Turtles 
and  Crocodiles,  and  a  few  Choristodera  to  represent  the  class.  But 
Cenozoic  life  is  not  distinguished  from  Mesozoic  merely  by  negative 
characters;  it  has  its  positive  features  as  well.  The  plants  and 
invertebrate  animals  nearly  all  belong  to  genera  which  are  still 
living,  and  the  proportion  of  modern  species  steadily  increases  as 
we  approximate  the  present  time.  The  Fishes,  Amphibia,  and 
Reptiles  differ  but  little  from  those  of  modern  times,  and  the  Birds 
take  on  the  diversity  and  relative  importance  which  characterize 
them  now.  Above  all,  the  Mammals  undergo  a  wonderful  ex- 
pansion and  take  the  place  of  the  vanished  reptiles,  giving  to  Ceno- 
zoic time  an  altogether  different  character  from  all  that  went  before 
it.  The  great  geographical  and  climatic  changes  produced  migra- 
tions of  land  animals  and  plants  upon  a  grand  scale,  from  continent 
to  continent  and  from  zone  to  zone,  the  result  of  which  is  the  dis- 
tribution of  living  beings  over  the  earth's  surface  as  we  find  it 
to-day. 

There  is  some  difference  of  usage  regarding  the  subdivisions 
of  the  Cenozoic  group,  though  the  difference  is  principally  with 
reference  to  the  rank  of  those  subdivisions.  We  shall  follow  the 
usual  American  practice  of  dividing  the  group  into  two  systems, 
the  Tertiary  and  Quaternary. 


724 


CENOZOIC   ERA— TERTIARY   PERIOD 


THE   TERTIARY   PERIOD 

The  names  Tertiary  and  Quaternary  are  remnants  of  an  old 
geological  nomenclature  which  has  lost  its  significance,  and  were 
proposed  when  the  whole  succession  of  strata  was  believed  to  be 
divisible  into  four  groups,  called  the  Primary,  Secondary,  Tertiary, 
and  Quaternary,  respectively.  When  it  was  learned  that  there  were 
groups  and  systems  much  older  than  the  so-called  Primary,  the 
name  Palaeozoic  was  substituted  for  Primary,  as  was  Mesozoic 
for  Secondary,  though  the  latter  term  is  still  used,  especially  in 
England.  The  name  Tertiary  has  thus  lost  its  meaning,  but  is 
nevertheless  retained  as  a  division  of  the  Cenozoic  group  or  era. 


TERTIARY  FORMATIONS  OF  THE  UNITED  STATES 


European 

Western  Interior 

Gulf  Border 

Pacific  Border 

Pliocene 

Sicilian 

Astian 
Plaisancian 
Messinian 

? 
Blanco 
Republican  River 

Florida 
or 
Caloosahatchie 

MERCED 

Miocene 

Tortonian 
Helvetian 
Langhian 

Loup  Fork 
Deep  River 
Arikaree 

Chesapeake 

MONTEREY  (Cal.) 
Empire  (Oregon) 

Oligocene 

Aquitanian 
Tongrian 
Ligurian 

John  Day 
White  River 
Uinta 

Chipola 
Chattahoochee 
Vicksburg 

Astoria  (Oregon) 
Kenai  (Alaska) 

Eocene 

Bartonian 
Lutetian 

Suessonian 

Bridger 
Wind  River 
(Green  River) 
Wasatch 

Jackson 
Claiborne 
Chickasaw 

TEJON 

Paleocene 

Thanetian 
Montian 

Torrejon  |  Fort 
Puerco      (  Union 

Midway 

? 

The  great  revolution  which  closed  the  Cretaceous  and  inaugu- 
rated the  Tertiary  has  left  its  effects  visible  in  all  the  continents, 
but  the  gap  between  the  two  periods  is  not  everywhere  the  same. 
This  revolution  gave  to  North  America  nearly  its  present  outlines, 
except  for  the  land  connections  with  Europe  and  Asia,  which  were 


THE  TERTIARY   PERIOD  725 


FIG.  303.  —  Map  of  North  America  in  the  Tertiary  period.  Black  areas  =  known 
exposures  of  marine  Tertiary ;  white  =  land ;  lined  areas  =  sea ;  dotted  areas 
=  continental  formations 


726  THE  TERTIARY   PERIOD 

from  time  to  time  interrupted  and  renewed.  In  consequence  of 
this,  marine  Tertiary  beds  occur  only  along  the  borders  of  the  con- 
tinent, while  the  Tertiary  of  the  interior  is  all  of  continental  origin. 
In  other  continents,  and  especially  in  Europe,  the  distribution  of 
land  and  sea  was  very  different  in  the  Tertiary  from  what  it  is 
now,  and  the  topography  of  the  land  was  profoundly  altered  in 
the  course  of  the  period.  Some  of  the  highest  mountain  ranges 
of  the  earth  were  upheaved  in  Tertiary  times,  such  as  the  Atlas, 
the  Alps,  the  Caucasus,  and  the  Himalayas,  and  many  ranges  of 
earlier  date  were  subjected  to  renewed  compression  and  upheaval. 
That  Tertiary  mountains  are  high- is  not  due  to  any  extreme  de- 
gree of  compression  as  compared  with  that  which  produced 
older  ranges,  but  merely  to  the  youth  of  the  former;  denudation 
has  not  yet  had  time  to  sweep  them  away. 

The  Tertiary  system  or  period  is  divisible  into  five  well-distin- 
guished series  or  epochs,  which  may  usually  be  identified  in  both 
the  marine  and  continental  formations;  but  for  lack  of  common 
fossils  .it  is  not  yet  possible  to  correlate  the  stages  and  substages 
of  the  interior  region  with  those  of  the  coast.  In  the  preceding 
table,  therefore,  no  exact  comparison  of  these  minor  subdivisions 
is  intended. 

The  name  Tertiary  was  given  by  Cuvier  and  Brongniart,  early 
in  the  last  century,  to  the  succession  of  marine,  brackish-water,  and 
fresh-water  beds  in  the  Paris  basin.  Sir  Charles  Lyell  many  years 
later  proposed  the  division  of  the  Tertiary  into  three  parts,  Eocene 
(from  the  Greek  eos,  the  dawn,  and  kainos,  recent),  Miocene 
(melon,  less,  and  kainos),  and  Pliocene  (pleion,  more,  and  kainos), 
a  scheme  which  is  still  used,  modified  by  Beyrich  through  the 
insertion  of  a  fourth  epoch,  the  Oligocene  (oligos,  little  or  in  small 
degree,  and  kainos).  Last  of  all,  the  lower  Eocene  has  been  sepa- 
rated under  the  name  Paleocene  (palaios,  ancient,  as  in  Palaeozoic) 
a  change  proposed  thirty  years  ago  by  the  botanist  Schimper,  but 
only  lately  coming  into  wider  favour.  It  has  become  customary 
to  (Jstinguish  between  the  older  and  newer  parts  of  the  Tertiary 
by  grouping  together  the  Eocene  and  Oligocene  into  the  Palceogene, 


THE    PALEOCENE    EPOCH  727 

and  the  Miocene  and  Pliocene  into  the  Neogene.  Eocene  and 
Neocene  are  employed  in  the  same  way,  but  this  is  objectionable 
because  it  is  using  Eocene  in  two  different  senses. 

THE  PALEOCENE  EPOCH 

The  term  Paleocene  has  not  been  used  by  American  geological 
writers,  who  have,  however,  frequently  employed  the  more  non- 
committal name  of  post-Cretaceous.  It  will  be  an  advantage  to 
follow  the  European  usage  wherever  this  can  be  done  to  express 
facts  of  correspondence  between  the  two  continents. 

American.  — Marine  formations  of  this  epoch  have  not  yet  been 
distinctly  identified  in  North  America,  though  in  the  table  the  Mid- 
way of  the  Gulf  region  has  been  provisionally  placed  in  that  series. 
On  the  other  hand,  extensive 'areas  in  the  western  interior  are  refer- 
able to  it.  In  the  region  of  the  Rocky  Mountains  and  northern 
plains,  the  Denver  and  Livingstone  beds  may  eventually  prove  to 
be  a  part  of  the  Paleocene  series,  a  correlation  which  is  favoured 
by  the  plants  which  they  contain.  They  also  contain,  remains 
of  Dinosaurs,  and  though  it  is  not  at  all  impossible  that  some  of 
these  great  reptiles  should  have  survived  in  the  earliest  Tertiary, 
they  are  not  yet  known  to  have  done  so.  The  oldest  known  beds 
which  are  definitely  assignable  to  the  Paleocene  are  those  of  the 
Fort  Union,  a  formation  with  a  maximum  thickness  of  2000  feet, 
which  covers  very  large  areas  in  Canada,  Montana,  North  Dakota, 
and  eastern  Wyoming,  and  is  composed  of  sandstones  and  clay 
rocks.  In  Montana  it  lies  conformably  on  the  Livingstone,  and 
in  Wyoming  there  is  an  apparently  unbroken  succession  from  the 
Laramie  into  the  Fort  Union,  barren  sandstones  between  the  two 
probably  representing  the  Livingstone-Denver  series.  Originally 
referred  to  the  Tertiary  on  account  of  its  plants,  the  position  of  the 
Fort  Union  has  been  confirmed  by  the  finding  of  a  considerable 
number  of  mammals  in  it.  The  conditions  under  which  these  beds 
were  formed  have  not  been  clearly  determined.  That  they  may 
have  been  partly  lacustrine  is  indicated  by  the  presence  of  fresh- 
water shells  in  some  localities.  Other  parts  are  probably  flood- 


728  THE  TERTIARY   PERIOD 

plain  deposits  and  others  again  entirely  subaerial.  Beds  with 
similar  plants  have  been  found  in  Greenland  and  Alaska. 

A  somewhat  different  fades  of  the  Paleocene  occurs  in  north- 
western New  Mexico  and  southwestern  Colorado  in  a  formation 
which  also  lies  in  apparent  conformity  upon  the  Laramie.  In  these 
beds,  which  are  about  800  feet  thick  and  mostly  barren  of  fossils, 
are  two  separate  horizons,  which  have  yielded  numerous  fossil 
mammals,  and  each  of  which  has  its  own  characteristic  fauna. 
The  lower  and  older  of  these  horizons  is  the  Puerco,  and  the  higher 
the  Torrejon,  the  Fort  Union  corresponding  to  both  together. 

Foreign.  —  In  northern  Patagonia  is  a  continental  formation, 
called,  from  one  of  its  most  characteristic  fossils,  the  Notostylops 
beds,  the  mammals  of  which  suggest  correlation  with  the  Puerco 
of  North  America. 

In  Europe  a  rapid  elevation  of  the  continent  had  occurred  in 
the  latest-  phases  of  the  Cretaceous,  followed  at  the  beginning  of 
the  Tertiary  by  numerous  minor  oscillations  of  level,  which  occa- 
sioned a  continual  struggle  between  land  and  sea  and,  as  a  result, 
"  the  Paleocene  does  not  consist  of  purely  marine  deposits,  but  of 
a  repeated  alternation  of  marine,  brackish,  and  fresh-water  for- 
mations." (Kayser.)  The  sands,  marls,  and  limestones  thus 
produced  cover  a  large  area  in  the  north  of  France,  Belgium,  and 
the  south  of  England.  The  upper  portion  of  the  series  near 
Rheims,  in  France,  contains  a  mammalian  fauna  so  like  that  of  the 
American  Torrejon  as  to  indicate  not  only  a  correlation  with  that 
horizon,  but  also  the  existence  of  a  land  bridge  between  the  two 
continents,  which  permitted  the  migration  of  terrestrial  animals 
from  one  to  the  other.  Paleocene  beds  occur  also  in  Denmark 
and  in  central  Russia  and  in  southern  Europe;  in  the  south  of 
France  and  the  Pyrenees,  it  is  represented  by  fresh-water  beds, 
while  in  Egypt  it  is  marine. 

Climate.  — The  earliest  Tertiary  floras  of  Greenland  and  Alaska 
show  that  the  equable  conditions  of  the  late  Cretaceous  continued, 
but  those  of  England  indicate  merely  a  temperate  climate  in  that 
latitude,  where  in  the  true  Eocene  it  became  tropical. 


THE  EOCENE  EPOCH  729 

Paleocene  Life 

The  vegetation  of  the  Paleocene  is  of  essentially  the  same  kind 
as  that  of  the  latest  Cretaceous,  the  difference  between  the  two 
being  principally  a  matter  of  species.  Among  the  marine  inver- 
tebrates, which  are  still  very  incompletely  known,  the  most  impor- 
tant feature  to  be  noted  is  a  negative  one ;  namely,  the  almost  uni- 
versal and  complete  absence  of  the  characteristic  Mesozoic  types, 
Ammonites,  Belemnites,  and  the  like.  Of  the  land  animals,  the 
more  or  less  aquatic  reptiles  allied  to  the  Rhynchocephalia,  the 
Choristodera,  survived,  but  all  the  other  Mesozoic  orders  of  rep- 
tiles vanished  in  what  appears  to  be  a  startlingly  sudden  way; 
no  trace  of  the  Dinosaurs,  Pterosaurs,  Ichthyosaurs,  Plesiosaurs, 
Mosasaurs,  etc.,  has  been  found  in  the  Paleocene.  On  the  con- 
trary, the  Mammals  have  greatly  increased  in  numbers  and  im- 
portance, but  are  still  extremely  primitive  and  very  like  those  of  the 
late  Cretaceous;  with  one,  or  perhaps  two,  exceptions,  they  all 
belong  to  extinct  orders  and  are  of  small  or  moderate  size,  not  a 
single  large  one  having  been  found.  The  close  relation  with  the 
Mesozoic  is  seen  in  the  presence  of  several  of  the  Multituberculata; 
the  last  and  largest  of  the  order,  Polymastodon  and  Ptilodus,  the 
latter  a  survival  from  the  Laramie,  are  the  common  genera.  The 
primitive  flesh-eaters,  Creodonta,  and  primitive  hoofed  animals, 
Condylarthra  and  Amblypoda,  are  abundant,  and  the  very  curious 
extinct  orders  of  the  Tillodonta  and  Tceniodonta,  and  primaeval 
Lemuroids,  and  perhaps  Insectivora,  complete  the  list.  The  as- 
semblage is  an  extremely  archaic  one  and  notable  for  the  absence 
of  the  characteristically  Tertiary  groups;  there  are  no  Monkeys, 
Rodents,  Bats,  true  Carnivores,  Artiodactyls,  Perissodactyls,  or 
any  of  the  other  higher  orders  of  mammals. 

THE  EOCENE  EPOCH 

American.  —  Along  the  Atlantic  and  Gulf  borders  the  coast-line 
of  the  Eocene  closely  follows  that  of  the  Cretaceous,  of  which  only 
a  narrow  strip  separates  the  Eocene  from  the  Triassic  and  crystal- 


730  THE  TERTIARY   PERIOD 

line  rocks  of  the  Piedmont  plain.  The  unconformity  between  the 
Cretaceous  and  Eocene  indicates  that  along  this  coast  the  latter 
period  had  been  inaugurated  by  an  encroachment  of  the  sea  upon 
the  land.  The  Mississippi  embayment  had  nearly  the  same  size 
and  form  as  before,  extending  up  to  the  mouth  of  the  Ohio.  Flor- 
ida was  entirely  submerged,  as  was  most  of  Central  America,  cutting 
off  the  northern  from  the  southern  continent.  On  the  Atlantic 
coast  the  Eocene  rocks  are  unconsolidated  sands  and  clays,  with 
some  glauconitic  greensand,  particularly  in  New  Jersey.  They 
form  a  narrow  belt  through  New  Jersey,  Maryland,  and  Virginia, 
widening  into  a  quite  broad  band  through  the  Carolinas  and  the 
Gulf  States,  and  extending  around  the  borders  of  the  Mississippi 
embayment  into  Texas.  In  the  Gulf  region  the  rocks  are  more 
consolidated,  and  are  quite  hard  limestones,  sandstones,  and  shales, 
with  extensive  deposits  of  lignite,  formed  in  ancient  peat  bogs 
which  followed  the  low-lying  Gulf  shores;  some  of  these  lignitic 
formations  may  be  more  properly  referable  to  the  Paleocene. 

On  the  Pacific  coast  a  long,  narrow  arm  of  the  sea  occupied  the 
great  valley  of  California,  extending  northward  into  Oregon  and 
Washington;  its  deposits  are  at  present  principally  displayed  along 
the  eastern  flank  of  the  Coast  Range.  These  deposits  form  a 
single  series,  the  Tejon,  which  lies  upon  the  Chico  in  apparent 
conformity;  but  the  lowest  Eocene  is  not  represented  in  the  Tejon, 
and  in  Oregon  an  unconformity  between  the  two  series  has  been 
detected.  The  presence  of  Atlantic  species  in  the  Eocene  of  Cali- 
fornia indicates  that  the  barrier  between  the  oceans  which  had 
existed  in  the  Cretaceous,  was  submerged  for  a  time. 

In  the  western  interior  the  Eocene  formations,  with  the  excep- 
tion of  a  few  small  areas  in  Colorado,  are  all  confined  to  the  plateau 
region  west  of  the  Rocky  Mountains.  Formerly,  they  were  con- 
sidered to  be  lacustrine  deposits,  but.  this  explanation  is  applic- 
able to  only  a  small  part  of  them;  they  are  much  more  generally 
due  to  the  subaerial  agents,  rivers,  rain,  and  the  wind,  in  the  work 
of  filling  basins  of  slow  depression.  In  some  instances,  at  least, 
the  material  of  these  beds  is  principally  fine  volcanic  ash  and  dust, 


THE   EOCENE   EPOCH  731 

so  extensively  distributed  as  to  prove  great  volcanic  activity  in 
the  region.  It  has  not  yet  been  determined  how  generally  the 
formations  are  built  up  of  these  volcanic  materials.  Whatever 
the  mode  of  their  accumulation,  the  Eocene  stages  are  remarkably 
complete  and  have  preserved  a  marvellous  record  of  the  successive 
faunas  of  the  epoch,  which  registers  not  only  the  changes  in  ani- 
rnal  life,  but  also  the  shifting  land  connections  with  the  other  con- 
tinents. 

The  oldest  and  most  widely  spread  of  these  Eocene  stages  is  the 
Wasatch,  the  principal  area  of  which  extends  from  New  Mexico, 
over  eastern  Utah  and  western  Colorado,  to  the  Uinta  Mountains^ 
around  the  eastern  end  of  which  it  passes  in  a  narrow  band,  ex- 
panding again  north  of  the  mountains  and  covering  southwestern 
Wyoming.  As  the  Wasatch  is  in  many  places  buried  under  newer 
rocks,  it  is  not  certain  that  the  beds  are  continuous  over  this  great 
area  of  450  miles  long  by  250  miles  wide  in  the  broadest  part,  but 
there  is  no  reason  to  suppose  that  they  were  not  originally  so. 
Another  great  body  of  Wasatch  strata  occupies  the  Big  Horn  Basin 
of  northwestern  Wyoming,  extending  into  southern  Montana,  and 
two  small  areas  in  southern  Colorado  are  referred  to  the  same  date. 
The  thickness  of  the  Wasatch  varies  from  1500  to  2500  feet. 

The  Wind  River,  the  second  of  the  Eocene  stages,  appears  to 
be  present  in  two  distinct  facies,  though,  as  the  two  contain  no 
common  fossils,  the  correlation  is  open  to  some  doubt.  The  beds 
of  the  typical  facies  occupy  the  Wind  River  Basin,  north  of  the 
mountains  of  that  name,  in  central  Wyoming,  and  contain  many 
remains  of  mammals,  and  also  occur  in  the  Huerfano  Canon, 
Colorado.  The  second  facies  (Green  River  Shales)  occupies  the 
same  position  stratigraphically  that  the  Wind  River  does  palaeon- 
tologically,  following  upon  the  Wasatch  and  overlaid  by  the  Bridger. 
This  facies  is  typically  displayed  in  southern  Wyoming  in  the  valley 
of  the  Green  River  and  is  a  thick  body  of  very  finely  laminated 
"  paper  shales,"  which  seem  to  have  been  deposited  in  a  very 
shallow  lake,  and  have  preserved  an  extraordinary  number  of 
plants,  insects,  and  fishes,  but  no  mammals  have  been  found, 


732  THE  TERTIARY  PERIOD 

except  in  the  form  of  footprints.  Occasional  crystals  of  gypsum 
indicate  that  the  water  became  salt,  at  least  locally,  while  the  pres- 
ence of  such  fishes  as  the  Rays  points  to  communication  with  the 
sea.  The  third  of  the  Eocene  stages  is  the  Bridger,  of  south- 
western Wyoming  and  northeastern  Utah,  which  lies  upon  the 
Green  River  Shales,  overlapping  these  east  and  west  and  extend- 
ing over  upon  the  Wasatch.  The  Bridger  beds  are  very  largely 
made  up  of.  volcanic  ash  and  dust  deposited  partly  on  land  and 
partly  in  shallow  or  periodical  lakes.'  The  remains  of  fishes, 
crocodiles,  and  non-marine  shells  scattered  through  the  beds  is 
a  proof  of  subaqueous  deposition,  and  the  large  selenite  crystals 
(see  p.  20),  which  frequently  occur,  indicate  that  the  water  was 
occasionally  strongly  saline. 

These  Eocene  continental  deposits  are  principally  sands  and 
clays,  with  occasional  banks  of  conglomerate.  They  are  more  or 
less  indurated,  but  usually  quite  soft  and  weather  rapidly,  giving 
rise  to  the  characteristic  bad-land  scenery  so  frequently  mentioned. 

While  the  mammals  of  the  Wasatch  age  are  so  largely  identical 
with  those  of  Europe  that  an  easy  way  of  land  migration  must  have 
been  open  between  the  continents,  the  similarity  becomes  less 
and  less  and  the  two  faunas  follow  such  divergent  lines  of  develop- 
ment through  the  Wind  River  and  Bridger  ages,  that  the  connec- 
tion between  America  and  Europe  must  evidently  have  been 
interrupted.  A  brief  connection  with  South  America  is  suggested 
by  the  very  unexpected  discovery  of  an  Armadillo  in  the  Bridger, 
though  it  is  possible  that  this  animal  was  a  survival  from  the  con- 
nection which  existed  between  North  and  South  America  in  late 
Cretaceous  or  Paleocene  times.  If  so,  the  ancestors  of  the  Bridger 
armadillo  yet  remain  to  be  discovered  in  the  Wind  River  and 
Wasatch. 

The  Eocene  epoch  was  brought  to  a  close  by  a  series  of  move- 
ments which  added  a  narrow  belt  of  land  to  the  Atlantic  and  Gulf 
coasts.  In  the  interior,  the  plateau  region  was  elevated  and  drained 
and  did  not  again  become  an  area  of  extensive  deposition.  A 
great  mountain-making  disturbance  followed  the  Bridger  age, 


THE  EOCENE  EPOCH  733 

elevating  the  mountain  ranges  to  which  the  post-Cretaceous  dis- 
turbances had  given  birth  and,  in  the  neighbourhood  of  these 
mountains,  tilting  and  upturning  the  Eocene  strata. 

Foreign.  — The  Old  World  Eocene  has  a  very  different  devel- 
opment from  that  of  North  America,  the  eastern  continents  not 
assuming  their  present  outlines  till  much  later.  At  the  close  of 
the  Cretaceous  period  extensive  geographical  changes  had  taken 
place  in  Europe,  consisting  chiefly  in  the  retreat  of  the  sea  from 
wide  areas  which  it  had  occupied  in  the  Cretaceous.  This  was 
especially  the  case  in  Russia,  northern  Germany,  and  France,  and 
southern  England,  and  in  place  of  the  great  gulf  which  had  occu- 
pied these  regions  (see  p.  712)  were  found  only  scattered  bodies 
of  fresh  and  brackish  water  in  which  the  Paleocene  deposits  were 
laid  down.  At  a  later  time  the  sea  again  advanced  over  part  of  these 
areas,  which  explains  the  general  unconformity  between  the  Cre- 
taceous and  Tertiary  strata.  In  southern  Europe  the  Mediter- 
ranean regained  the  great  expansion  which  it  had  partly  lost  in  the 
latter  part  of  the  Cretaceous,  extending  far  over  northern  Africa, 
where  nearly  the  whole  continent  north  of  the  equator  was  sub- 
merged in  the  early  Eocene  sea,  and  transgressing  over  southern 
Europe.  A  long,  narrow  arm  of  this  sea  extended  from  southern 
France,  past  the  north  side  of  the  future  Alps  and  Carpathians, 
into  western  Asia.  Another  narrow  sea,  or  strait,  extended  down 
the  east  side  of  the  Ural  Mountains,  from  the  Arctic  Ocean  to  the 
expanded  Mediterranean,  completely  cutting  off  Europe  from  Asia. 
This  complete  severance  of  Europe  from  Asia  necessitates  an  in- 
dependent land  connection  of  the  former  with  North  America  to 
explain  the  community  of  terrestrial  animals  and  plants  between 
these  continents.  From  Asia  Minor  the  Mediterranean  extended 
across  Persia  and  Turkistan,  northern  India,  Borneo,  and  Java, 
to  the  Pacific,  separating  the  southern  peninsulas  from  the  Asiatic 
mainland.  There  was  thus  a  continuous  sea  around  the  earth, 
everywhere  separating  the  southern  continents  from  the  northern, 
though  transient  connections  between  them  may  have  been  es^ 
tablished. 


734  THE  TERTIARY    PERIOD 

In  the  Alpine  and  north  African  regions  were  accumulated  thick 
masses  of  limestone,  largely  composed  of  the  gigantic  foraminiferal 
shells  called  Nummulites,  a  hard  massive  limestone  which  reaches 
a  thickness  of  several  thousands  of  feet.  Closely  associated  with 
the  Nummulitic  facies  of  the  Eocene  is  the  Flysch,  an  extremely 
thick  mass  of  sandstones  and  shales,  which  occurs  in  the  Alps 
and  Apennines,  the  Carpathians  and  Balkan  Peninsula,  the 
Caucasus,  Asia  Minor,  and  southern  Asia  generally.  In  the  Alpine 
region  the  Flysch  contains  enormous  erratic  blocks  of  granite, 
gneiss,  etc.,  which  appear  to  have  come  from  southern  Bohemia, 
and  which  have  been  interpreted  as  due  to  transportation  by 
glaciers.  This  interpretation  has  not,  however,  been  established. 
In  northern  Europe  no  such  widely  spread  formations  occur. 
After  the  Eocene  had  continued  for  some  time,  a  marine  basin, 
the  Anglo-Gallic,  was  formed  over  southern  England,  northern 
France,  and  Belgium,  which  contains  a  succession  of  alternating 
marine,  brackish,  and  fresh-water  strata.  This  basin  is  classic 
ground,  for  in  it  were  made  the  studies  of  Cuvier  and  Brongniart, 
which  led  to  the  recognition  of  the  Tertiary  as  a  distinct  system 
and  founded  the  science  of  Palaeontology. 

On  the  west  coast  of  Africa  the  sea  encroached  in  a'  narrow 
belt.  The  correlations  of  the  early  Tertiary  rocks  of  Australia 
and  New  Zealand  are  still  the  subject  of  debate,  but  there  seems 
to  be  little  doubt  that  the  Eocene  is  present.  In  South  America 
the  Eocene  of  Patagonia  consists  of  a  series  of  continental  deposits 
containing  a  highly  interesting  succession  of  mammals.  These 
grow  more  and  more  divergent  from  the  mammals  of  the  north- 
ern continents. 

The  Eocene  thus  had  broad  seas  where  now  is  land,  and  con- 
;inents  now  connected  were  then  separated  by  straits  and  sounds. 
On  the  other  hand,  there  were  then  land  bridges  joining  land 
areas  which  are  now  far  apart.  Some  of  these  land  bridges  may 
be  reconstructed  with  much  confidence,  while  others  are  more  or 
less  probable.  America  was  connected  with  Asia  across  what  is 
now  Bering's  Sea,  and  also  with  Europe,  probably  by  an  extension 


EOCENE   LIFE  735 

of  Greenland  and  Iceland.  The  Antarctic  continent  apparently 
had  a  much  greater  extension  than  it  has  now,  and  seems  to  have 
been  joined  with  both  Australia  and  South  America.  It  is  quite 
possible  that  Africa  was  more  or  less  directly  connected  with  the 
same  land  mass.  If  this  be  true,  then  in  Eocene  times  the  north- 
ern continents,  Europe  and  Asia,  were  joined  in  the  Arctic  latitudes 
by  way  of  North  America,  while  South  America,  Africa,  and  Aus- 
tralia radiated  in  three  great  lines  from  the  South  Pole.  Between 
the  two  series  of  continents,  northern  and  southern,  swept  the 
transverse  seas,  of  which  the  Mediterranean  and  Caribbean  are 
remnants. 

Climate.  — The  Eocene  climate,  especially  as  inferred  from  the 
plants,  was  warmer  than  that  of  the  Paleocene.  In  England,  for 
example,  the  temperate  flora  of  the  latter  epoch  was  followed  by 
one  of  subtropical  character,  and  in  North  America  the  subtropi- 
cal zone  extended  much  farther  north  than  in  recent  times. 

Eocene  Life 

Except  for  the  Vertebrates,  Eocene  life  is  chiefly  instructive 
from  the  manner  of  its  distribution  over  the  globe.  Invertebrates 
and  plants  are  nearly  the  same  as  modern  forms,  the  genera,  for 
the  most  part,  still  existing,  though  the  species  are  nearly  all 
extinct. 

Plants.  — The  Eocene  flora  of  North  America,  which  is  very  rich 
and  varied,  is  found  preserved  in  widely  separated  localities,  — 
Canada,  Montana,  Wyoming,  and  Idaho.  Besides  Ferns  and 
Horsetails,  this  flora  includes  some  Grasses,  Bananas,  and  many 
noble  Palms  (Fig.  304),  Myrtles,  Beeches,  Oaks,  Willows  (XVI,  i), 
Poplars,  Elms,  Sycamores,  Laurels,  Magnolias,  Maples,  Walnuts, 
Pines,  Spruces,  Arbor  Vitae,  and  the  like.  Even  in  Greenland  and 
Alaska  was  a  luxuriant  growth  of  forests  of  a  temperate  character, 
such  as  could  not  exist  there  now. 

The  European  flora  has  a  more  decidedly  tropical  character 
than  that  of  North  America,  and  contains  plants  whose  nearest 


736 


THE  TERTIARY   PERIOD 


living  allies  are  now  widely  scattered,  occurring  in  the  warmer 
parts     of     America,    Africa,    Asia,    and    Australia.    Cypresses, 

Yews,  and  Pines 
are  abundant,  in- 
cluding the  Se- 
quoia, now  con- 
fined to  California, 
and  the  Gingko 
of  China  and 
Japan.  Aloes, 
Palms,  and  Screw- 
pines  occur, 
mingled  with  the 
ordinary  temper- 
ate forest  trees, 
Elms,  Poplars, 
Willows,  Oaks,  etc. 
The  distribution 
of  plants  in  the 
Eocene  was  thus  very  different  from  what  it  is  at  present. 

Animals.  —  Foraminifera  of  relatively  enormous  size  abounded, 
and  their  shells  make  up  great  rock  masses.  Orbitolites  is  a  con- 
spicuous genus  along  our  Gulf  coasts,  Nummulites  in  the  Old 
World.  Corals  are  completely  modern  in  character.  The  Sea- 
urchins  and  especially  the  Irregulares  are  much  the  most  important 
representatives  of  the  Echinoderms.  Of  the  Mollusca  both  Bivalves 
(PL  XVI,  Figs.  2,  3)  and  Gastropods  (XVI,  4,  5)  increase  greatly 
and  are  very  rich  in  species.  Nautiloid  Cephalopods  are  more 
varied  and  widely  distributed  than  now  (XVI,  8),  and  in  a  few  lo- 
calities, particularly  in  India,  Ammonites  and  Belemnites  have  been 
found,  but  these  are  mere  belated  stragglers  from  the  Cretaceous 
and  are  much  too  rare  to  be  at  all  characteristic.  Among  the 
Crustacea  should  be  noted  the  increase  of  the  Crabs,  which  are 
more  numerous  and  varied  than  in  the  Cretaceous. 
The  Fishes,  both  fresh-water  and  marine,  differ  only  in  minor 


FlG.  304.  —  Flabellaria  sp.,  x  1/12.     Green  River  Shales 


PLATE  XVI.  — AMERICAN  TERTIARY  FOSSILS 

Fig.  i,  Oslrca  virginiana,  x  %,  Miocene.  2,  Pecten  madisonicus,  X  %,  Miocene.  3,  Car- 
<iiVa  perantiqua,  Eocene.  Whitfield.  4,  Volutolithes  sayana,  X  %>  Eocene.  5,  Oliva  caroli- 
nensis,  x  %,  Miocene.  6,  Helix  dalli,  Miocene.  White.  7,  Planorbis  convoluta,  ?  Fort 
Union.  Meek.  8,  Aturia  vanuxemi,  x  l/^,  Eocene.  9,  Glyptostrobus  ungeri,  x  %,  F-ocene. 
Lesquereux.  10,  Salix  sp.,  x  %.  Miocene.  (Figs.  1-5,  and  8,  after  Whitfield.) 


738  THE  TERTIARY   PERIOD 

details  from  modern  fishes.  The  Reptiles  are  likewise  essentially 
modern  in  character,  the  Choristodera  having  died  out  with  the 
Paleocene,  and  only  two  groups,  the  Lizards  and  Snakes,  are  more 
numerous  than  they  had  been  in  Mesozoic  times,  though  the 
venomous  snakes  had  not  yet  appeared.  The  Eocene  beds  of  the 
West  contained  multitudes  of  large  Crocodiles  and  a  great  variety 
of  Turtles. 

Eocene  Birds  are  very  much  more  numerous,  advanced,  and 
diversified  than  those  of  the  Cretaceous;  one  characteristic  feature 
of  the  times  was  the  presence  in  Europe  and  America  of  extremely 
large,  flightless  birds,  more  or  less  like  the  ostriches  in  appearance. 
Of  flying  birds  there  were  many  kinds;  Owls,  Eagles,  Buzzards, 
Vultures,  Gulls,  Waders,  Woodcock,  Quail,  Ibis,  and  Pelicans  are 
represented  by  ancestral  forms,  somewhat  different  from  their 
modern  descendants. 

The  Mammals  have  developed  in  a  marvellous  way  since  the 
Cretaceous,  assuming  in  terrestrial  life  that  dominant  place  which 
they  have  ever  since  held.  Compared  with  the  evolution  of  other 
animal  groups,  that  of  the  mammals  has  been  so  rapid  that  each 
stage  of  the  Eocene  has  its  own  mammalian  fauna,  differing  from 
those  of  the  preceding  and  succeeding  stages.  Besides  these 
geological  differences  between  the  successive  mammalian  assem- 
blages, there  are  often  marked  geographical  differences  between 
the  faunas  which  are  of  approximately  contemporaneous  age, 
but  widely  separated  in  space.  Comparing  Europe  and  North 
America  in  this  respect,  we  find  that  in  the  Eocene  each  con- 
tinent had  its  own  peculiarities,  but  that  the  land  connection  be- 
tween them  allowed  intermigration  and  thus  kept  up  a  close 
general  similarity  in  their  mammals  in  the  Lower  Eocene,  but  this 
connection  was  interrupted  and  the  faunas  of  the  middle  and  later 
portions  of  the  epoch  diverge  more  and  more  in  the  two  continents. 
The  southern  continents,  on  the  other  hand,  had  altogether  different 
mammalian  faunas,  due  to  their  long  separation  from  the  northern 
lands. 

The  change   from  the  Paleocene  mammals  to   those   of   the 


EOCENE  LIFE  739 

WASATCH  was  very  abrupt,  though  no  great  time  interval  is  involved, 
and  in  Europe  the  change  was  equally  sudden,  most  of  the  archaic 
Mesozoic  types  going  out  and  those  of  more  modern  character 
replacing  them.  Evidently  this  was  due  to  an  influx  of  mammals 
from  some  region  still  unknown,  and  hardly  at  all  to  a  develop- 
ment of  the  Paleocene  mammals.  Rodents  come  in  for  the  first 
time  in  North  America.  Perissodactyls  make  their  first  appearance 
with  ancestral  members  of  the  Horse  family  (Hyracotherium), 
the  tapirs  (Systemodori),  and  other  families  now  extinct.  The 
Wasatch  horse  was  a  curious  little  creature,  not  larger  than  a 
domestic  cat,  with  four  toes  on  the  fore  foot  and  three  on  the  hind, 
instead  of  having  only  a  single  functional  toe,  like  the  modern 
horses.  The  curious  extinct  group  of  hoofed  animals  called  the 
Amblypoda  greatly  increases  in  numbers  and  in  stature,  and  both 
in  Europe  and  America  the  predominant  genus  is  Coryphodon, 
Artiodactyls  also  appear  for  the  first  time  in  ancestral  members 
of  the  Pigs  (Eohyus),  and  the  Ruminants  (Trigonolestes) .  The 
Creodonts  increase  in  numbers  and  in  the  size  and  strength  of  the 
individuals,  Pachyczna  being  as  large  as  a  bear.  Numerous 
Lemuroids  and  primitive  types  of  Monkeys  (Anaptomorphus) 
swarmed  in  the  trees.  The  correspondence  between  the  mammals 
of  Europe  and  North  America  was  never  closer  than  in  Wasatch 
times. 

The  WIND  RIVER  fauna  is  a  development  of  the  Wasatch,  ap- 
parently without  the  admixture  of  foreign  elements  by  immigra- 
tion, and  there  is  no  such  complete  change  as  at  the  end  of  the 
Paleocene.  Noteworthy  is  the  appearance  of  the  first  known 
member  of  the  Perissodactyl  family  of  the  Titanotheres,  a  family 
which  was  destined  to  great  expansion  in  the  Upper  Eocene  and 
Lower  Oligocene ;  also  of  the  earliest  true  Carnivora. 

The  BRIDGER  mammals  represent  a  steady  advance  upon  those 
of  the  Wind  River.  The  Perissodactyls  may  be  said  to  culminate 
in  the  Bridger;  for  though  they  afterwards  reached  much  higher 
stages  of  development,  they  never  again  had  the  same  relative  im- 
portance. Horses,  still  of  minute  size,  but  more  highly  developed 


THE  OLIGOCENE   EPOCH  74! 

than  those  of  the  Wasatch,  Tapirs,  Rhinoceroses,  and  Titan- 
otheres  (Pal&osyops)  are  extraordinarily  abundant.  Corypho- 
don  has  vanished,  but  the  wonderful  elephantine,  six-horned 
Uintatherium  and  Eobasileus  (Fig.  305)  take  its  place  in  North 
America,  though  not  in  Europe.  Artiodactyls,  Creodonts,  Rodents, 
Tillodonts,  and  Lemurs  were  more  diversified  than  ever,  and 
Bats  are  found  here  for  the  first  time.  The  remarkable  discov- 
ery in  the  Bridger  of  such  a  distinctively  South  American  type  as 
an  armadillo  has  already  been  mentioned. 

In  the  Upper  Eocene  seas  great  whale-like  forms  (Zeuglodon) 
of  extraordinary  appearance  and  structure  had  grown  abundant. 

The  recently  discovered  Middle  and  Upper  Eocene  fauna  of 
Egypt  is  of  very  great  interest.  The  mammals  differ  much  from 
those  of  Europe,  but  there  are  some  forms  common  to  both  regions. 
The  long-sought  ancestors  of  the  Elephants  have  been  found  in  the 
Egyptian  beds,  and  a  very  curious  animal  (Arsinoetherium),  which 
might  be  described  as  a  small  elephant  with  a  pair  of  huge  horns 
upon  its  narrow  head,  accompanies  them. 

Volcanic  eruptions  continued  in  the  Rocky  Mountain  region 
during  the  Eocene.  The  Yellowstone  Park  was  a  centre  of  great 
activity,  with  numerous  cones  ejecting  acid  lavas  and  tuffs. 

THE  OLIGOCENE  EPOCH 

American.  —  The  marine  Oligocene  is  better  developed  and 
better  understood  on  the  Gulf  coast  than  elsewhere,  and  there- 
fore forms  the  standard  of  comparison.  "  It  was  a  period  of 
profuse  invertebrate  life  and  steady  sedimentation,  especially  of 
oceanic  deposits  in  water  of  not  always  great  depth.  Some  2000 
feet  of  strata,  formed  almost  wholly  of  organic  debris,  were  de- 
posited in  the  peninsular  region  of  Florida."  (Ball.)  In  the  Gulf 
region  there  is  no  decided  stratigraphic  break  between  the  Eocene 
and  Oligocene,  but  a  change  in  the  marine  fauna.  The  Oligocene 
is  but  scantily  shown  on  the  Atlantic  coast;  some  beds  in  the 
Carolinas  are  referred  to  it  and  traces  of  it  occur  in  New  Jersey, 


742  THE  TERTIARY   PERIOD 

but  it  is  generally  concealed  beneath  the  overlying  Miocene.  In 
the  western  Gulf  region  certain  fluviatile  beds  are  placed  in  this 
epoch,  but  upon  insufficient  reasons.  Oligocene  limestones  are 
found  in  the  Greater  Antilles  and  very  extensively  in  Central 
America,  which  seems  to  have  been  nearly  or  quite  submerged. 
At  the  end  of  the  lower  division  of  the  series  there  was  some 
disturbance,  raising  northern  Florida  into  an  island,  and  shoaling 
the  water  where  deposition  continued.  The  marine  fauna  of  the 
Oligocene  is  an  assemblage  of  warm-water  animds,  very  much 
like  those  which  now  live  on  the  coasts  of  the  West  Indies  and 
Central  America,  some  of  the  West  Indian  forms  extending  as  far 
north  as  New  Jersey. 

On  the  Pacific  coast  the  Oligocene  is  found  in  western  Oregon 
and  British  Columbia  and  very  extensively  in  Alaska,  where  the 
Kenai  formation,  10,000  feet  thick  and  containing  beds  of  lignite, 
is  exposed  along  the  coast  and  at  many  places  in  the  interior  down 
to  British  Columbia.  The  overlying  Miocene  follows  in  apparent 
conformity. 

In  the  interior,  Oligocene  formations  are  among  the  most  impor- 
tant of  all  the  continental  Tertiaries.  The  lower  division,  the 
Uinta,  is  found  in  a  relatively  small  area  of  northeastern  Utah 
and  northwestern  Colorado,  where  it  lies  unconformably  upon  the 
Bridger,  overlapping  the  latter  upon  the  southern  flanks  of  the 
Uinta  Mountains.  The  Uinta,  which  is  the  last  of  the  Tertiary 
horizons  in  the  plateau  region,  is  usually  regarded  as  uppermost 
Eocene,  but  its  fauna  allies  it  more  closely  with  the  Oligocene. 
Its  mammals  show  that  the  isolation  from  Europe,  which  had 
begun  after  the  Wasatch,  still  continued. 

The  Middle  Oligocene,  or  White  River,  covers  a  vast  area, 
northeastern  Colorado,  western  Nebraska,  eastern  Wyoming, 
and  southwestern  South  Dakota,  with  outliers  in  the  Black  Hills 
and  North  Dakota,  and  a  separate  area  in  the  Northwest  Territory 
of  Canada.  The  mode  of  formation  of  the  White  River  beds  has  long 
been  a  subject  of  discussion;  originally  they  were  considered  to  be 
lacustrine,  a  view  which  is  supported  by  their  very  regular  strati- 


THE  OLIGOCENE   EPOCH  743 

fication,  but  it  is  now  very  generally  believed  that  they  are  chiefly 
fluviatile,  and  several  of  the  old  stream-channels,  filled  with  cross- 
bedded  sandstone  and  banks  of  conglomerate,  have  been  observed 
(see  Fig.  36,  p.  no).  The  fine  clays  which  make  up  most  of  the 
beds  are  chiefly  flood-plain  deposits,  but  there  are  also  beds  of  pure 
white  volcanic  ash,  showing  that  the  volcanic  activity  which  was 
so  marked  in  the  Eocene  still  continued.  A  system  of  streams 
meandering  over  a  nearly  base-levelled  plain,  with  very  low 
divides  between  them,  would  in  times  of  flood  unite  into  a  vast, 
but  shallow  and  temporary  lake,  and  such  would  appear  to  have 
been  the  conditions  under  which  the  White  River  beds  were  laid 
down. 

The  mammals  of  the  White  River  prove  that  a  Way  of  intermigra- 
tion  for  terrestrial  animals  had  again  been  established  with  Europe. 
While  many  families  did  not  join  in  this  migration  and  each  con- 
tinent had  several  groups  peculiar  to  itself,  the  number  of  identical 
and  closely  allied  genera  common  to  both,  and  the  appearance  in 
America  of  types  which  Europe  had  had  in  earlier  times,  is  suffi- 
cient proof  of  the  renewed  connection.  The  contrast  between  the 
Uinta  and  White  River  in  this  respect  is  very  marked. 

The  Upper  part  of  the  interior  Oligocene  is  the  John  Day,  which 
covers  a  large  part  of  eastern  Oregon,  and  a  small  area  in  central 
Montana.  The  Oregon  beds  are  a  very  thick  mass  (3000-4000 
feet)  of  stratified  volcanic  ash  and  tuff,  with  some  fresh-water  beds 
at  the  top.  Evidently,  gigantic  eruptions  were  in  progress  and  the 
vents  were  at  no  great  distance,  though  too  far  away  for  the  forma- 
tion of  a  coarse  agglomerate. 

Foreign.  —  During  the  Eocene  nearly  all  Germany  had  been 
land,  but  in  the  Oligocene  it  was  invaded  by  the  sea,  which  broke 
in  from  the  north  and  covered  all  the  northern  plain,  extending 
into  Belgium,  and  sending  long  bays  to  the  south.  One  of  these 
reached  to  the  strait  on  the  north  of  the  Alps,  expanding  into 
a  large  basin  near  Mayence  and  Frankfort.  Over  Germany  the 
sea  was  shallow,  permitting  the  formation  of  extensive  peat-bogs, 
where  were  accumulated  masses  of  lignite  or  brown  coal.  The 


744  THE  TERTIARY   PERIOD 

Oligocene  is  very  extensively  displayed  in  southern  Russia, 
marine  below  and  lignitic  above.  In  the  basin  of  Paris  the  sea 
had  a  greater  extent  than  in  Eocene  times,  though  with  lacustrine 
beds  intercalated.  The  Lower  Oligocene  of  the  Parisian  area  con- 
tains thick  bodies  of  gypsum,  which  were  formed  in  very  strongly 
saline  lagoons.  In  England  the  beds  are  more  of  brackish-  and 
fresh-water  origin.  In  southern  Europe  the  sea  retreated  from 
wide  areas,  and  in  its  place  were  extensive  bodies  of  fresh  and 
brackish  water,  in  many  of  which  peat-bogs  accumulated  masses 
of  lignite.  Such  lignitic  deposits  occur  at  intervals  in  the  south 
of  France,  Switzerland,  and  Bavaria.  In  the  Alps,  Apennines, 
Carpathians,  Caucasus,  Asia  Minor,  and  southern  Asia,  the  Oligo- 
cene is  represented  by  the  upper  part  of  the  Flysch,  the  formation 
of  which  began  in  the  Eocene. 

The  Oligocene  is  found  in  north  Africa,  but  in  the  other  con- 
tinents, beside  those  enumerated,  it  has  not  yet  been  separated 
from  the  Eocene  below  or  the  Miocene  above. 


Oligocene  Life 

The  marine  invertebrates  so  resemble  those  of  the  Eocene  that 
any  general  statement  of  the  differences  is  difficult;  these  differ- 
ences are,  for  the  most  part,  of  species  only. 

The  UINTA  contains  large  and  numerous  crocodiles,  their  last 
appearance  in  the  northern  interior,  and  a  highly  interesting 
mammalian  fauna,  which,  however,  is  only  partially  known  and 
demands  further  exploration.  The  great  Uintatheres,  which 
dominated  the  Upper  Eocene,  have  disappeared,  and  the  Peris- 
sodactyls  have  begun  to  decline  in  relative  importance,  though  not 
absolutely;  the  small  three-toed  Horses  continue  to  develop  steadily; 
Rhinoceroses  and  Tapirs  are  abundant,  and  the  Titanotheres  in- 
crease notably  in  stature  and  in  the  prominence  of  their  horns. 
The  most  characteristic  feature  of  the  Uinta  fauna,  however,  is 
the  great  increase  in  the  Artiodactyls,  which  then  began  to  assume 
the  place  they  have  ever  since  held  as  the  most  numerous  and 


OLIGOCENE   LIFE  745 

important  of  the  hoofed  animals.  In  the  Uinta  the  Artiodactyls 
mostly  belong  to  a  great  and  typically  American  group  (Tylopoda), 
of  which  the  camel  and  llama  are  among  the  few  modern  survivors. 
The  most  primitive  known  ancestor  of  the  camels  and  llamas  is 
found  in  these  beds  (Protylopus)  associated  with  a  curious  extinct 
family,  the  Oreodonts,  which  were  extremely  abundant  and  varied 
throughout  the  American  Oligocene  and  Miocene,  and  with  other 
families  of  small,  graceful  animals,  which  throve  also  in  the  White 
River  and  John  Day.  This  large  assemblage  of  the  Artiodactyls 
distinguishes  the  Uinta  fauna  very  sharply  from  that  of  the  Bridger. 

The  Creodonts  are  still  common,  though  distinctly  less  so  than, 
they  had  been  in  the  Eocene,  and  the  true  Carnivora  are  beginning 
to  replace  them. 

In  the  WHITE  RIVER,  or  Middle  Oligocene,  the  Crocodiles  have 
become  extremely  rare,  and  only  a  dwarf  species  is  known,  but 
Lizards  are  much  more  numerous.  The  Mammals,  which  are 
preserved  in  astonishing  numbers,  resembled  those  of  the  Uinta, 
but  had  made  great  progress  since  that  time.  The  Creodonts  had 
almost  disappeared,  leaving  only  one  or  two  curious  genera 
(e.g.  Hycenodon),  while  the  Carnivora  became  abundant,  Dogs, 
Sabre-tooth  Cats,  Weasels,  and  primitive  Raccoons,  being  rep- 
resented. The  Lemurs  and  Monkeys  have  vanished  from  North 
America.  The  Perissodactyls  continue  to  be  abundant;  the 
Horses  are  represented  by  the  little  three-toed  Mesohippus, 
about  as  large  as  a  sheep,  the  Tapirs  by  Protapirus,  and  Rhi- 
noceroses by  three  very  distinct  series:  thus,  Metamynodon  was  a 
heavy,  short-legged,  aquatic  animal,  not  unlike  a  hippopotamus 
in  appearance;  C&nopus  a  more  slender,  terrestrial  animal  with 
the  proportions  of  a  tapir,  and  Hyracodon  was  a  long-necked, 
long-limbed,  lightly  built,  running  type,  yet  still  a  rhinoceros. 
The  Titanotheres  culminate  in  the  massive,  elephantine  Titano- 
therium  and  its  allied  genera,  which  developed  huge  nasal  horns 
(see  Fig.  306)  and  died  out  early  in  the  White  River. 

The  Artiodactyls  continue  to  increase;  the  native  stock  which 
came  over  from  the  Uinta  age  shows  a  distinct  advance  in  develop- 


OLIGOCENE  LIFE  747 

ment;  the  Camels  (Poebrotherium)  and  allied  families  are  very 
common,  among  them  Protoceras,  a  very  curious  animal,  the  male 
of  which  had  four  horns,  and  a  pair  of  tusks  in  the  upper  jaw, 
while  the  Oreodonts  must  have  covered  the  plains  in  great  herds, 
so  abundant  are  their  remains.  The  Peccaries,  or  American  rep- 
resentatives of  the  pigs,  are  not  yet  known  from  the  Uinta,  but 
occur  in  the  White  River  (Perchcerus),  and  the  extraordinary, 
long-limbed,  two-toed,  pig-like  Elotherium  may  have  descended 
from  Uinta  ancestors,  or  may  have  been  a  migrant  from  the  Old 
World,  as  certainly  were  the  members  of  the  European  family  of 
Anthracotheres  (Anthracotherium  and  Hyopotamus)  which  appear 
in  the  White  River  beds;  there  is  nothing  like  them  in  the  Uinta. 
The  Rodents  of  the  White  River  are  much  more  numerous  and 
varied  than  they  had  been  before;  Marmots,  Squirrels,  Beavers, 
Mice,  Pocket-gophers,  and  Rabbits  were  already  well  established. 

The  Mammals  of  the  JOHN  DAY  are  much  like  those  of  the  White 
River,  but  are  more  advanced  and  modernized,  and  some  ancient 
groups  have  vanished,  among  them  the  Creodonts,  the  aquatic  and 
cursorial  Rhinoceroses,  the  immigrant  Anthracotheres,  and  the 
huge  Titanotheres.  On  the  other  hand,  the  Carnivora,  especially 
the  Dogs  and  Sabre-tooth  Cats,  greatly  increase  in  numbers  and 
diversity,  and  the  same  is  true  of  the  Rodents.  The  Horses  and 
true  Camels  are  larger  than  those  of  the  White  River,  as  are  also 
the  Oreodonts,  but  the  Rhinoceroses  are  reduced  to  the  two-horned 
Diceratherium. 

The  Oligocene  Mammals  of  Europe  have  much  in  common  with 
those  of  North  America,  but  there  are  many  local  differences.  In 
Europe,  the  Weasels  were  much  more  varied  and  common  than 
in  America,  and  the  Civet-cats,  a  family  which  never  reached  this 
continent  at  all,  were  well  represented.  Some  families  of  Peris- 
sodactyls,  such  as  the  Palaeotheres,  and  a  host  of  Artiodactyl 
Tamilies,  some  extinct,  like  the  Anoplotheres,  Cainotheres,  and 
Xiphodonts,  and  the  true  Ruminants,  were  then  peculiar  to  the 
Old  World. 

Climate.  — The  disappearance  of  the  crocodiles  from  the  north- 


748  THE  TERTIARY  PERIOD 

ern  interior  seems  to  show  that  the  climate  had  grown  rather  cooler, 
though  on  the  Atlantic  coast  warm-water  conditions  still  continued, 
and  the  vegetation  shows  that  Europe  still  had  a  subtropical 
climate,  palms  growing  up  to  the  north  of  Germany.  The  Kenai 
beds  of  Alaska  contain  a  temperate  vegetation,  and  probably 
the  leaf-bearing  beds  which  are  distributed  so  generally  around 
the  Arctic  Sea  and  have  yielded  similar  plants,  should  be  referred 
to  the  Oligocene,  though  they  are  usually  called  Miocene. 

THE  MIOCENE  EPOCH 

American.  — The  marine  Miocene  rocks,  which  have  an  enor- 
mous development  on  the  Pacific  coast,  are  rather  scantily  dis- 
played along  the  Atlantic  and  Gulf  borders.  The  eastern  coast, 
which  had  emerged  during  the  Oligocene,  was  slightly  depressed, 
and  the  Miocene  beds  were  deposited  unconformably  upon  the 
Eocene,  in  some  places  overlapping  the  latter  landward,  and  it 
may  be  that  the  narrow  belt  of  coastal  Eocene  has  all  been  exposed 
by  the  denudation  of  the  overlying  Miocene.  In  any  event,  the 
Miocene  coast-line  was  nearly  the  same  as  that  of  the  Eocene  had 
been,  save  for  the  reduction  of  the  Mississippi  embayment  and  the 
presence  of  the  Florida  island.  Miocene  beds  occur  in  the  island 
of  Martha's  Vineyard,  apparently  are  concealed  beneath  the  sea 
along  the  New  England  coast,  and,  from  New  Jersey  southward, 
are  almost  continuous.  In  New  Jersey  their  thickness  is  only 
700  feet,  thinning  to  400  feet  in  Maryland,  but  reaching  1500  feet 
in  eastern  Texas,  where  they  are  concealed  under  later  deposits, 
but  their  presence  is  revealed  by  deep  borings.  In  the  North 
the  strata  are  unconsolidated  sands  and  clays  with  local  accumula- 
tions of  diatom  ooze,  as  at  Richmond,  Va.  (see  p.  3 14),  but  in  Florida 
they  are  compact  limestones,  and  in  Georgia,  limestones  and  con- 
glomerates. Owing  to  the  nearly  complete  closing  of  the  Missis- 
sippi embayment,  Miocene  strata  do  not  extend  into  Tennessee 
and  Arkansas. 

The  Oligocene  of  the  Atlantic  coast  had  been  a  time  of  warm 


THE   MIOCENE  EPOCH  749 

waters,  but  in  the  Miocene  a  cool  current  flowed  southward  along 
the  shore  and  through  the  straits  between  the  Florida  island  and 
the  mainland  into  the  Gulf  of  Mexico.  "  The  change  by  which 
the  Oligocene  was  brought  to  a  close  and  the  typical  Miocene  in- 
augurated, caused  .  .  .  the  most  remarkable  faunal  break  in  the 
geological  history  of  the  United  States  after  the  Cretaceous." 
(Ball.) 

On  the  Pacific  coast  the  Miocene  rocks,  though  reaching  the 
enormous  thickness  of  5000  to  7000  feet,  form  only  a  narrow  belt, 
and  lie  unconformably  upon  the  Eocene.  The  Coast  Range 
formed  a  chain  of  reefs  and  islands  in  the  Miocene  sea.  Volcanoes 
were  very  active  and  showered  great  quantities  of  ash  into  the  sea, 
where  it  was  extensively  mingled  with  diatoms,  which  largely 
compose  the  Monterey  series,  though  sandstones  and  bituminous 
shales  also  occur.  The  sea  did  not  extend  into  the  northern  part 
of  the  Sacramento  valley,  which  is  filled  with  continental  sedi- 
ments, fluviatile  and  subaerial  and  perhaps  partly  lacustrine. 
Orogenic  disturbances  took  place  in  California,  for  the  older  part 
of  the  series  in  the  Santa  Cruz  Mountains  near  San  Francisco  is 
folded  and  metamorphosed  and  the  newer  part  there  rests  uncon- 
formably upon  it. 

The  foothills  of  the  Sierras  had  been  worn  down  to  a  peneplain, 
which  was  elevated,  perhaps  early  in  the  Miocene,  and  carved  into 
valleys  and  ridges,  and  in  the  lower  stream  courses  the  "  deep 
Auriferous  Gravels  "  were  laid  down.  In  the  Upper  Miocene 
came  a  depression  and  very  thick  masses  of  the  "  bench  Aurif- 
erous Gravels  "  accumulated  in  the  valleys.  Then  followed  a 
time  of  great  volcanic  activity  in  the  Sierras,  at  first  forming  lava- 
flows  and  tuffs  of  rhyolite,  then,  after  an  interval,  andesite  tuffs 
and  breccias,  which  poured  down  the  valleys  as  great  torrents  of 
mud. 

The  coast  of  Washington  and  Oregon  was  covered  by  the  sea, 
which  extended  up  the  valley  of  the  Columbia  and  its  tributary 
the  Willamette,  but  the  beds  are  far  thinner  than  in  California  and, 
in  places,  lie  upon  folded  and  eroded  Eocene.  The  sea  also 


750  THE  TERTIARY  PERIOD 

extended  over  parts  of  British  Columbia.  Early  in  the  Miocene, 
Alaska  was  depressed,  especially  to  the  north,  and  the  valley  of  the 
Yukon  invaded  by  the  sea  and  much  of  western  Alaska  was  sub- 
merged, yet  in  the  Middle  and  Upper  Miocene,  at  least,  some  land 
connection  with  the  Old  World  must  have  existed. 

The  Miocene  fauna  of  California  was  largely  indigenous  and 
shows  no  evidence  of  communication  with  Asia,  which  would 
indicate  that  Bering  Strait  was  open;  if  so,  the  undoubted  con- 
nection of  America  with  Eurasia  must  have  been  by  some  other 
route,  perhaps  by  way  of  Greenland  and  the  north  Atlantic. 
Atlantic  Miocene  species  are  not  known  in  the  Pacific  fauna, 
whence  it  may  be  inferred  that  the  upheaval  of  Central  America 
and  the  Isthmus  of  Panama,  joining  South  and  North  America 
and  separating  the  two  oceans,  took  place  at  the  close  of  the  Oligo- 
cene,  though  there  are  some  difficulties  in  accepting  this  view. 

In  the  interior  region  Miocene  continental  deposits,  mostly 
fluviatile,  cover  a  vast  area,  though  to  no  very  great  depth.  The 
Arikaree,  or  Rosebud,  stage,  which  is  in  part  transitional  from  the 
uppermost  Oligocene,  is  found  overlying  the  White  River  beds  in 
South  Dakota,  western  Nebraska,  and  eastern  Wyoming,  with 
small  areas  in  northeastern  Colorado  and  Montana.  The  middle 
Miocene  Deep  River  stage,  occurs  in  widely  scattered  areas  of  re- 
stricted extent,  in  central  Montana,  central  Wyoming,  northeastern 
Colorado,  northwestern  Texas,  and  eastern  Oregon.  In  this  stage 
the  migration  of  land  mammals  from  the  Old  World,  which  ceased 
at  the  close  of  the  White  River  Oligocene,  was  resumed,  bringing 
in  several  new  types,  particularly  the  primitive  elephants,  which 
migrated  from  Africa  to  Asia  and  reached  Europe  and  North 
America  at  nearly  the  same  time.  In  this  stage  also  appear  the 
first  forerunners  of  the  migration  from  South  America  for  which 
the  junction  of  the  two  Americas  opened  the  way.  The  Loup 
Fork  stage  covers  much  of  the  Great  Plains  region  with  a  thin  sheet 
of  fine  sands  and  marls,  in  successive  disconnected  areas  from 
South  Dakota  far  into  Mexico,  with  outlying  areas  in  Montana 
and  New  Mexico. 


THE   MIOCENE   EPOCH 


751 


In  addition  to  these  comparatively  well-known  and  well-defined 
stages  of  the  Miocene,  there  are  several  others  which  are  referred 


UPPER 
ROSEBUD    250 


Merycochderus 


5teneoFiber 
Promerycochderus 

Lcptauchenia 
Protoceras 
Oreodon  (Upper) 

Oreodon  (Middle) 
Oreodon  (Lower) 

Titanotherium  (Upper) 


TitanotheriumjMiddle) 


Tilanotheriu  m  (Lower) 


FIG.  307.  —  Idealized  section  of  the  great  Bad  Lands  of  South  Dakota.     (Osborn) 

to  the  Miocene,  although  for  no  very  convincing  reasons.  In 
southwestern  Nevada  is  an  immense  thickness  (14,000  feet)  .of 
supposably  Miocene  beds,  described  as  being  lacustrine,  but  con- 


752  THE  TERTIARY   PERIOD 

taining  some  coal  and  sulphur.  Several  other  areas  are  found  in 
Nevada,  Washington,  and  British  Columbia.  A  small  area  of 
probably  Miocene  rocks  occurs  in  the  South  Park  of  Colorado, 
the  Florissant  beds,  which  have  usually  been  called  Oligocene, 
but  which  recent  and  more  extended  studies  have  shown  to  be 
probably  Miocene.  The  deposits  are  thin,  papery  shales,  com- 
posed of  fine  volcanic  ash  showered  into  a  small  body  of  water, 
and  have  preserved  countless  insects  and  plants,  many  fish  and  a 
few  birds,  but  no  mammals. 

The  Miocene  was  a  time  of  great  volcanic  activity  in  the  Pacific 
mountain  ranges  and  along  the  principal  range  of  the  Rocky 
Mountains;  the  great  volcanoes  of  the  Cascades  and  of  Mexico 
are  believed  to  date  from  this  epoch,  and  in  the  Yellowstone  Park 
were  immense  eruptions  of  andesites  and  basalts,  both  lavas 
and  tuffs.  The  great  Columbia  River  basaltic  flows  are  of  early 
Miocene  date,  for  they  lie  upon  the  slightly  disturbed  and  eroded 
John  Day,  while  Middle  Miocene  beds  were  deposited  upon  them. 
Vulcanism  was  also  displayed  in  the  West  Indies,  the  Andes,  and 
Patagonia. 

As  we  have  seen,  erogenic  movements  went  on  in  California  be- 
tween the  Lower  and  Upper  Miocene.  Later  in  the  epoch  and  at  its 
close,  these  movements  grew  very  important  and  widely  extended, 
affecting  the  mountains  of  all  the  Pacific  States  and  causing  the 
principal  upheaval  of  the  Coast  Range  in  California  and  Oregon. 
The  great  fault  bounding  the  Sierras  on  the  east  was  made  and  the 
block  mountains  of  the  Great  Basin  raised  by  an  extensive  system 
of  faults.  The  high  plateaus  of  southern  Utah  and  northern 
Arizona  were  raised,  beginning  the  great  erosion-cycle  of  the 
Colorado  River.  In  the  East  the  West  Indian  islands  were  raised 
and  the  Florida  island  was  joined  to  the  mainland. 

Foreign.  —  In  the  north  of  Europe  the  sea  retreated  from  large 
areas;  northern  Germany  was  now  dry  land,  with  only  a  relatively 
small  bay  invading  it,  while  England  was  entirely  above  water, 
anfl  has  no  marine  Miocene  beds.  On  the  west  coast  of  Europe, 
the  Atlantic  encroached  largely,  as  in  France,  Spain,  Portugal, 


THE   MIOCENE   EPOCH  753 

and  also  the  northwest  of  Africa.  Spain  was  joined  to  Africa, 
but  straits  across  northern  Spain  and  southern  France  connected 
the  Atlantic  with  the  Mediterranean.  Another  change  of  great 
importance  wa"s  the  shutting  off  of  the  long-standing  connection  of 
the  Mediterranean  with  the  Indian  seas.  The  former  covered 
much  of  eastern  Spain,  and  flooded  the  lower  Rhone  valley,  send- 
ing an  arm  along  the  northern  border  of  the  Alps  to  the  neigh- 
bourhood of  Vienna.  Here  it  expanded  into  a  broad  basin,  con- 
nected with  another  great  basin  covering  Hungary.  Most  of 
Italy,  Sicily,  and  a  large  part  of  northern  Asia  Minor  were  under 
water,  but  the  Adriatic  and  ^Egean  Seas  were  mostly  land,  and 
the  Alps  formed  a  chain  of  islands,  mountainous,  but  not  nearly 
so  high  as  at  present. 

At  the  end  of  the  Lower  Miocene  came  a  great  upheaval  of  the 
Alps,  by  which  the  sea  was  again  excluded  from  that  region,  and, 
just  as  in  the  Oligocene,  inland  seas  and  lakes  took  the  place  of 
the  marine  straits.  The  basins  of  Vienna  and  Hungary  had  a  very 
complex  history,  with  repeated  changes  of  size  and  position,  re- 
sulting in  the  formation  of  an  immense  inland  sea  (the  Sarmatian 
Sea),  which  reached  from  Vienna  to  the  Black,  Aral,  and  ^Egean 
Seas,  and  was  nearly  as  large  as  the  present  Mediterranean.  This 
vast  basin  had  but  a  limited  connection  with  the  ocean,  and  repre- 
sented conditions  much  like  those  of  the  Black  Sea  at  present. 
Europe  had  also  a  number  of  fresh-water  lakes,  particularly  in 
France,  Switzerland,  and  Germany,  which  have  preserved  a  very 
interesting  record  of  Miocene  land  life.  A  comparison  with  that 
of  North  America  shows  that  a  way  of  migration  was  still  open 
between  the  two  continents.  In  the  basin  of  the  Ebro,  in  Spain, 
the  Miocene  consists  of  red  sandstones  and  marls,  with  beds  of 
gypsum  and  salt,  demonstrating  arid  conditions  which  were  no 
doubt  only  local. 

In  the  Old  World  the  Miocene  was  a  time  of  mountain  making. 
The  Pyrenees  had  been  elevated  in  the  later  Eocene;  the  Alps 
received  nearly  their  present  altitude  in  the  Miocene.  The  Apen- 
nines had  two  distinct  phases  of  upheaval,  one  in  the  Eocene  and 


754  THE  TERTIARY   PERIOD 

one  in  the  Miocene,  the  latter  coinciding  with  that  of  the  Alps. 
The  Caucasus  dates  from  the  close  of  the  Miocene,  while  the  date 
of  the  Himalayas  is  yet  uncertain,  but  was  either  Eocene,  or 
Miocene. 

Marine  Miocene  beds  occur  in  north  Africa,  on  the  coast  of 
the  Soudan  and  in  Asia  Minor.  In  Asia  marine  Miocene  is  known 
to  be  present  in  northwestern  India,  in  Burmah  and  Japan,  also 
in  the  island  of  Java. 

The  whole  of  Patagonia  was  submerged  in  a  great  transgression 
of  a  shallow,  epicontinental  sea,  the  Patagonian  stage,  and  after 
some  oscillation  the  sea  withdrew  and  the  terrestrial  Santa  Cruz 
beds  were  deposited.  These  are  very  largely  composed  of  volcanic 
tuffs,  but  also  contain  cross-bedded  sandstones  and  other  fluviatile 
deposits.  Marine  deposits  which  are  correlated  with  the  Pata- 
gonian stage  are  on  the  west  coast  of  Chili. 

The  Tertiary  rocks  of  Australia  and  New  Zealand  which  cover 
extensive  regions,  especially  in  Victoria,  have  not  yet  been  defi- 
nitely classified.  Certain  of  these,  usually  referred  to  the  Oligocene 
but  more  probably  Lower  Miocene,  show  so  close  a  resemblance  in 
their  fossils  to  those  of  Patagonia,  as  to  require  the  assumption 
of  a  continuous  coast-line  with  South  America,  probably  by  way 
of  Antarctica.  The  probability  of  this  assumption  is  much 
strengthened  by  the  occurrence  of  the  marine  Patagonian  with 
its  characteristic  fossils  in  the  South  Shetland  Islands,  an  Antarctic 
group. 

Miocene  Life 

The  life  of  the  Miocene  is  in  all  respects  a  great  advance  upon 
that  of  the  Eocene  and  Oligocene.  The  Grasses  greatly  multiply 
and  take  possession  of  the  open  spaces,  producing  a  revolution 
in  the  conditions  of  food  for  the  herbivorous  animals.  The  vege- 
tation of  North  America,  as  far  north  as  Montana,  perhaps  even 
to  northern  British  Columbia,  still  bore  a  southern  character.  In 
the  UpperMiocene  tuffs  of  the  Yellowstone  Park  and  contemporary 
strata  of  Oregon  are  found  such  trees  as  Poplars,  Walnuts,  Hicko- 


FIG.  308.  ~&i$r  Sp,    A  sumach  from  the  Florissant  Shales. 


756 


THE  TERTIARY  PERIOD 


ries,  Oaks,  Elms,  Maples,  Beeches,  noble  forms  of  Magnolias  and 
Sycamores.  One  species  of  Aralia  had  leaves  2  feet  long  by 
3  inches  wide.  Curiously  enough,  the  Breadfruit  (Artocarpus) 
flourished  in  Oregon,  and  probably  on  the  Yellowstone  also. 
Conifers  were  numerous  and  varied.  At  Florissant  the  plants  are 
of  a  similar  warm-temperate  character,  with  very  few  palms, 
but  with  Sequoia,  the  California  Redwood,  abundant. 

In  Europe  the  Lower  Miocene  flora  was  quite  like  that  of  modern 
India;  over  the  central  and  western  regions  Palms  continue  to 
flourish,  together  with  Live  Oaks,  Myrtles,  Magnolias,  Figs,  etc. 

In  the  latter  part  of  the  epoch  a 
change  is  noted,  and  such  trees  as 
Beeches,  Poplars,  Elms,  Maples, 
Laurels,  and  the  like  become 
dominant. 

Marine  Invertebrates  belong 
almost  entirely  to  genera  which 
still  live  in  the  seas,  and  many  of 
the  species  persist  to  our  own  day. 
In  Europe  the  older  Miocene  has 
numbers  of  shells  such  as  now  live 
only  in  warm  seas,  like  Cyprcea, 
Mitra,  Purpura,  Strombus,  etc.  (See 
PI.  XVII,  p.  765.)  The  Miocene 
of  our  Atlantic  coast  was  evidently 
a  time  of  cooler  waters,  and  a  similar  change  took  place  in 
Europe  in  the  Upper  Miocene.  A  very  characteristic  shell  of 
the  Atlantic  coast  Miocene  is  Ecphora  quadricostata  (Fig.  309). 

The  terrestrial  Vertebrates  of  the  interior  are  of  much  interest. 
Little  is  known  of  Miocene  Birds  in  this  country,  but  in  Europe 
they  are  abundantly  preserved  and  are  of  distinctly  African 
character.  Parrots,  Indian  Swallows,  Secretary  Birds,  Adjutants, 
Cranes,  Flamingoes,  Ibises,  Pelicans,  Sand-grouse,  and  numerous 
Gallinaceous  birds,  were  mingled  with  birds  of  European,  type, 
such  as  Eagles,  Owls,  Woodpeckers,  Gulls,  Ducks,  etc. 


FlG.  309.  —  Ecphora  quadricostata 
Say,  X  2/3,  Yorktown,  Va. 


MIOCENE   LIFE  757 

The  Lower  Miocene  (Arikaree)  Mammals  of  the  interior  have  only 
lately  been  discovered  and  are  not  yet  fully  described.  In  general, 
these  animals  are  a  continuation  of  %  the  John  Day  fauna,  in  a 
higher  stage  of  advancement,  without  admixture  of  exotic 
elements.  The  Ancylopoda,  a  very  curious  group  of  hoofed 
animals  in  which  the  hoofs  had  been  converted  into  huge  claws, 
and  of  which  a  few  traces  have  been  found  in  the  White  River 
and  John  Day,  assume  great  importance  in  these  beds. 

In  the  Middle  Miocene  (Deep  River)  came  a  renewed  migration 
from  the  Old  World,  bringing  in  the  first  of  the  elephant  group 
(Proboscidea)  which  had  simpler  teeth  than  the  modern  elephants 
and  a  pair  of  tusks  in  the  lower  jaw,  as  well  as  in  the  upper. 
The  genus  is  Tetrabelodon.  The  first  of  the  true  Ruminants  to 
appear  in  North  America  came  in  with  this  migration  and  were, 
in  a  measure,  intermediate  between  deer  and  antelopes,  while 
European  Rhinoceroses  accompany  them.  Of  the  native  stock, 
the  Horses  and  Camels  deserve  particular  mention  as  having  in- 
creased in  size  and  in  variety  and  having  made  great  advances  to- 
ward the  modern  standard.  The  Oreodonts  in  a  variety  of  bizarre 
genera,  some  of  them  aquatic,  are  very  common. 

The  Upper  Miocene  (Loup  Fork)  Mammals  resemble  closely 
those  of  the  Deep  River  stage,  rather  more  advanced  and  modern- 
ized. The  true  Cats  and  a  number  of  weasel-  and  otter-like 
Carnivores  came  in  from  the  Old  World,  while  the  Wolves,  Pan- 
thers, and  Sabre-tooth  Tigers  were  very  numerous.  Besides  the 
true  Ruminants,  the  American  type  of  Camels  and  Llamas  con- 
tinued to  flourish  in  such  genera  as  Procamelus,  Pliauchenia, 
and  others.  One  very  remarkable  camel,  Alticamelus,  had 
nearly  the  same  proportions  as  the  giraffe.  The  extraordinary 
four-horned  genus,  Protoceras,  of  the  White  River,  is  represented 
by  Syndyoceras,  in  which  the  four  horns  are  much  increased  in 
length,  but  the  tusks  are  reduced.  The  Loup  Fork  Horses 
(Protohippus  and  Hippariori)  are  much  more  modern  in  char- 
acter and  larger  in  size  than  their  predecessors,  but  still  have 
three  toes  on  each  foot.  The  Rhinoceroses  are  very  abundant, 


758  THE  TERTIARY   PERIOD 

and  form  a  peculiar  American  genus  (Aphelops)  of  massive, 
hornless  animals.  The  Peccaries,  or  American  swine,  were  com- 
moner in  the  Loup  Fork  than  in  the  earlier  Miocene  stages. 
The  Atlantic  coast  Miocene  has  yielded  numbers  of  Dolphins, 
Sperm  and  Whalebone  Whales. 

In  Europe  the  Upper  Miocene  mammals  were,  in  general,  like 
those  of  North  America,  but  a  salient  difference  is  in  the  much 
greater  number  of  early  types  of  Deer  and  Antelopes  which  are 
found  there,  together  with  various  forms  of  Swine  and  ancestral 
Bears.  Besides  the  Mastodons,  which  were  common  to  both  con- 
tinents, Europe  had  in  Dinotherium  a  remarkable  kind  of  elephant; 
this  animal  had  a  much  flattened  head  and  a  pair  of  massive, 
backwardly  curved  tusks  in  the  lower  jaw.  The  weasel  and  otter 
tribe  of  Carnivora  was  much  more  abundant  and  varied  in  Europe, 
and  the  Civet-cats,  which  were  also  common  there,  did  not  migrate 
to  America. 

Little  is  known  of  the  Miocene  Mammals  of  other  continents 
except  South  America,  where  a  magnificent  assemblage  has  been 
preserved  in  the  Santa  Cruz  tuffs  of  Patagonia.  This  fauna  is  so 
entirely  different  from  that  of  the  northern  hemisphere  that  it 
seems  to  belong  to  another  world.  It  contains  no  Carnivora, 
Proboscidea,  Artiodactyla,  or  Perissodactyla.  The  flesh-eaters 
were  carnivorous  Marsupials,  like  those  of  Australia,  and  another 
family  of  Marsupials  like  the  Australian  Phalangers,  was  also 
present  in  addition  to  the  American  Opossums.  The  Rodents, 
of  which  there  were  very  many,  all  belong  to  the  great  porcupine 
group  (Hystricomorpha)  and  closely  resemble  modern  South 
American  types,  but  among  them  are  no  rats  or  mice,  squirrels, 
marmots,  beavers,  hares,  or  rabbits.  Edentates  are  extraordi- 
narily numerous  and  varied,  Armadillos,  Glyptodonts,  and 
Ground  Sloths  forming  one  of  the  most  conspicuous  elements  of 
the  fauna.  Hoofed  animals  were  present  in  multitudes,  but 
though  having  a  certain  likeness  to  those  of  the  northern  conti- 
nents, they  are  but  remotely  related  to  them.  The  Toxodontia 
(Nesodon)  were  slow  and  massive  animals,  and  the  little  Typo- 


THE   PLIOCENE   EPOCH  759 

theria  had  a  superficial  resemblance  to  Rodents.  The  Litopterna 
had  one  group  which  imitated  the  horses  in  a  surprising  manner 
and  another  which  had  some  likeness  to  the  llamas  (see  Fig.  310). 
The  Homalodotheria  were  a  parallel  to  the  northern  Ancylopoda 
and  the  Astrapotheria,  largest  of  Santa  Cruz  mammals,  were  not 
altogether  unlike  Rhinoceroses. 

The  climate  of  the  early  Miocene  was  much  like  that  of  the  Olig- 
ocene  and  decidedly  warmer  in  Europe  than  in  North  America, 
though  it  was  mild  even  in  the  latter.  The  difference  seems  to  have 
been  largely  due  to  the  manner  in  which  Europe  was  intersected 
by  arms  and  gulfs  of  the  warm  southern  sea.  In  the  Upper 
Miocene  the  climate  became  distinctly  cooler  on  both  sides  of  the 
ocean. 

THE  PLIOCENE  EPOCH 

American.  —  The  Pliocene  is  not  a  conspicuous  formation  in 
this  country,  and  only  of  comparatively  late  years  has  it  been 
recognized  at  all  on  the  Atlantic  coast.  The  movements  which 
closed  the  Miocene  gave  to  the  Atlantic  and  Gulf  shores  nearly 
their  present  outlines,  but  some  differences  may  be  noted.  Much 
of  southern  Florida  was  still  under  water,  and  a  gulf  invaded 
northern  Florida,  covering  a  narrow  strip  of  Georgia  and  South 
Carolina.  Isolated  patches  of  Pliocene  rocks  in  North  Carolina 
and  Virginia  may  be  remnants  of  a  continuous  band.  The  Gay 
Head  Sands  on  the  island  of  Martha's  Vineyard  have  marine 
fossils  and  lie  unconformably  on  the  Miocene,  forming  the  most 
northerly  known  exposure  of  marine  Pliocene  on  the  Atlantic 
coast.  All  of  these  marine  formations  in  the  eastern  United 
States  are  very  thin  and  in  notable  contrast  to  the  Pacific  coast. 
Florida  also  has  some  fresh-water  Pliocene.  A  small  part  of 
eastern  Mexico,  much  of  Yucatan,  and  some  of  Central  America 
were  still  submerged. 

On  the  Pacific  coast  the  post-Miocene  upheaval  had  laid  bare 
the  western  foothills  of  the  Sierra  and  greatly  disturbed  the  Mio- 
cene strata  of  the  Coast  Range.  The  latter  range  sank  again  early 


THE   PLIOCENE   EPOCH  761 

in  the  Pliocene,  whose  strata  lie  unconformably  upon  the  Miocene, 
and  extend  over  upon  older  beds.  The  transgression  of  the  sea 
was  limited,  and  Pliocene  rocks  form  only  a  narrow  band  along 
the  coast  in  California,  Oregon,  and  Washington.  The  San  Fran- 
cisco peninsula  was  an  area  of  subsidence  and  maximum  deposi- 
tion, for  here  no  less  than  5800  feet  of  sandstone  (the  Merced 
series)  were  formed,  and  quite  lately  Professor  Lawson  has  de- 
scribed a  series  of  beds,  containing  much  volcanic  material,  7000 
feet  thick,  lying  below  the  Merced  and  above  the  Monterey  Mio- 
cene. This  would  make  the  Pliocene  near  San  Francisco  have 
a  thickness  of  nearly  13,000  feet,  by  far  the  thickest  mass  of 
Pliocene  in  North  America.  On  the  other  hand,  a  deduction, 
perhaps  a  very  considerable  one,  should  probably  be  made  from 
this  thickness,  for  the  upper  part  of  the  Merced  appears  to  be 
Quaternary.  (Dall,  Arnold.)  The  mountains  of  British  Colum- 
bia are  believed  to  have  been  at  a  higher  level  than  now,  an  ele- 
vation which  probably  connected  Vancouver's  and  the  Queen 
Charlotte  Islands  with  the  mainland.  Marine  Pliocene  also  occurs 
in  southern  Alaska.  The  marine  Pliocene  faunas  of  California 
and  Japan  became  closely  similar,  due  to  a  migration  along  the 
shore  around  the  North  Pacific,  where  the  climate  was  temperate, 
no  Indian  species  joining  in  the  migration  of  the  Japanese  forms. 
In  the  Upper  Pliocene  the  waters  of  the  California  coast  appear 
to  have  been  somewhat  colder  than  they  are  now. 

In  the  interior  region  a  few  areas  of  Pliocene,  resembling  the 
Upper  Miocene  in  physical  character  and  in  mode  of  formation, 
have  been  described.  The  oldest  of  these,  the  Republican  River 
stage,  overlies  and  is  intimately  associated  with  the  upper  Loup 
Fork,  in  northwestern  Kansas,  northern  Nebraska,  and  eastern 
Oregon.  The  Blanco  stage  is  typically  displayed  in  the  Staked 
Plains  of  northwestern  Texas,  where  it  contains  South  American 
mammals,  and  is  also  found  in  Nebraska  and  Oregon.  The 
Upper  Pliocene  is  not  definitely  known  to  be  represented  in  the 
interior,  but  its  presence  is  suspected  in  Texas  and  elsewhere. 

Some  isolated  areas  of  Pliocene  which  cannot  yet  be  correlated 


762  THE  TERTIARY  PERIOD 

with  the  stages  mentioned,  are  found  in  southern  Idaho,  eastern 
Washington,  etc.,  and  no  doubt  much  of  the  surface  deposits  of 
the  Great  Basin  and  other  regions  is  Pliocene,  but  lack  of  fossils 
prevents  their  determination. 

The  volcanic  activity  in  the  Rocky  Mountain  and  Pacific  coast 
regions,  which  had  begun  in  the  Cretaceous,  continued  through 
the  Pliocene.  The  great  outflow  of  rhyolite  which  built  up  the 
Yellowstone  Park  plateau  is  referred  to  the  Pliocene.  Some  of 
the  enormous  fissure  eruptions,  which  flooded  northern  Cali- 
fornia and  Nevada,  southern  Idaho,  eastern  Oregon,  and  Wash- 
ington, with  thick  sheets  of  basalt,  obliterating  the  valleys  and 
revolutionizing  the  system  of  drainage,  are  probably  Pliocene, 
as  some  are  demonstrably  Miocene. 

A  problematical  formation  is  the  Lafayette,  whose  geological 
position  and  mode  of  origin  are  still  debated.  The  Lafayette  is 
a  belt  of  sands  and  gravels  which  runs  through  Maryland,  Vir- 
ginia, the  Carolinas,  and  the  Gulf  States,  around  the  southern  end 
of  the  Appalachians,  up  to  southern  Illinois,  whence  it  turns  south- 
westward  to  Texas.  As  in  the  typical  exposures  the  Lafayette 
rests  unconformably  upon  the  Miocene  and  is  uncomformably 
overlaid  by  the  Pleistocene,  many  authorities  refer  it  to  the  Plio- 
cene. The  mode  of  formation  is  somewhat  obscured  by  the  ab- 
sence of  fossils,  but  this  very  lack  and  the  physical  characters  of 
the  beds  make  a  marine  origin  improbable.  It  is  more  likely 
that  the  deposition  was  subaerial,  "  resulting  from  a  compara- 
tively rapid  Pliocene  uplift  in  the  Appalachian  region."  (Dall.) 
According  to  another  view,  the  Lafayette  is  due  to  a  depression 
of  the  coastal  plain  while  the  Piedmont  region  was  elevated, 
"  and  streams  gorged  with  detritus  from  the  decayed,  uplifted 
Piedmont  above  rushed  down  to  the  sea  and  poured  their  con- 
tents into  the  ocean.  Either  the  waves  were  weak  or  the  sea 
advanced  rapidly  or  this  decayed  material  was  discharged  in 
enormous  quantities,  for  the  sea  was  unable  to  cope  with  it  and 
deposited  it  unsorted  on  the  bottom."  (Shattuck.)  , 

At  or  near  the  close  of  the  Pliocene,  extensive  upheavals  took 


THE   PLIOCENE  EPOCH  763 

place  in  several  different  parts  of  the  continent,  especially  on  the 
Pacific  slope.  The  rise  of  the  Rocky  Mountains  continued,  rais- 
ing the  western  part  of  the  Miocene  beds  3000  feet  higher  than  the 
eastern.  The  height  of  the  Sierra  was  greatly  increased  by  the 
rise  of  the  mountains  along  the  eastern  fault-plane  and  the  tilting 
of  the  whole  block  westward.  The  new  valleys  cut  through  the 
late  basalt  sheets  of  the  Sierras  are  much  deeper  than  the  older 
valleys  excavated  in  Cretaceous  and  Tertiary  times,  which  is 
due  to  the  greater  height  of  the  mountains  and  consequent  greater 
fall  of  the  streams.  The  fault-blocks  which  form  the  Basin  Ranges 
were  still  further  displaced,  increasing  their  height.  The  Wasatch 
Mountains  and  the  High  Plateaus  of  Utah  and  Arizona  were  again 
upraised.  The  great  mountain  range  of  the  St.  Elias  Alps,  in 
southeastern  Alaska,  was  upheaved  at  this  time,  or  even  later, 
and  the  mountains  of  British  Columbia  were  probably  raised  still 
higher.  In  Washington  and  Oregon  the  uplift  was  small,  but 
became  much  greater  in  southern  California,  reaching  2500  feet 
in  the  Monte  Diablo  range.  On  the  eastern  side  of  the  continent 
the  uplift  was  on  a  much  more  restricted  scale,  not  generally  ex- 
ceeding 100  feet.  The  Florida  anticline  underwent  renewed 
compression,  which  increased  its  height;  in  Georgia,  the  con- 
tinuation of  this  fold  rose  to  400  feet.  The  same  movement  ex- 
tended the  coast  of  Mexico  and  Central  America  and  brought  the 
continent  to  nearly  its  present  outlines. 

It  is  not  necessary  to  suppose  that  all  these  movements  were 
contemporaneous;  merely  that  they  occurred,  now  in  one  place, 
now  in  another,  at  or  near  the  end  of  the  Pliocene  epoch. 

Foreign.  —  In  Europe  the  sea  generally  retreated  at  the  end  of 
the  Miocene,  leaving  in  the  north  only  Belgium  and  a  small  part  of 
northern  France  under  water.  In  England  the  sea  advanced  upon 
the  land;  while  in  the  Mediterranean  region  large  areas  remained 
submerged,  as  in  Spain,  Algeria,  nearly  all  of  central  and  southern 
Italy  and  Sicily,  and  Greece.  In  this  region  volcanic  activity 
was  intense,  and  y£tna,  Vesuvius,  and  the  volcanoes  of  central 
Italy  had  begun  their  operations.  Germany  has  no  marine 


764  THE  TERTIARY   PERIOD 

Pliocene,  but  extensive  areas  of  fluviatile  and  other  continental 
deposits  belong  to  this  epoch;  especially  famous  are  the  stratified 
sands  of  the  Eppelsheim  basin,  called  Dinotherium  sands,  which 
also  occur  in  several  other  parts  of  South  Germany.  In  the  lower 
Main  valley  are  Pliocene  lignites,  the  plants  of  which  are  nearly 
one-half  Conifers,  but  also  include  many  American  trees,  such  as 
Walnuts  and  Hickories.  Continental  Pliocene,  containing  the 
same  land  mammals,  occurs  in  many  separate  areas  of  southern 
Europe  and  Asia  Minor,  Mt.  Leberon  in  the  south  of  France, 
Pikermi  near  Athens,  the  island  of  Samos,  which  was  then  part 
of  the  continent,  and  Maragha  in  Persia,  are  all  celebrated  locali- 
ties of  the  older  Pliocene,  while  the  newer  Pliocene  fauna  is  found 
in  great  abundance  in  the  river  deposits  of  the  lower  Arno  valley 
in  Italy  (Val  d'Arno  stage).  Over  the  region  of  the  great  Sar- 
matian  Sea  of  the  Upper  Miocene  were  numerous  bodies  of  brack- 
ish water,  in  which  lived  shells  much  like  those  which  now  inhabit 
the  Caspian. 

On  the  south  side  of  the  Himalayas,  in  northern  India, 
are  several  thousand  feet  of  sandstones  and  conglomerate,  with 
some  clay  and  lignite,  formed  principally  from  the  piedmont 
accumulations  transported  from  the  Himalayas  during  the 
Pliocene,  though  probably  the  process  of  accumulation  began  in 
the  Upper  Miocene.  These  deposits  now  make  the  Siwalik 
Hills,  famous  for  their  fossil  bones;  and  similar  deposits  with  the 
same  fossils  occur  in  Borneo,  and  probably  Java,  which  then 
were  connected  with  Asia.  In  South  America  a  Pliocene  trans- 
gression of  the  sea  took  place,  submerging  the  entire  eastern 
coast  of  Argentina  and  Patagonia  (Parana,  or  Cape  Fairweather 
stage)  and  .along  the  line  of  47°  S.  lat.,  at  least,  extending  to 
the  foothills  of  the  Andes.  The  marine  Pliocene  beds  were 
involved  in  the  last  upheaval  of  the  southern  Andes. 

Pliocene  Life 

The  life  of  the  Pliocene  is  very  modern  in  character.  Little  is 
known  of  the  vegetation  in  North  America,  but  in  Europe  it  is 


PLIOCENE  LIFE 


76S 


marked  by  the  continued  disappearance  of  the  characteristically 
tropical  plants  and  by  an  approximation  to  the  modern  European 
flora.  Many  trees  persisted,  however,  which  are  no  longer  native 
to  that  continent,  but  are  still  found  in  eastern  Asia  or  in  North 
America,  such  as  Tulip  Trees,  Walnuts,  Hickories,  Magnolias, 
Sequoias,  etc. 


PLATE  XVII.  —  TERTIARY  FOSSILS  FROM  FLORIDA 

Fig.  i,  Marginella  aurora,  x  3/4,  Miocene.  2,  Nassa  bidentata,  x  3/4,  Miocene  and  Plio- 
cene. 3,  Purpura  conradi,  x  2/3,  Miocene.  4,  Natica  floridana,  x  1/2,  Miocene.  5,  Mitra 
wilcoxi,  x  1/2,  Miocene.  6,  Fasciolaria  fulipa,  x  1/2,  Pliocene.  7,  Ty phis  floridana,  Plio- 
cene. 8,  Turbo  rectogrammicus,  x  1/2,  Pliocene.  (After  Dall) 

Marine  Invertebrates  are  nearly  identical  with  modern  forms, 
and  the  great  majority  of  Pliocene  species  of  shells  are  still  living. 

The  Mammals  are  still  somewhat  behind  their  modern  succes- 
sors, though  much  more  advanced  than  their  predecessors, 


766  THE  TERTIARY   PERIOD 

Those  of  North  America  are  still  incompletely  known,  and  the 
list  is  a  short  one.  Mastodons,  Horses,  Rhinoceroses,  Peccaries, 
and  very  large  Llamas  represent  the  hoofed  animals,  beside  the 
Dogs,  Cats,  and  Mustelines.  The  effects  of  the  connection  with 
South  America  are  seen  in  the  appearance  of  the  gigantic  Ground 
Sloths  and  Armadillos,  and  of  southern  families  of  Rodents.  If 
this  connection  was  actually  established  at  the  close  of  the 
Oligocene,  it  is  difficult  to  see  why  the  South  American  mam- 
mals should  so  long  have  delayed  their  migration  to  the  north. 

The  early  Pliocene  mammals  of  southern  Europe  closely  re- 
semble those  of  modern  Africa,  —  Wolves,  Cats,  Civets,  Hyaenas, 
Monkeys,  Rhinoceroses,  three-toed  Horses,  Deer  (of  which  Africa 
has  none),  a  great  variety  of  Antelopes  and  of  Giraffe-like  forms, 
and  Swine.  Mastodon  and  Dinotherium  persisted,  the  latter 
attaining  great  size.  India  had  a  similar  fauna,  with  certain 
geographical  differences.  Especially  to  be  noted  are  the  great 
variety  of  Oxen,  the  presence  of  Bears,  true  Elephants,  and  the 
Hippopotamus,  of  the  first  Old  World  Camels,  and  of  the  ex- 
traordinary Sivathcrium  and  Brahmatherium,  great  four-horned 
creatures  allied  to  the  Giraffes.  In  the  Upper  Pliocene  the  true 
Elephants,  Oxen,  Hippopotamus,  and  Bears  had  extended  their 
range  to  Europe,  but  not,  so  far  as  we  know,  to  North  America. 
In  volcanic  tuffs  of  probably  Pliocene  age  on  the  island  of  Java  was 
discovered  a  fossil,  Pithecanthropus  erectus,  which  nas  attracted 
great  interest  and  has  been  the  subject  of  much  discussion.  Ac- 
cording to  one  view,  these  bones,  part  of  the  skull  and  a  thigh  bone, 
represent  one  of  the  "  missing  links  "  in  the  ancestry  of  Man;  ac- 
cording to  another,  they  are  human,  but  abnormal,  while  a  third 
opinion  regards  them  as  belonging  to  a  large  ape.  The  material 
is  insufficient  for  a  definite  solution  of  the  problem. 

On  the  coast  of  Zululand,  southeastern  Africa,  a  small  area 
of  continental  Upper  Pliocene  has  yielded  a  few  fossil  mammals, 
which  suffice  to  show  that  Africa  had  already  acquired  the  fauna 
which  characterizes  it  now.  The  list  includes  an  elephant,  rhi- 
noceroses, hippopotamuses,  and  several  antelopes. 


PLIOCENE  LIFE  767 

The  Climate  of  the  Pliocene  was,  on  the  whole,  evidently  cooler 
than  that  of  the  Miocene,  as  is  shown  by  the  changes  in  the  char- 
acter of  the  vegetation  and  of  the  marine  shells.  On  the  Ameri- 
can Atlantic  coast  this  is  not  true,  for  here  the  Miocene  waters 
were  exceptionally  cold  and  the  Pliocene  was  warmer,  but,  on  the 
other  hand,  the  thin  beds  of  Florida  and  the  Carolinas  represent 
but  a  small  part  of  the  Pliocene.  In  the  English  Pliocene  the 
proportion  of  Arctic  shells  rises  from  5  %  in  the  oldest  to  over 
60  %  in  the  newest  beds.  The  refrigeration  was  greater  in  the 
sea  than  on  the  land,  for  the  vegetation  shows  that  the  air  had 
not  yet  grown  cold.  That  was  to  come  later. 


CHAPTER    XXXVI 
THE   QUATERNARY    PERIOD 

THE  Quaternary  is  the  last  of  the  great  divisions  of  geological 
time  and  may  be  said  to  be  still  in  progress,  for  its  events  led  by 
gradual  steps  to  the  present  climatic  and  geographical  order  of 
things,  and  to  the  present  geographical  distribution  of  animals 
and  plants  over  the  surface  of  the  earth.  Quaternary  deposits 
are  to  a  very  large  extent  continental  in  their  origin,  marine  sedi- 
ments in  most  regions  being  of  very  subordinate  extent,  and  con- 
sist generally  of  loose,  uncompacted  sands,  gravels,  boulder  clays, 
clays,  and  the  like.  These  deposits  never  reach  any  very  great 
thickness,  but  their  horizontal  extent  is  at  least  equal  to  that  at- 
tained by  any  preceding  system,  for  in  one  form  or  other  they 
cover  almost  the  entire  surface  of  the  globe.  In  an  even  greater 
degree  than  in  the  Tertiary  are  the  Quaternary  formations  of 
different  areas  difficult  to  correlate,  because  of  the  locally  re- 
stricted character  of  many  of  them,  the  frequent  and  radical 
changes  of  facies  from  point  to  point,  and  the  scantiness  of  fossils 
or  their  absence  over  wide  regions. 

The  line  of  division  between  the  Tertiary  and  Quaternary  is 
not  easy  to  draw,  especially  in  those  regions  which  were  not 
reached  by  the  great  glaciers  and  ice-sheets  of  the  Pleistocene. 
The  seas  at  the  end  of  the  Pliocene  had  much  the  same  extent  as 
later,  and  on  the  same  coasts  the  same  kinds  of  material  continued 
to  accumulate,  and  some  of  the  Pliocene  coral  reefs  continued  to 
grow  uninterruptedly  into  the  Pleistocene.  Even  in  the  regions  of 
glaciation,  the  end  of  the  Tertiary  is  fixed  differently  by  different 
authorities. 

768 


THE   PLEISTOCENE  OR   GLACIAL   EPOCH  769 

According  to  general  practice,  the  Quaternary  is  divided  into 
two  epochs,  (i)  the  Pleistocene,  and  (2)  the  Recent,  though  various 
names  are  used  for  them. 


THE  PLEISTOCENE  OR  GLACIAL  EPOCH 

The  conception  of  immense  ice-sheets,  like  those  of  Greenland 
and  Antarctica,  covering  large  parts  of  North  America  and  Europe 
at  comparatively  recent  geological  dates,  is  one  that  at  first  seems  to 
be  incredible,  and  made  its  way  very  slowly  in  the  face  of  determined 
opposition.  Originally  suggested  by  Agassiz  about  1846,  it  required 
nearly  thirty  years  to  gain  the  general  acceptance  of  geologists. 
This  change  of  view  was  brought  about  by  evidence  too  strong  to 
be  resisted;  not  that  all  difficulties  have  been  removed  and  all 
problems  solved,  but  no  other  hypothesis  can  rival  the  glacial  in 
satisfactorily  explaining  so  many  and  such  varied  classes  of  phe- 
nomena. It  will  be  well  to  summarize  this  evidence  briefly.  The 
work  of  erosion,  transportation,  and  deposition  accomplished  by 
existing  glaciers  was  described  in  Chapters  VI  and  VIII,  but  many 
of  the  illustrations  and  descriptions  of  those  chapters  were  drawn 
from  the  areas  of  Pleistocene  glaciation,  partly  for  the  convenience 
of  dealing  with  things  near  home,  and  partly  in  order  to  bring 
the  past  and  present  into  immediate  juxtaposition.  The  char- 
acteristics of  glacial  erosion,  the  rounded  and  flowing  outlines,  the 
smoothed  and  polished  rocks,  the  parallel  striae  cut  by  the  stones 
and  boulders  held  in  the  bottom  of  the  ice,  are  all  to  be  found 
abundantly  in  the  glaciated  area  wherever  the  rocks  are  hard 
enough  to  receive  and  retain  the  marks  and  have  been  protected 
from  the  weather  since  the  withdrawal  of  the  ice;  in  many  in- 
stances, even  prolonged  exposure  to  weathering  has  not  sufficed 
to  destroy  the  markings.  The  striae,  which  are  parallel  in  small 
areas,  when  examined  on  a  large  scale,  are  found  to  be  arranged 
in  definite  systems,  which  show  the  outward  movement  of  the  ice 
from  the  centres  of  dispersion.  The  roches  moutonnees  and 
hummocks  of  rock,  gently  sloping  and  smoothed  on  the  side  against 


7/0  THE  QUATERNARY   PERIOD 

which  the  ice  impinged  (stoss  side)  abrupt  and  often  rough  on  the 
sheltered  side  (lee  side)  characterize  the  areas  of  Pleistocene 
glaciation,  just  as  they  do  the  rocky  beds  recently  abandoned  by 
retreating  glaciers.  In  short,  all  the  characteristic  features  of 
glacial  erosion,  which  can  be  reproduced  by  no  other  known  agency, 
occur  where,  by  the  theory,  they  should  be  found.  When  the 
contact  of  the  drift  with  the  smoothed  and  striated  ice-floor  can 
be  observed  (see  Fig.  69,  p.  160),  the  change  is  sudden  and  abrupt, 
the  drift  resting  upon  the  hard,  clean,  unaltered  rock,  not  at  all 
like  the  gradual  transition  of  soil,  subsoil,  and  rotten  rock  (cf .  Fig. 
34,  p.  1 06)  which  occurs  when  the  soil  arises  from  the  decompo- 
sition of  rock  in  place.  There  are  some  exceptions  to  this  rule, 
where  the  erosive  action  of  the  ice  was  feeble  and  insufficient  to 
remove  all  the  old  soil  and  rotten  rock,  but  such  exceptions  offer 
no  difficulty  of  explanation. 

The  drift  itself  is  as  convincing  in  its  testimony  of  glacial  de- 
position as  the  ice-floors  are  in  their  evidence  of  glacial  erosion. 
The  unstratified  drift,  made  by  the  ice  alone,  in  the  form  of 
moraines,  chiefly  terminal,  but  also  lateral  around  projecting 
lobes  of  the  ice,  are  highly  characteristic  of  glaciers,  as  are  the 
huge  erratic  and  perched  blocks,  which  often  have  travelled 
hundreds  of  miles.  The  ground  moraine,  or  till,  made  up  of 
finely  ground  rock-flour,  in  which  are  embedded  boulders  large 
and  small,  many  of  them  faceted,  smoothed,  and  striated,  as  only 
ice-worn  boulders  can  be,  and  spread  out  in  sheets  of  very  variable 
thickness,  its  large  boulders  often  deposited  high  above  the  points 
whence  they  were  taken,  testifies  eloquently  that  it  was  accumulated 
by  moving  ice,  which  alone  can  deposit  the  finest  and  the  coarsest 
materials  together  and  can  move  to  a  large  extent  independently 
of  the  topographical  features.  The  materials  of  the  till  are  mostly 
of  local  origin  and  have  travelled  but  a  few  miles,  but  there  is 
almost  always  a  greater  or  less  admixture  of  stones  from  a  long 
distance,  and  generally  these  are  smaller  in  proportion  to  the 
distance  travelled,  because  of  the  wear  to  which  they  have  been 
subjected. 


THE  PLEISTOCENE  OR   GLACIAL  EPOCH  771 

The  stratified  drift  is  no  less  indicative  of  glacial  action.  Whether 
advancing  or  retreating,  the  ice  margin  was  melting,  and  the  drift 
left  by  retreating  ice-sheets  was  more  or  less  worked  over  by  water. 
Subglacial  streams  discharging  in  valleys  made  valley  trains  (see 
p.  234),  and  water  descending  in  broad,  shallow  sheets,  where 
the  ice  ended  on  a  plain,  made  overwash  plains,  both  con- 
nected with  morainic  deposits  at  their  head.  Eskers  are  gravel- 
filled  subglacial  tunnels,  but  drumlins  and  kames  offer  much 
difficulty  of  interpretation  and  have  not  yet  been  explained  in  a 
way  that  commands  general  assent.  The  ice  barriers  frequently 
formed  lakes  large  and  small,  and  in  these  lakes  water-made  and 
ice-made  deposits  were  intimately  associated.  The  glacial  theory 
"  distinctly  affirms  that  rivers,  lakes,  the  sea,  icebergs,  and  pan-ice 
must  have  cooperated  with  glacier  ice  in  the  production  of  the 
drift,  each  in  its  appropriate  way  and  measure."  (Chamberlin 
and  Salisbury.) 

Finally,  the  evidence  of  the  fossils,  marine  and  terrestrial, 
animal  and  plant,  strongly  supports  the  glacial  theory,  by  demon- 
strating a  general  refrigeration  of  the  climate,  when  Arctic  mol- 
luscs lived  on  the  coasts  of  New  England  and  northern  Europe, 
and  Arctic  vegetation  covered  the  lands  in  low  latitudes,  and  Arctic 
mammals,  like  the  reindeer  and  the  musk-ox,  descended  to  the 
south  of  France  and  to  Arkansas.  The  testimony  is  thus  all  har- 
monious as  to  the  great  expansion  of  the  Pleistocene  glaciers  and 
ice-sheets. 

No  one  would  pretend  that  there  are  no  difficulties,  still  unex- 
plained, in  the  way  of  accepting  the  theory.  Some  of  these  have 
been  alluded  to  above;  another  is  the  enormous  thickness  of 
the  ice-sheets  required  by  the  evidence,  several  thousand  feet,  for 
the  glacial  marks  sweep  over  the  tops  of  the  highest  mountains  in 
New  England  and  New  York.  On  the  other  hand,  it  is  held  by 
physicists  that  i6op  feet  is  the  maximum  possible  thickness  of  ice, 
as  a  greater  amount  would  cause  the  bottom  to  melt  from  pressure, 
and  in  confirmation  of  this  it  is  pointed  out  that  the  Antarctic  ice- 
cap does  not  exceed  the  theoretical  maximum,  and  that  "  at  the 


7/2  THE    QUATERNARY   PERIOD 

present  day  no  ice  more  than  1600  feet  (thick)  has  been  recorded." 
(Ferrar.)  This  conflict  of  evidence  it  must  be  left  to  future  in- 
vestigations to  reconcile,  but  the  probable  solution  is  to  be  found 
in  the  temperature  relations  of  the  ice.  The  theoretical  maximum 
depends  upon  the  assumption  that  the  bottom  of  the  ice  is  at  or 
near  32°  F.,  but  if  it  were  considerably  below  this,  the  thickness 
might  be  greatly  increased. 

Distribution  of  Pleistocene  Glaciers.  — The  ice-sheets  were  local- 
ized, not  universal,  though  it  is  probable  that  the  entire  world  felt 
the  effects  of  the  lowered  temperature.  At  the  time  of  maximum 
extension  of  the  ice,  it  covered  nearly  4,000,000  square  miles  in 
North  America,  especially  toward  the  northeast  of  the  continent; 
in  Europe  the  ice-cap  which  covered  the  north,  Great  Britain, 
Scandinavia,  North  Germany,  etc.,  is  computed  at  770,000  square 
miles  (A.  Geikie),  and  the  Alps  were  deeply  buried  in  ice,  which 
flowed  far  out  over  the  surrounding  lowlands.  Glaciation  in  Asia 
was  principally  confined  to  the  mountain  ranges,  as  in  Asia  Minor; 
on  the  south  side  of  the  Himalayas  the  ice  descended  to  within 
3000  feet  of  the  present  sea-level.  On  Mt.  Kenya,  which  is 
almost  on  the  equator  in  eastern  Africa  and  still  has  glaciers,  the 
presumably  Pleistocene  ice  covered  the  whole  mountain  like  a  cap, 
descending  5400  feet  below  the  present  glacier  limit.  In  New 
Zealand  the  ice  also  descended  below  the  present  sea-level  and 
some  of  the  old  moraines  stand  in  the  sea.  The  Australian  Alps 
and  the  western  highlands  of  Tasmania  bore  extensive  glaciers, 
which,  however,  ended  1000  to  2000  feet  above  the  sea.  The 
glaciers  of  the  Patagonian  Andes  extended  to  the  foot  of  the 
mountains  and  out  upon  the  plains,  which  were  then  probably 
submerged.  Thus,  the  northern  hemisphere,  above  all,  North 
America,  was  the  region  of  the  most  extensive  Pleistocene  glacia- 
tion,  but  in  the  southern  hemisphere,  and  even  in  the  tropics,  its 
effects  are  visible. 

Glacial  and  Interglacial  Stages.  —  It  is  still  a  debated  question, 
whether  there  was  a  single  Glacial  age,  or  epoch,  during  which  the 
ice-sheet,  though  having  many  episodes  of  advance,  never  entirely 


GLACIAL  AND   INTERGLACIAL  STAGES 


773 


FlG.  311.  —  North  America  in  the  time  of  maximum  glaciation.  The  letters  indi- 
cate the  centres  of  dispersal  of  the  ice.  C  =  Cordilleran  Glacier;  K  =  Kewa- 
tin  Glacier;  L  =  Laurentide  Glacier ;  N.F.  =  Newfoundland  Glacier 


7/4  THE  QUATERNARY   PERIOD 

disappeared,  or  whether  there  were  several  distinct  Glacial  ages, 
when  the  snow  accumulated  to  form  an  ice-cap  which  spread  out 
widely  from  its  centres  of  dispersal,  separated  by  Interglacial  ages, 
when  the  ice-cap  completely  melted  away.  Among  students  of 
these  problems  the  present  tendency  is  to  accept  the  multiple 
character  of  the  Glacial  and  Interglacial  stages,  one  of  the  strongest 
arguments  for  which  is  the  evidence  of  fossils  showing  the  return 
of  mild  and  even  warm  conditions  in  some  of  the  Interglacial  ages. 
At  the  same  time,  there  is  much  difference  of  opinion  regarding 
the  number  of  these  disappearances  and  reappearances  of  the  ice. 

Obviously,  the  problem  is  one  of  much  difficulty,  because  each 
advance  of  the  ice  would  tend  to  remove  the  older  drift,  or  to  bury 
it  out  of  sight  under  new  accumulations,  when  erosion  was  in- 
sufficient to  remove  it.  Only  on  the  margins  of  the  successive 
ice-sheets,  where  they  but  partially  coincided,  should  we  expect  to 
find  the  evidence  preserved.  A  series  of  such  advances  and  re- 
treats of  the  ice  must  have  produced  an  exceedingly  complex  suc- 
cession of  stratified  and  unstratified  drift,  and  it  is  not  surprising 
that  the  interpretations  of  such  obscure  phenomena  should  differ. 
If  the  superposed  sheets  of  drift,  one  over  the  other  (an  arrange- 
ment which  is  not  questioned),  were  separated  by  long,  truly 
interglacial  times,  then  each  drift-sheet  in  turn  must  have  been 
exposed  to  the  denuding  agencies  for  corresponding  lengths  of 
time  and  should  exhibit  the  various  stages  of  chemical  and  me- 
chanical disintegration  proportioned  to  the  length  of  exposure. 
Between  the  earlier  and  later  drifts  there  should  be  manifest  differ- 
ences in  this  respect.  Further,  to  complete  the  evidence,  inter- 
glacial deposits,  with  testimony  of  climatic  amelioration  from  the 
fossils,  should  be  observed. 

The  following  comparative  table  gives  the  views  on  this  subject 
of  Professors  Chamberlin  and  Salisbury  (I)  for  the  Mississippi 
valley,  of  Professor  James  Geikie  (II)  for  Europe,  and  of  the 
Prussian  Geological  Survey  (III)  for  North  Germany. 


GLACIAL  AND   INTERGLACIAL  STAGES 


775 


6.  Later  Wisconsin  (glacial) 
e.  5th  Interglacial  stage 

5.  Earlier  Wisconsin  (glacial) 
d.  Peorian  (interglacial) 

4.  lowan  (glacial) 

c.  Sangamon   (intergla- 
cial) 
3.  Ittinoian  (glacial) 

b.  Yarmouth,  or  Bucha- 
nan (interglacial) 
2.  Kansan  (glacial) 


II 

6.  Upper  Turbarian  (glacial) 
e.  Upper  Forestian 

(interglacial) 

5.  Lower  Turbarian  (glacial) 
d.  Lower  Forestian 

(interglacial) 

4.  Mecklenburgian  (glacial) 
c.  Neudeckian    (inter- 
glacial) 

3.  Polandian  (glacial) 
b.  Helvetian  (intergla- 
cial) 
2.  Saxonian  (glacial) 


in 


a.  Aftonian  (interglacial)          a.  Norfolkian      (inter- 
glacial) 

t.  Sub-Aftonian,  or  Jerseyan       i.  Scanian  (glacial) 
(glacial) 


3.  Last  Glacial 
age 

b.  Later  Inter- 
glacial 
2.  Main    Glacial 

age 

a.  Earlier   In- 
terglacial 
i.  First     Glacial 
age 


In  the  Alps,  Professor  Penck  has  determined  three  Glacial 
stages,  and  Huntington  has  found  evidence  of  five  in  the  moun- 
tains of  Turkestan. 

The  table  must  not  be  understood  as  attempting  to  correlate  the 
events  in  Europe  and  America,  as  that  would  be  premature.  At 
any  rate,  the  events  in  the  two  continents  did  not  correspond  in 
the  way  which  the  table  seems  to  imply.  For  example,  in  the 
last  of  Geikie's  Glacial  stages,  the  Upper  and  Lower  Turbarian, 
the  glaciers  are  described  as  being  restricted  to  the  high  lands  and 
mountains,  not  forming  a  general  ice-sheet.  The  Mecklenburgian 
and  Polandian  more  nearly  correspond  to  the  Wisconsin. 

American.  —  At  the  time  of  greatest  expansion  the  ice-sheets 
covered  nearly  all  of  North  America  down  to  lat.  40°  N.,  an- 
ticipating the  conditions  of  modern  Greenland,  though  on  a  vastly 
larger  scale.  Three  distinct  centres  or  areas  of  maximum  accumu- 
lation of  the  ice  have  been  identified  in  northern  Canada,  from 
which  the  great  ice-sheets  flowed  outward  in  all  directions,  though 
each  one  of  the  sheets  had  its  own  episodes  of  advance  and  retreat, 
so  that  the  same  region  of  country  was  overflowed,  now  by  exten- 


776  THE  QUATERNARY  PERIOD 

sions  from  one  sheet,  and  again  by  those  from  another.  One  oi 
these  centres  of  accumulation  and  distribution  lay  to  the  north  of 
the  St.  Lawrence  River,  and  on  the  highlands  of  Labrador,  send- 
ing its  ice-mantle  southward  over  the  Maritime  Provinces,  New 
England,  and  the  Middle  States,  as  far  west  as  the  Mississippi, 
River.  This  is  called  the  Laurentide,  or  Labradorean  Ice-sheet  or 
Glacier.  A  second  centre  was  near  the  west  coast  of  Hudson's  Bay, 
and  from  this  area  the  ice  streamed  outward  in  all  directions  west- 
ward toward  the  Rocky  Mountains,  northward  to  the  Arctic  Ocean, 
eastward  into  Hudson's  Bay,  southward  through  Manitoba  into  the 
Dakotas,  Minnesota,  and  Iowa.  This  great  ice-sheet  has  been 
named  the  Keewatin  Glacier,  from  the  Canadian  district  of  that 
name.  A  third  centre  was  formed  by  the  Cordillera  of  British 
Columbia,  which  for  a  distance  of  1200  miles  was  buried  under 
a  great  ice-mantle  that  flowed  both  to  the  northwestward  and 
southeastward.  To  these  large  and  well-defined  centres  should 
probably  be  added  a  fourth,  Newfoundland,  from  which,  there  is 
reason  to  think,  came  the  ice  which  crossed  Cape  Cod  and  extended 
over  Nantucket  Sound  to  the  island. 

In  addition  to  the  great  northern  ice-cap,  large  local  glaciers 
accumulated  in  all  the  western  mountains  ranges:  the  Rocky 
Mountains,  as  far  south  as  New  Mexico,  the  Uinta,  Wasatch,  and 
Bighorn  ranges,  and  the  Sierras  and  the  Cascades,  even  the  San 
Francisco  Mountains  of  northern  Arizona,  and  the  other  ranges  of 
the  western  Cordillera,  all  bore  thousands  of  glaciers.  In  these 
mountains  almost  every  valley  shows  the  evidences  of  former 
glaciation,  in  cirques  at  the  head,  in  the  smoothed  and  striated 
walls  and  bed,  and  in  the  moraines  at  the  foot.  The  mountains 
of  Alaska  were  heavily  glaciated,  but  not  the  lowlands. 

In  the  Mississippi  valley  the  Pleistocene  sequence  is  best  dis- 
played. The  first  known  advance  of  the  ice  (sub-Aftonian)  is 
registered  in  much  disintegrated  drift,  which  is  exposed  by  denu- 
dation in  Iowa.  A  similar  sheet  of  very  old  and  much  worn 
drift  which  extends  from  beneath  much  later  drift  in  New  Jersey 
and  Pennsylvania  may  be  of  the  same  date. 


GLACIAL  AND   INTERGLACIAL  STAGES  777 

A  great  retreat  of  the  ice,  if  not  its  entire  disappearance,  brought 
about  interglacial  conditions  at  least  in  the  Mississippi  valley 
(Af Ionian  stage).  The  surface  exposed  by  the  retiring  ice  was 
occupied  by  vegetation,  which  in  many  places  in  Iowa  formed 
accumulations  of  peat,  sometimes  to  the  depth  of  25  feet.  The 
Kansan  stage  represents  the  greatest  extension  southwestward  of 
the  ice-sheet,  when  the  glacier  descended  from  the  north  (perhaps 
the  Keewatin  glacier)  nearly  to  the  mouth  of  the  Ohio  River,  and 
spread  across  Iowa  and  Missouri  far  into  Kansas  and  Nebraska. 
East  of  the  Mississippi  the  Kansan  drift  has  not  been  recognized. 
Again  came  a  time  of  retreat,  when  soil  was  formed,  and  the 
Kansan  drift  was  eroded  and  deeply  decomposed,  and  peat  de- 
posited upon  it  (  Yarmouth,  or  Buchanan  stage).  A  renewed  ex- 
tension of  the  ice  laid  down  the  Illinois  till-sheet,  which  is  found 
not  only  in  that  State,  but  in  Iowa  also,  overlapping  the  Kansan 
drift,  and  it  extends  to  Wisconsin,  eastward  into  Ohio  and 
Indiana,  and  passes  under  later  till-sheets  to  the  northeast. 
This  Illinoian  drift  appears  to  be  derived  from  the  Laurentide 
glacier. 

The  Sangamon  interglacial  deposits  are  of  peat,  old  soil,  etc. 
A  fourth  recrudescence  of  the  glacier  (lowan  stage)  occasioned 
the  deposit  of  another  till-sheet,  of  an  extent  not  yet  determined, 
which  is  best  displayed  in  northeastern  Iowa,  where  it  is  intimately 
associated  with  the  largest  accumulations  of  loess  in  the  Mississippi 
valley.  The  lowan  till-sheet  is  followed  (Peorian  stage)  by 
interglacial  deposits  which  are  perhaps  contemporaneous  with 
those  so  well  shown  near  Toronto  on  Lake  Ontario.  The  latter 
beds  form  a  succession  of  fine  shales  and  sand  that  lie  between 
two  sheets  of  glacial  drift  and  are  divisible  into  two  parts;  the 
lower  (Don  formation)  contains  many  fossils  of  which  the  plants, 
such  as  the  Pawpaw  and  the  Mock-orange,  indicate  a  climate 
distinctly  warmer  than  that  of  the  region  at  present  and  about 
like  that  of  the  middle  United  States.  The  fossils  of  the  upper 
part  (Scarboro  formation)  indicate  a  cold  temperate  climate  and 
herald  the  approach  of  a  renewed  glaciation,  which  in  turn  is 


7/8  THE  QUATERNARY   PERIOD 

recorded  in  the  overlying  till.  Such  facts  are  difficult  to  explain, 
except  as  the  result  of  truly  interglacial  conditions.  The  Wiscon- 
sin stages  are  much  the  most  conspicuous  and  best  known  of  all, 
and  the  sheets  of  till  and  drift  are  far  thicker  than  those  of  the  other 
Gfeacial  stages.  Especially  conspicuous  is  the  great  terminal 
moraine,  or  rather  morainic  belt,  which  itself  records  many  episodes 
in  the  history  of  the  ice  and  which  has  been  traced  across  the 
continent.  Beginning  at  Nantucket,  the  moraine  runs  through 
Long  Island  and  Staten  Island  to  New  Jersey,  which  it  crosses  into 
Pennsylvania;  here  it  bends  sharply  to  the  northwest  to  the 
boundary  of  New  York,  but  turns  southwest  almost  at  a  right 
angle,  reaching  nearly  to  the  Ohio  River  at  Cincinnati.  It  crosses 
in  an  irregular,  sinuous  line  the  states  of  Indiana,  Illinois,  Iowa, 
and  thence  northwestward  through  the  Dakotas  into  Montana, 
where  it  nearly  follows  the  international  boundary  to  the  Pacific 
coast  mountains  (see  Fig.  311). 

In  the  Eastern  States  there  is  no  such  clear  indication  of  several 
successive  ice-invasions  as  in  the  Mississippi  valley,  the  Wisconsin 
erosion  and  its  thick  mantle  of  drift  removing  or  obscuring  the 
records  of  earlier  events.  The  remnants  of  very  ancient  till-sheets 
in  New  Jersey  and  Pennsylvania  have  been  mentioned,  and  in  New 
York  and  New  England  not  more  than  two  or  three  invasions  can 
be  identified.  In  part,  this  may  be  due  to  the  later  development 
of  the  Laurentide  glacier.  The  geologists  of  the  Canadian  Survey 
believe  that,  "  beginning  at  the  west  and  going  eastward,  these 
three  great  glaciers  [i.e.  the  Cordilleran,  Keewatin,  and  Laurentide] 
reached  their  widest  extent  and  retired  in  succession."  (Tyrrell.) 

The  final  retreat  of  the  ice  was  by  slow  stages  with  many  halts. 
In  the  central  West  are  preserved  many  lines  of  moraine,  with 
kettle-holes,  kames,  and  drumlins,  which  mark  readvances  and 
pauses  in  the  retreat. 

Probably  every  retreat  of  the  ice  was  accompanied  by  the 
formation  of  barrier  lakes  held  in  by  the  ice-front,  but  only  those 
of  the  final  recession  have  left  intelligible  records  of  themselves. 
A  comparatively  simple  case  is  that  of  Lake  Agassiz,  which 


GLACIAL  AND   INTERGLACIAL   STAGES  779 

covered  Manitoba  and  Minnesota  with  a  great  sheet  of  water,  700 
miles  from  north  to  south.  The  lake  was  formed  when  the 
Keewatin  glacier  in  its  retreat  had  freed  nearly  all  of  Manitoba 
from  the  ice  and  was  joined  by  the  Laurentide  glacier  from  the 
east,  making  a  great  wall  of  ice  which  shut  off  the  drainage  toward 
Hudson's  Bay,  while  to  the  south  high  land  held  back  the  lake  in 
that  direction.  The  water  of  the  lake  rose  until  it  overflowed  the 
lowest  point  in  the  southern  barrier  and  formed  a  river  (Warren 
River,  now  abandoned)  which  joined  the  Mississippi.  The  level 
of  the  lake  was  gradually  lowered  as  Warren  River  deepened  its 
bed,  and  was  finally  discharged  when  the  retreat  of  the  ice  opened 
the  course  to  Hudson's  Bay. 

The  history  of  the  great  Laurentian  lakes  is  exteemely  complex 
and  is  slowly  being  deciphered  by  the  combineji^eTforts^of  many 
workers.  The  changing  positions  of  thKJ0&es  which  projected-— "" 
from  the  ice-front,  the  numerous  basins/now  connected  and  now 
severed,  as  the  water  rose  and  fell,  combmed  with  slow  diastrophic 
movements,  make  up  a  very  intricate  succession  of  temporary 
lakes  and  shifting  outlets.  Considerations  of  space  forbid  more 
than  a  brief  and  simplified  outline  of  this  interesting  story.  When 
the  ice  had  retreated  so  far  as  to  uncover  land  to  the  north  of  the 
divide  between  the  basins  of  the  St.  Lawrence  and  the  Mississippi 
and  eastward  the  Hudson,  the  waters  produced  by  the  melting  of 
the  ice  were  held  in  between  those  divides  and  the  ice-front,  form- 
ing a  great  number  of  small  lakes  from  New  York  to  Minnesota, 
three  of  which  require  mention  as  the  earliest  recorded  stages  of 
the  Great  Lakes  at  a  time  when  most  of  their  present  basins  was 
filled  by  the  ice.  Of  these  three  lakes  which  embraced  three  promi- 
nent lobes  of  the  ice-front,  one  was  in  the  axis  of  Lake  Superior, 
one  at  the  southern  end  of  Lake  Michigan,  and  the  third  in  a  line 
with  Lake  Erie,  but  west  of  it,  and  each  discharged  by  a  separate 
outlet  to  the  Mississippi. 

Omitting  several  intermediate  stages,  and  coming  to  a  time  when 
the  basins  of  Lake  Michigan  and  Lake  Erie  and  part  of  that  of 
Lake  Ontario  had  been  freed  by  the  retreat  of  the  ice,  we  find  Lake 


780  THE  QUATERNARY  PERIOD 

Whittlesey,  which  filled  the  basin  of  Lake  Erie,  but  was  very  much 
larger;  it  was  connected  along  the  ice-front  to  the  north  with  the 
crescentic  Lake  Saginaw,  that  discharged  westward  into  Lake 
Chicago,  a  larger  Lake  Michigan,  which  retained  its  original  outlet 
to  the  Mississippi.  Lake  Whittlesey  was  succeeded  by  Lake 
Warren,  which  was  formed  by  a  junction  with  Lake  Saginaw  on 
the  northwest,  and  by  an  extension  along  the  ice-front,  eastward 
into  New  York  and  northeastward  into  Ontario,  but  still  discharg- 
ing westward  to  Lake  Chicago  and  the  Mississippi.  Later,  how- 
ever, Lake  Warren  extended  into  central  New  York  and  emptied 
by  way  of  the  Mohawk  into  the  Hudson.  The  condition  of  the 
Erie  basin  now  becomes  obscure,  for  when,  after  several  changes, 
Lake  Iroquois  was  established,  it  was  merely  an  enlarged  Lake 
Ontario,  and  the  three  upper  lakes,  now  clear  of  ice,  had  coalesced 
into  the  great,  irregular  Lake  Algonquin,  which  had  lost  connec- 
tion with  the  Mississippi  and  discharged  into  Lake  Iroquois,  at 
first  probably  by  the  line  of  the  St.  Clair  and  the  Erie  basin  and 
later  probably  along  the  course  of  the  present  Trent  across  the 
province  of  Ontario.  As  the  Mohawk  outlet  persisted,  the  entire 
discharge  of  the  lakes  was  into  the  Hudson,  but  whether  the  Erie 
basin  was  filled  with  water  as  it  certainly  was  somewhat  later,  has 
not  been  clearly  made  out.  Lake  Algonquin  was  eventually  cut 
off  from  its  connection  with  Lake  Iroquois  and  found  an  outlet  by 
way  of  Lake  Nipissing  and  the  Ottawa  River,  thus  severing  the 
series  of  lakes  into  two  independent  systems,  while  the  lowering 
of  the  water  level  in  Lake  Algonquin  had  divided  it  into  three 
lakes  which  had  very  nearly  the  present  outlines  of  Superior, 
Michigan,  and  Huron.  The  final  stage  in  the  history  is  connected 
with  the  Champlain  subsidence  and  reelevation  presently  to  be 
described. 

The  Champlain  Subsidence.  —  In  the  Glacial  epoch  a  subsi- 
dence had  begun  which  continued  until  it  became  a  very  marked 
feature  of  the  times.  The  depression  was  greatest  toward  the 
north  and  especially  in  the  valley  of  the  St.  Lawrence;  at  the  mouth 
of  the  Hudson,  for  example,  the  land  stood  about  70  feet  below 


THE  NON-GLACIATED   AREAS  78 1 

its  present  level,  on  the  coast  of  Maine  150  to  300  feet,  and  in  the 
St.  Lawrence  valley  500  to  600  feet  below.  The  consequence  of 
the  depression  was  that  an  arm  of  the  sea  extended  up  the  St. 
Lawrence  to  Lake  Ontario,  which  was  little,  if  at  all,  above  sea- 
level.  Two  long  and  narrow  gulfs  reached  out  from  this  sea,  one 
up  the  valley  of  the  Ottawa  River  and  the  other  over  Lake  Cham- 
plain,  while  the  Hudson  River  appears  to  have  been  converted 
into  a  narrow  strait  connecting  the  marine  waters  of  the  Champlain 
basin  with  those  of  the  Atlantic  where  New  York  Bay  now  is. 
The  lines  of  raised  beaches,  the  sands  and  gravels  filled  with  marine 
shells,  and  the  bones  of  whales  and  walruses,  are  the  present  evi- 
dences of  this  submergence. 

The  Champlain  subsidence  and  the  reelevation  which  expelled  the 
sea  from  the  Hudson  and  Ottawa  rivers  and  from  the  basins  of 
Lakes  Ontario  and  Champlain  also  affected  the  Great  Lakes.  Lake 
Iroquois  had  found  a  lower  outlet  than  the  Mohawk,  when  the  ice 
withdrew  from  the  Adirondacks,  into  Lake  Champlain,  which  then 
discharged  into  the  Hudson,  because  of  the  ice  barrier  to  the  north. 
Subsequently  this  outlet  of  Lake  Iroquois  was  into  the  Champlain 
sea,  when  the  subsidence  had  drowned  the  St.  Lawrence  valley. 
Just  when  the  Niagara  began  to  flow  is  not  certain,  nor  when  the 
basin  of  Lake  Erie  was  refilled,  if  it  were  ever  emptied,  but  so  long 
as  the  upper  lakes  had  their  outlet  through  the  Ottawa,  the  Niagara 
carried  only  the  discharge  of  Lake  Erie.  The  elevation  which 
followed  the  Champlain  subsidence  was  accompanied  by  a  slight 
tilt  of  the  lake  region  to  the  southwest,  cutting  off  the  Ottawa  out- 
let and  causing  the  three  upper  lakes  to  discharge  into  Lake  Erie. 
The  consequent  change  in  the  volume  of  the  Niagara  is  registered 
in  the  sudden  increase  in  the  width  of  its  gorge. 

THE  NON-GLACIATED  AREAS 

In  the  non-glaciated  parts  of  the  continent  occur  stratified 
Pleistocene  deposits,  which  it  is  very  difficult  to  associate  with  the 
events  taking  place  in  the  glaciated  area,  for  lack  of  any  means 


782  THE  QUATERNARY   PERIOD 

of  direct  comparison.  On  the  Atlantic  slope  from  New  Jersey 
southward  a  succession  of  Pleistocene  gravels  and  sands  constitutes 
the  Columbian  formation,  so  called  because  of  its  typical  develop- 
ment in  the  District  of  Columbia.  These  deposits  are  differently 
interpreted  by  those  who  have  examined  them,  but  they  appear  to 
be  largely  fluviatile  and  subaerial  much  like  the  Pliocene  Lafayette. 
Three  parts  of  the  Columbia  formation  have  been  recognized 
which  by  some  authorities  are  regarded  as  three  successive  de- 
pressions and  emergences  from  the  sea,  but  the  difficulties  in  this 
interpretation  are  such  that  a  non-marine  origin  is  more  probable. 
On  the  other  hand,  marine  fossils  in  the  uppermost  of  the  three 
divisions  in  the  Chesapeake  Bay  region  indicate  some  depression 
in  that  area.  All  the  divisions  contain  large  boulders  transported 
by  floating  ice. 

Over  the  Great  Plains  from  South  Dakota  to  Texas  the  surface 
formation  is  a  fine,  calcareous,  sandy  clay,  which  lies  unconform- 
ably  upon  the  eroded  surfaces  of  older  strata,  from  the  Blanco  to 
the  Cretaceous.  This  formation  has  been  called  the  Sheridan  stage 
("Equus  Beds"),  from  Sheridan  County,  Nebraska,  where  it  is  typi- 
cally displayed.  It  is,  to  a  large  extent,  of  aeolian  origin  and  in 
places  contains  great  numbers  of  fossil  bones.  In  South  Dakota  the 
Sheridan  passes  under  a  drift  sheet,  and  probably  it  corresponds  to 
one  of  the  earlier  interglacial  stages. 

In  the  Great  Basin,  the  later  Pleistocene  had,  temporarily  at 
least,  a  much  less  arid  climate  than  at  present,  as  is  indicated  by 
the  many  lakes  which  it  contained,  and  two  of  these,  Lakes  Bonne- 
ville  andLahontan,  were  very  large  (see  p.  219).  The  former,  which 
was  in  the  eastern  part  of  the  Great  Basin,  had  an  outlet  north- 
ward to  the  Snake  River,  and  had  two  periods  of  expansion, 
separated  by  one  of  almost  complete  desiccation.  Lake  Lahontan, 
which  had  no  outlet,  had  similar  episodes  of  rise  and  fall. 

On  the  Pacific  coast,  marine  Pleistocene  in  two  unconformable 
stages  occurs  in  southern  California;  the  fauna  of  the  lower  stage 
has  still  a  somewhat  northern  character,  but  in  the  upper  stage 
the  water  became  warmer  than  it  is  now,  and  tropical  species 


THE  NON-GLACIATED   AREAS  783 

which  no  longer  live  on  the  California  coast  were  present.  Pleis- 
tocene movements  affected  the  Pacific  coast  to  the  amount  of 
3000  feet  in  the  Inyo  Mountains  of  California,  200  feet  or  more 
on  the  coast  of  Oregon,  and  4000  feet  in  southeastern  Alaska,  and 
increased  the  height  of  the  Sierra,  Wasatch,  and  Basin  ranges  and 
of  the  high  plateaus  of  Utah  and  Arizona. 

The  volcanic  activity  which  had  been  so  very  striking  during 
the  Tertiary  period  in  the  western  region  continued  into  the  Pleis- 
tocene, as  is  to  be  seen  in  the  lava  flows  of  the  Great  Basin,  Arizona, 
New  Mexico,  Idaho,  all  the  Pacific  States,  and  Alaska. 

The  late  Pleistocene  was  a  time  of  ameliorated  climate  and 
heavy  rainfall,  when  the  flooded  rivers  moved  sluggishly,  owing 
to  the  diminished  slope,  and  spread  sheets  of  sands,  gravels,  and 
clays  over  their  flood  plains  and  in  their  estuaries,  through  which 
they  have  subsequently  cut  terraces,  when  elevation  had  given 
them  renewed  power. 

The  events  of  the  Glacial  epoch,  and  the  diastrophic  movements 
which  accompanied  and  followed  it,  have  had  the  most  impor- 
tant and  widespread  effects  upon  the  topography  of  the  glaciated 
regions.  The  sheets  of  drift,  stratified  and  unstratified,  have  com- 
pletely changed  the  surface  of  the  country,  and  by  filling  up  the 
pre-Glacial  valleys,  have  revolutionized  the  drainage,  only  the 
largest  streams  being  able  to  regain  their  old  courses.  Innumer- 
able lakes,  large  and  small,  were  formed  in  depressions,  rock  basins, 
and  behind  morainic  dams,  the  contrast  between  the  glaciated  and 
non-glaciated  regions  in  regard  to  the  number  of  lakes  in  each 
being  very  striking. 

The  Pleistocene  was  closed  and  the  Recent  epoch  inaugurated 
by  a  movement  of  upheaval  which  raised  the  continent  to  its 
present  height.  These  Pleistocene  movements  have  been  corre- 
lated with  the  accumulation  and  removal  of  the  ice,  and  it  is  at 
least  a  curious  coincidence  that  the  continent  should  have  slowly 
sunk  under  the  maximum  load  of  ice  and  have  risen  again  after 
the  ice  had  melted.  These  movements  were  greatest  where  the 
ice  was  thickest. 


784  THE  QUATERNARY   PERIOD 

FOREIGN  PLEISTOCENE 

The  Glacial  epoch  in  Europe  ran  a  course  remarkably  parallel 
with  its  history  in  North  America.  After  the  first  Glacial  and 
Interglacial  stages  (perhaps  representing  the  sub-Aftonian  and 
Aftonian),  came  the  time  of  the  greatest  expansion  of  the  ice,  the 
Saxonian  stage  of  Geikie,  which  is  believed  to  correspond  to  the 
Kansan  of  America.  The  great  centre  of  dispersion  was  the  Scan- 
dinavian peninsula,  where  the  ice  was  probably  6000  to  7000  feet 
thick,  and  whence  it  flowed  outward,  filling  the  Baltic  and  North 
seas,  and  covering  Finland,  northwestern  Russia,  the  lowlands  of 
Germany,  and  extending  to  England.  The  Highlands  of  Scotland 
were  a  secondary  centre,  its  ice-sheets  flowing  into  the  North  Sea 
and  uniting  with  those  from  Scandinavia,  and  westward  to  the 
ocean.  The  Irish  Channel  was  also  filled  up.  From  the  south- 
west of  Ireland  to  the  North  Cape  of  Norway,  a  distance  of  2000 
miles,  was  probably  a  continuous  wall  of  ice  fronting  the  sea,  like 
that  which  now  surrounds  the  Antarctic  continent.  At  the  same  time 
the  Alps  were  the  seat  of  enormous  glaciers,  only  the  highest  peaks 
rising  above  the  sheets  of  ice,  and  these  great  glaciers  extended 
far  out  from  the  foot  of  the  mountains,  covering  all  the  lowlands 
of  Switzerland  and  extending  from  Austria  and  Bavaria,  on  the 
east,  to  the  Rhone  valley  near  Lyons,  on  the  west.  The  high 
plateau  of  Asia,  from  the  Himalaya  to  Bering's  Sea,  shows  evi- 
dences of  glaciation,  and  great  valley  glaciers  were  formed  on  the 
southern  slopes  of  the  Himalayas,  extending  in  some  places  to 
within  2000  feet  of  the  sea-level. 

A  second  great  Glacial  stage  (the  fourth  Glacial  or  Mecklen- 
burgian  of  Geikie)  is  generally  recognized  in  Europe  and  corre- 
lated with  the  Wisconsin  stage  of  America.  This  ice-sheet  was 
much  less  extensive  than  the  former  one,  being  confined  prin- 
cipally to  Finland,  Scandinavia,  the  Baltic  Sea,  which  it  filled, 
Denmark,  and  a  little  of  north  Germany.  The  prevailing  motion 
of  this  sheet  was  from  east  to  west.  The  Alpine  glaciers  were  also 
extended  far  beyond  their  present  limits,  but  not  so  widely  as 
before. 


PLEISTOCENE  LIFE  785 

Following  the  Mecklenburgian  stage  came  alternating  periods 
of  milder  and  colder  climates,  the  fourth  and  fifth  Interglacial, 
and  fifth  and  sixth  Glacial  stages  of  Geikie,  the  Glacial  stages 
marked,  not  by  the  formation  of  great  continental  ice-sheets,  but 
by  the  extension  or  recrudescence  of  local  snow-fields  and  valley 
glaciers.  Oscillations  of  level  also  occurred  along  the  coasts, 
allowing  limited  transgressions  of  the  sea. 

The  Pleistocene  of  the  other  continents  has  been  considered 
in  the  general  introductory  statements. 

Causes  of  the  Glacial  Climates.  —  This  is  but  a  special  case  of 
the  general  problem  of  climatic  changes  in  the  history  of  the  earth. 
We  now  know  that  the  Pleistocene  glaciation  is  not  what  it  was 
once  supposed  to  be,  a  unique  phenomenon  in  .geological  history. 
On  the  contrary,  in  at  least  three  other  periods,  Algonkian,  Cam- 
brian, and  Permian,  we  have  found  evidence  of  glaciation  rivalling 
or  equalling  that  of  the  Pleistocene.  At  other  times  mild  and 
equable  climates  have  extended  far  into  the  Arctic  regions. 

In  attempting  to  explain  these  remarkable  changes,  three  kinds 
of  agencies  have  been  called  upon:  (i)  Astronomical,  or  change 
in  the  position  of  the  earth's  axis,  in  the  eccentricity  of  its  orbit, 
in  the  heat  radiated  from  the  sun,  etc.,  but  these  are  now  very 
generally  discarded.  (2)  Geographical,  or  changes  in  the  arrange- 
ment of  land  and  sea,  in  the  height  of  the  land,  direction  of  ocean 
currents,  etc.,  but  none  of  these  seems  to  afford  any  real  help  in 
solving  the  problem.  (3)  The  most  promising  and  widely  favoured 
agency  of  climatic  changes  is  now  sought  in  variations  in  the  com- 
position of  the  atmosphere,  especially  in  the  quantity  of  carbon 
dioxide  present.  Whatever  may  eventually  come  of  this,  it  has 
not  yet  advanced  beyond  the  stage  of  an  interesting  speculation. 

Pleistocene  Life 

The  frequent  and  extreme  climatic  changes,  of  which  we  find 
such   abundant   evidence   in   the   Pleistocene,    caused   extensive 
migrations  and  dispersions  of  animals  and  plants,  and  the  rapid 
3E 


786  THE  QUATERNARY   PERIOD 

succession  of  Arctic  and  temperate  forms  in  the  same  region. 
Many  land  bridges  between  different  continents,  or  between  con- 
tinents and  what  are  now  islands,  were  not  severed  until  the  end 
of  the  Pleistocene,  permitting  migrations  which  are  no  longer 
possible.  The  extension  of  the  ice-sheets  brought  with  them 
Arctic  floras  and  faunas,  which  retreated  in  the  Interglacial  times, 
while  temperate  animals  and  plants  spread  northward  to  replace 
them.  These  conditions  produced  a  very  severe  struggle  for  exist- 
ence and  were  fatal  to  a  great  many  large  mammals,  causing 
numerous  extinctions  over  the  larger  part  of  fhe  world. 

Pleistocene  Plants  are  almost  all  of  the  same  species  as  those 
now  living,  but  they  are  often  very  differently  distributed.  The 
Glacial  cold  greatly  impoverished  the  European  forests,  which  in 
the  Pliocene  had  many  kinds  of  trees  now  found  only  in  North 
America  or  in  eastern  Asia.  Owing  to  the  east  and  west  trend  of 
the  European  mountains,  the  forests  could  not  retire  before  the 
ice,  and  return,  as  they  did  in  the  United  States,  where  no  moun- 
tain barriers  shut  them  off  from  the  warm  latitudes  of  the  south. 
When  the  ice-sheets  melted  and  the  climate  was  ameliorated,  the 
Arctic  flora  and  fauna  were  forced  to  retreat  in  their  turn;  they 
did  so  not  only  by  following  the  retiring  ice-front,  but  also  by 
ascending  the  mountains  as  the  latter  were  cleared  of  ice.  Thus, 
high  mountains  in  the  northern  hemisphere  have  on  their  upper 
slopes  an  Arctic  flora  and  fauna,  separated,  perhaps,  by  hundreds 
of  miles  from  the  nearest  similar  colony.  For  example,  the  higher 
parts  of  the  White  Mountains  have  plants  which  do  not  occur 
on  the  lowlands  until  Labrador  is  reached,  and  the  snowy  Alps 
have  truly  Arctic  plants  and  animals.  In  Europe  the  disappear- 
ance of  the  ice-sheets  was  followed  by  a  dry  climate,  when  a  fauna 
like  that  of  the  Russian  Steppes  extended  to  western  Europe. 

Of  Pleistocene  animals  it  is  only  the  mammals  that  require 
mention.  Here  also  we  find  the  same  mingling  of  northern  and 
southern  forms,  and  association  of  types  now  widely  separated. 
North  America  had  Mastodons  (i.e.  an  extinct  type  of  elephant 
which  had  smaller  and  simpler  grinding  teeth  than  the  true  ele- 


PLEISTOCENE   LIFE  787 

phants),  Elephants,  Horses,  Tapirs,  the  first  Bisons  (which  had 
migrated  from  the  Old  World,  as  had  several  kinds  of  Deer  and 
the  Musk-ox),  Peccaries  and  huge  Llamas,  Wolves,  great  Cats 
as  large  as  lions,  Sabre-tooth  Tigers,  and  the  first  Bears,  also 
immigrants.  A  great  variety  of  Rodents  is  found,  most  of  them 
kinds  which  still  inhabit  the  country,  but  mingled  with  these  are 
South  American  forms  like  the  Cavies  and  Water  Hog  (Hydro- 
chasms),  and  the  giant  Beaver  (Castor vides)  is  an  altogether 
peculiar  form.  Enormous  Ground  Sloths  (Megatherium,  Mylodon, 
and  Megalonyx)  and  Glyptodonts  show  that  the  way  of  migra- 
tion from  the  south  was  still  open. 

In  South  America  were  an  astonishing  number  of  huge  Eden- 
tates: Sloths  nearly  as  large  as  elephants,  Ant-eaters,  and  a  mar- 
vellous variety  of  giant  Armadillos.  Some  of  the  immigrants 
from  the  north,  which  are  now  extinct,  still  lingered  in  the  Pleis- 
tocene, such  as  the  Mastodons  and  Horses,  as  did  also  some  of 
the  peculiarly  South  American  hoofed  animals,  Typotheria, 
Toxodontia,  and  Litopterna,  the  ancestors  of  which  may  be  traced 
back  almost  continuously  to  the  Notostylops  beds  of  the  Paleocene. 

Europe  was  the  meeting-ground  of  mammalian  types  now 
widely  scattered.  Together  with  Arctic  forms  like  the  Reindeer, 
Musk-ox,  Mammoth  (Hairy  Elephant),  Hairy  Rhinoceros,  and 
the  Lemming  (Myodes)  were  found  southern  animals,  such  as 
the  Hippopotamus,  several  kinds  of  Elephants  and  Rhinoceroses, 
Lions,  and  Hyaenas,  and  likewise  species  allied  to  those  still  liv- 
ing in  Europe,  such  as  the  huge  Cave  Bear,  the  gigantic  Irish  Deer 
(Megaceros),  and  great  Oxen.  Northern  Africa  was  joined  to 
Europe  by  way  of  Malta  and  Sicily,  and  probably  at  Gibraltar 
also,  permitting  frequent  intermigrations.  The  junction  of  Ire- 
land with  Great  Britain  and  of  both  with  the  continent  continued 
until  after  the  ice-sheets  had  disappeared,  so  that  these  islands, 
and  especially  Great  Britain,  were  stocked  by  the  continental 
animals  and  plants. 

In  the  Pleistocene  of  India  are  found  many  animals  which  now 
live  only  in  Africa,  §uch.  as,  the  Baboon,  Spotted  Hyaena,  etc. 


788  THE  QUATERNARY   PERIOD 

Australia  had  a  Pleistocene  mammalian  fauna  composed,  with 
the  exception  of  the  Wild  Dog  (Canis  dingo),  of  Marsupials,  allied 
to  those  which  still  inhabit  that  region,  but  many  of  them  were  of 
vastly  greater  size  than  the  living  forms. 

The  Pleistocene  Mammals  are  remarkable  for  the  great  size 
which  distinguishes  many  of  them,  and  it  is  just  these  which  have 
passed  away,  leaving  a  world  that  is  "  zoologically  impoverished, " 
but  is  nevertheless  a  much  more  agreeable  place  of  residence  with- 
out them.  Further  we  note,  (i)  that  the  Pleistocene  mammals 
are  in  general  like  the  smaller  forms  which  have  succeeded  them 
in  the  same  regions,  but  (2)  that  in  Europe  and  North  America 
there  was  a  commingling  of  types  now  found  only  in  widely  sepa- 
rated regions. 

In  Europe  Man  first  appears  in  the  early  Pleistocene.  It  is 
altogether  probable  that  the  human  race  originated  in  Asia,  quite 
aside  from  the  doubtful  testimony  of  the  Pliocene  Pithecanthropus, 
and  reached  Europe  by  migration.  The  most  ancient  European 
men,  such  as  the  "Man  of  Spy  "  in  Belgium,  are  of  a  much  lower 
type  physically  than  those  of  the  later  Pleistocene;  only  in  the 
Recent  age  do  human  remains  and  implements  become  at  all 
common  and  so  the  Recent  or  Postglacial  time  is  frequently  called 
the  Human  Period,  the  description  of  which  is  rather  in  the  prov- 
ince of  prehistoric  Archaeology  than  in  that  of  Geology.  Whether 
Man  reached  North  America  in  the  Pleistocene  is  still  an  open 
question,  though  there  is  no  reason  why  he  should  not  have  accom- 
panied the  Old  World  mammals  in  their  frequent  migrations,  and 
there  is  some  evidence  that  he  did  and  that  a  race  older  than  the 
American  Indian  occupied  this  continent.  This  evidence,  how- 
ever, is  not  altogether  conclusive  and  has  been  subjected  to  a 
vigorous  destructive  criticism,  so  that  many  authorities  are  alto- 
gether sceptical.  On  the  other  hand,  the  undoubted  presence  of 
human  bones  in  the  Pleistocene  of  South  America,  associated 
with  a  mammalian  fauna  which  is  almost  entirely  extinct,  lends 
additional  strength  to  the  position  of  those  who  contend  that  Man 
had  reached  North  America  before  the  ice  finally  disappeared. 


APPENDIX 

FOR  convenience  of  reference,  the  system  of  classification  of  the 
animals  and  plants  which  has  been  used  in  the  book  is  here  given 
in  tabular  form,  omitting  those  groups  which  possess  no  importance 
as  fossils.  Groups  marked  with  an  asterisk  (*)  are  extinct. 

ANIMAL    KINGDOM 

Sub-Kingdom  I.     PROTOZOA. 
Class  i.  Rhizopoda. 

Order  i.  Foraminfera. 

2.  Radiolaria. 
Sub-Kingdom  II.     CCELENTERATA. 

Sub-Branch  A.     PORIFERA. 
Class  i.  Spongia,  Sponges. 

Sub-Class  3.  Silicispongia,  Siliceous  Sponges. 

4.  Calcispongia,  Calcareous  Sponges. 
Sub-Branch  B.     CNIDARIA. 
Class  i.  Anthozoa. 

Sub-Class  i.  *Tetracoralla,  Palaeozoic  Corals. 

2.  Hexacoralla,  Modern  Corals. 

3.  Octocoralla,  Modern  Corals. 
Class  2.  Hydrozoa. 

Sub-Class  i.  Hydromedusae. 

Order  2.  Hydrocorallince,  Hydroid  Corals. 

3.  Tubularia,  Organ  Corals. 

4.  *Graptolitoidea,  Graptolites. 
Sub-Class  2.  Acalephae,  Jellyfishes. 

Sub-Kingdom  III.     ECHINODERMATA. 
Sub-Branch  A.     PELMATOZOA. 
Class  i.  Crinoidea,  Sea  Lilies. 
Order  i.  *Inadunata. 

2.  *Camerata. 

3.  Articulata. 
Class  2.  *Cystoidea. 

3.  *Blastoidea. 
Sub-Branch  B.     ASTEROZOA. 

Class  i.  Ophiuroidea,  Brittle  Stars. 

2.  Asteroidea,  Starfishes. 
Sub-Branch  C.     ECHINOZOA. 

Class  i.  Echinoidea,  Sea-urchins. 
Sub-Class  i.  *Palaeechinoidea. 

2.  Euechinoidea. 
Order  i.  Regtdares,  Regular  Sea-urchins. 

2.  Irregulares,  Span  tan  goids,  Sand-dollars. 
Class  2.  Holothuroidea,  Sea  Cucumbers. 
Sub-Kingdom  IV.  VERMES,  Worms. 

789 


79°  APPENDIX 

Sub-Kingdom  V.     BRYOZOA,  Sea  Mosses. 
Sub-Kingdom  VI.  BRACHIOPODA,  Lamp  Shells. 
Order  i.  Inarticulata. 

2.  Articulata. 

Sub-Kingdom  VII.     MOLLUSCA. 
Class  i.  Pelecypoda,  Bivalves. 

2.  Scaphoda,  Tusk -shells. 

3.  Amphineura^ 

4.  Gastropoda,  Conchs,  Whelks,  Cowries,  etc. 

5.  Pteropoda. 

6.  Cephalopoda. 
Sub-Class  i.  Tetrabranchiata. 

Order  i«  Nautiloidea,  Pearly  Nautilus. 

2.  *Ammonoidea,  Ammonites. 

Sub-Class  2.  Dibranchiata. 

Order  i.  *Belemnoidea,  Belemnites. 

2.  Sepioidea,  Cuttlefishes,  Squids. 

3.  Octopoda,  Octopuses. 
Sub-Kingdom  VIII.     ARTHROPODA. 

Class  i.  Crustacea. 

Sub-Class   i.  Merostomata. 
Order  i.  *Eurypterida. 

2.  Xiphosura,  Horse-shoe  Crabs. 

3.  *Synxiphosura. 
Sub-Class  2.  *Trilobita. 

Sub-Class  3.  Eucrustacea,  Typical  Crustacea. 
Super-Order  i.  Phyllopoda. 

3.  Ostracoda. 

4.  Cirripedia,  Barnacles. 

5.  Malacostraca. 
Order  i.  Phyllocarida. 

2.  Schizopoda. 

3.  Stomatopoda. 

4.  Decapoda. 

Sub-Order  a.  Macrura,  Lobsters,  etc. 

b.  Brachyura,  Crabs. 
Class  2.  Arachnida,  Spiders  and  Scorpions. 

3.  Myriapoda,  Centipedes. 

4.  Insecta,  Insects. 

Order  i.  Orthoptera,  Cockroaches,  Grasshoppers,  etec 

2.  Neuroptera,  Caddis-flies,  Ant-lions,  etc. 

3.  Hemiptera,  Cicadas,  etc. 

4.  Diptera,  Flies. 

5.  Lepidoptera,  Butterflies  and  Moths. 

6.  Coleoptera,  Beetles. 

7.  Hymenoptera,  Bees,  Wasps,  Ants,  etc. 
Sub-Kingdom  IX.     VERTEBRATA. 

Class  2.  *Ostracodermata. 

3.  Pisces,  Fishes. 
Sub-Class  i.  Selachii,  Sharks,  Rays,  etc. 

2.  Holocephali,  Chimaeras,  or  Spook-fishes. 

3.  Dipnoi,  Lung-fishes. 


APPENDIX  791 

4.  *Arthrodira. 

5.  Teleostomi. 
Order  i.  Crossopterygii. 

2.  Actinopteri. 

Sub-Order  i.  Chondrostei  or  Ganoids,  Sturgeon,  Gar-pike,  etc 
2.    Teleocephali  or  Teleosts,  Herring,  Salmon,  etc. 
Class  3.   Amphibia. 

Order  i.  *Stegocephali. 

3.  Urodela,  Mud-puppies,  Salamanders. 

4.  Anura,  Frogs  and  Toads. 
Class  4.  Reptilia. 

Super-Order  *THERIODONTIA. 

{Order  i.  *Cotylosauria. 
2.  *  Therocephalia. 
3.  *Cynodontia. 
4.  *Anomod&ntia. 
Order  5.  *Placodontia. 

6.  *Plesiosauria. 

7.  Testudinata,  Tortoises  and  Turtles. 
Super-Order  DIAPTOSAURIA. 

'Order    8.  *Procolophonia. 
9.  *Proterosauria. 

10.  *Proganosauria. 

11.  *Gnathodontia. 

12.  *Pelycosauria. 

13.  *Choristodera. 

14.  Rhynchocephalia,  New  Zealand  Lizard. 
Order  15.  *Parasuchia. 

1 6.  *Ichthyosauria. 

17.  *  Thalattosauria. 

18.  Crocodilia. 

Sub-Order  a.  Mesosuchia,  Crocodiles,  Alligators. 

6.  *Thalattosuchia. 
Super-Order  *DiNOSAURiA. 
Order  19.  *Theropoda. 

20.  *Opisthoccelia. 

21.  *Orthopoda. 
Super-Order  SQUAMATA. 

Order  22.  Lacertilia,  Lizards. 

23.  *Mosasauria. 

24.  Ophidia,  Snakes. 
Order  25.  *Pterosauria. 

Class  5.  Aves,  Birds. 

Order  i.  *Saururce  (Archaeopteryx) . 

2.  RatitcB,  Wingless  Birds,  Ostriches,  etc. 

3.  Carinata,  Flying  Birds. 
Class  6.   Mammalia. 

Sub-Class  i.  PROTOTHERIA. 

Order  i.  Monotremata,  Spiny  Ant-eater,  Duck-billed  Mole. 

2.  *Multituberculata. 
Sub-Class  2.  EUTHERIA. 

Super-Order  i.  MARSUPIALIA,  Opossum,  Kangaroo,  etc. 


792  APPENDIX 

2.  PLACENTALIA,  Placentals. 
Order    i.  Edentata,  Sloths,  Armadillos,  etc. 

2.  Cetacea,  Whales,  etc. 

3.  Sirenia,  Sea-cows  and  Dugongs. 

4.  Insectivora,  Moles,  Hedgehogs,  etc. 

5.  Cheiroptera,  Bats. 

6  *Creodonta,  Primitive  Flesh-eaters. 

7.  Carnwora,  Dogs,  Cats,  Bears,  Seals,  etc. 

8.  *Tillodonta. 
g.  *  Tceniodonta. 

10.  Rodentia,  Rats,  Porcupines,  Squirrels,  Hares,  etc, 
n.  *Condylarthra. 

12.  *Amblypoda. 

13.  *Typotheria. 

14.  *Toxodontia. 

15.  *Homalodotheria. 

1 6.  *^4  strapotheria. 

17.  *Litopterna. 

18.  Hyracoidea,  Dassies. 

19.  Proboscidea,  Elephants. 

20.  Artiodactyla,  Pigs,  Camels,  Deer,  Antelopes,  etc. 

21.  Perissodactyia,  Horses,  Tapirs,  Rhinoceroses. 

22.  *Ancylopoda. 

23.  Lemuroidea,  Lemurs. 

24.  Primates,  Monkeys,  Apes,  Man. 

VEGETABLE   KINGDOM 

Sub-Kingdom  A.     CRYPTOGAMS,  Flowerless  Plants. 
I.     THALLOPHYTA. 
Class  i.  Algae,  Seaweeds,  etc. 

2.  Fungi,  Mushrooms,  etc. 
II.     BRYOPHYTA,  Mosses. 

III.  PTERIDOPHYTA. 
Class  i.  Filicales,  Ferns. 

2.  Equisetales,  Horsetails. 

3.  Lycopodiales,  Club  Mosses. 

4.  *Sphenophyllales. 

5.  *Cycadofilices. 

Sub-Kingdom  B.    PHANEROGAMS,  Flowering  Plants. 

IV.  GYMNOSPERM^:. 

Order  i.  Cycadales. 

Sub-Order  i.  Cycadacece. 

2.  ZamiecR. 

3.  *BennettitecB 
Order  2.  *Cordaitece. 

3.  Gingkoacece,  Maiden-hair  Tree. 

4.  Conifera,  Pines,  Spruces,  etc. 
V.     ANGIOSPERM^E. 

Class  i.  Monocotyledones,  Palms,  Grasses,  Lilies,  etc. 
2.  Dicotyledones,  Oaks,  Ro^es,  Crucifers,  etc. 


INDEX 


Page  numbers  marked  with  an  asterisk  (*)  indicate  figures.  Where  several  references  are  given 
ander  one  heading,  the  most  important  is  in  heavy-face  type.  Names  of  the  genera,  of  animals  and 
plants  are  in  italics;  species  are  not  listed. 


Abysmal  deposits,  245,  269 

Agassiz,   A,   260,   270,   271 

Aloes,  736 

Abyssal  rocks,  284 

Agassiz,  L,  769 

Alpine  ranges,  denudation 

Acadian  epoch,  549  ;  prov- 

Age, geological,  530;  topo- 

of, 510 

ince,    613,    620;    Range, 

graphical,  439 

Alps,  506,  510,   514;   com- 

5°4 

Agencies,  diastrophic,  436; 

pression,  506;    elevation, 

Acanthodes,  635 

dynamical,       25,        281; 

726,   753;    glaciers,  156, 

Accidents  to  rivers,  491 

igneous,     26,    28;     sub- 

165;   Pleistocene  glacia- 

Accordance     of     mountain 

terranean,    26,   28;    sur- 

tion, 772,  775,  784 

summits,   513;     of    river 

face,  26,  97 

Altai  Mts,  538 

valleys,  140,  443 

Agglomerate,   volcanic,   81, 

Alteration  of  minerals,  18, 

Accumulations,         organic, 

286,   300,   389,  459 

290,  406,  431 

309,  residual,  186 

Aggradation  of  land,  436 

Alticamelus,  757 

Acervularia,  *6o2 

Agnostus,  *5S7,  558 

Alumina,  294,  407 

Addas  pis,   573,  *576,   603 

Aktian  deposits,   245,  266 

Aluminium,  6,  427 

Acid  rocks,  292,  293,  294 

Alabaster,  20 

Aluminous    silicates,     104, 

Actinocrinus,   602,   631 

Alarm,  688 

1  86 

Actinolite,  16 

Alashuk  River,  141 

Amazon,   205,   269;    delta, 

Actinopteri,  608,  635 

Alaska,     earthquakes,     41. 

213;   material  carried  byu 

Adirondack       Mts,       534; 

46;  fjords,  496;   glaciers, 

147 

faults    in,     465 

*i54,   *i57 

Amblypoda,  729,  *74o,  741 

Adjutant  birds,  756 
&goceras,  689 

Albany  stage,  640 
Albian  series,  702 

Ambonychia,  *574 
Ammonium  carbonate,  266; 

^olian    rocks,     191,    316, 

Albite,  13,  14 

chloride,  82 

3T7 

Aleutian    Ids,    68;     earth- 

Ammonoidea,    6O4,     634, 

Msiocrinus,  631 

quakes,  41;    frost  action, 

651,    656,  671,  689,  717, 

^Etna,  50,  141,  763 

IJ5 

723,  729,   736 

Aetosaurus,  674 

Algae,    192,    194,   309,   314, 

Amorphous  substances,  8 

Africa,  117,  187,  472;    Ar- 

570; calcareous,  670 

Amphibia,   548,    608,    636, 

chaean,    538;     Carbonif- 
erous,   623  ;     continental 

Algonkian,   531,  54O 
"Alkali"  lakes,  *226 

652,  656,  673,  692,  723 
Amphibole-trachyte,  297 

formations,   567;     Creta- 

Alkalies,    294,     297,     298; 

Amphiboles,    15,    16,    290, 

ceous,     713;     Devonian, 

precipitates  of,  307 

297,  409 

599;     Eocene,   734;    ice- 

Alkaline,   carbonates,    192; 

Analcites,  *672,  673 

ages,  543;  Jurassic,  683; 
Miocene,        754;        Oli- 

earths,     precipitates     of, 
307;       sulphides,       194; 

Anaptomorphus,  739 
Anchisaurus,  675 

gocene,  744;  Ordovician, 

waters,  194 

Anchura,  *7i5 

567  ;  Permian,  643  ;  Pleis- 
tocene, 772;   Rift  Valley, 

Allegheny  stage,  610 
Allodon,  698 

Ancyloceras,  717 
Ancylopoda,  757,  759 

469;    Silurian,  584;   Tri- 

Allosaurus,  *6g5,  696 

Andes,  203,  298,  712;    ele- 

assic,    666  ;      volcanoes, 

Allotriomorphic  grains,  286 

vation,  712,  764 

54 

Alluvial   cones,   2O2,  *203, 

Andesine,  13,  14 

Aftershocks,  44 

205,  *479;  fans,  202,  *203 

Andesite,    293,    298,     299, 

Aftonian   stage,    775,    777, 

Alluvium,  river,  279 

393;  -breccia,  301;  meta- 

784 

Almandine,  17 

morphism   of,   420;   -ob- 

793 


794 


INDEX 


sidian,    293,    398;    -por- 
phyry, 293;   -tuff,  301 
Andreae,  A.,  698 
Angiospermse,  655,  687 
Anglo-Gallic  Basin,  734 
Anhydrite,  20,  308 
Animals,     destruction     by, 

179 

Anisian  stage,  658 

Annularia,  627 

Anomodontia,  654,  676 

Anoplotheres,  747 

Anorthic  system,  7 

Anorthite,  13,  14,  298 

Anorthoclase,   13,  14 

Anorthosite,  299 

Antarctic  continent,  735, 
754;  ice-sheet,  155,  771 

Ant-eaters,  787 

Antelopes,  757,  758,  766 

Anthracite,  314,  315,  409, 
418,  619, 620 

Anthracopalamon,  632 

Anthracotherium,  747 

Anticlines,  316,  *327,  *328, 
*329,  *33i,  333,  357, 
436,  452,  456,  457,  478, 
489,  508;  denuded,  384, 
*458 ;  experimental,  *363, 
364,  *36s;  faulted,  349; 
joints  in,  374;  modern, 
362;  overthrust,  357 

Anticlinorium,  330 

Anticosti  Id,  567 
' Antinomia,  *685,  687 

Antiquity  of  land  surfaces, 

2  79 

Ants,  work  of,  179 
Apatite,   n,  20,   289,   296, 

297 
Apennines,     elevation     of, 

Aphelops,  758 
Apiocrinus,  686 
Aplite,  296,  *395 
Apophyses,  398,  400 
Appalachia,  *522,  561 
Appalachian,         coal-field, 

620;  cycles,  511 
Appalachian  Mts,  490,  581 ; 
absence  of  intrusions 
403;  Algonkian,  543; 
Cambrian,  549;  denuda- 
tion, 1 80;  elevation,  511, 
512,  647;  folds,  505; 
geosyncline,  330;  Ordo- 
vician, 564,  565;  Palaeo- 
zoic, 545;  ridges,  513; 
strike  in,  326;  thermal 
springs,  133;  thickness 


of    beds    in,     330,-   505 ; 

thrusts,    355,    356,  358 
Appalachian,    Range,    504; 

River,   490;   System,  504 
Aptian  series,  702 
Aqueous  rocks,  303 
Aquitanian  stage,  724 
Aragonite,    19,    266,    307, 

A   3I7''    4I3< 

Arafoa,  756 

Arapahoe  stage,  710 

Araucarian  pines,  669,  684 

Araucarites,  669 

Arbor  vitae,  735 

Arcestes,  673 

Archaean,  531,  534 

Archceocidaris,  631 

Archaocyathellus,  *554 

Archaocyathus,  *554,  555 

Archaopteryx,  *698,  721 

Archegosaurus,  636 

Archimedes,  *6^o,  634 

Arctic  Ids,  Jurassic,  678; 
Ordovician,  566;  Silu- 
rian, 584;  Triassic,  661 

Arctic  plants,  771,  786 

Arctic  regions,  coast  ice, 
1 66;  boulders,  239 

Arctic  Sea,  limestone  banks, 
2S7>  258;  suspended 
mud  in,  267 

Ardwick  series,  610 

Arenig  series,   560 

Argillaceous,  deposits,  303, 
305;  materials,  302 

Arid  regions,  chemical  de- 
posits, 187;  continental 
deposits,  278;  denuda- 
tion in,  446;  lakes,  216, 
220;  rivers,  146;  snow 
in,  150;  soil,  1 86;  wind 
erosion,  120,  *i2i,  447, 
448 

Arietites,  689 

Arikaree   stage,    724,    750, 

757 

Aristozoe,  *557 
Arkose,  304 
Armadillos,  732,  741,    758, 

766,  787 
Arnold,  R,  761 
Arrangement  of  rocks,  462 
Arrhenius,  93 
A  rsinoet  her  mm  ,741 
Artesian  wells,  132,  *i34 
Arthrodira,  606,  635 
Arthropoda,  555,  573,  587, 

602,  631,  650,   656,   670, 

687,  717 
Artinsk  stage,   642,   643 


Artiodactyla,  729,  739,  741 
744,  745,  747,  758 

Artocarpus,  756 

Arundel  stage,  702 

Asaphus,  573 

Asar,  234 

Ash  (volcanic),  55,  *56,  57, 
*59,  60,  61,  62,  65,  81, 
83,  286,  390;  cementing 
of,  81,  277;  730,  732; 
743,761;  stratified,  183 

Asia,  Archaean  of,  538; 
Cambrian,  553;  Carbo- 
niferous, 623 ;  Creta- 
ceous, 713;  Devonian,  598, 
19;  dust  storms,  189; 
:ne>  733.  7345  fault- 
systems,  466 ;  Jurassic, 
682;  Miocene,  754;  Oli- 
gocene,  744;  Ordovician, 
567;  Permian,  642 ;  Pleis- 
tocene, 772,  784;  Plio- 
cene, 764;  Silurian,  584, 
Triassic,  66 1 

Asphalt,  316 

Aspidorhynchus,  *6gi 

Assam  earthquake,  42,   46 

Assimilation,  magmatic, 
291,  405 

Asteroidea,  573,  586,  655, 
686 

Astian  stage,  724 

Astoria  stage,  724 

Astreeospongia,  *585,  586 

Astrapotheria,  759 

Astrophyllites,  627 

A  stylos pongia,  586 

Asymmetric  system,  7 

Athyris,  595,  603,  671 

Atlantic  c^ast,  Cretaceous, 
702,  706,  708;  Eocene, 
729;  estuaries,  274;  Mio- 
cene, 748;  Pliocene,  759; 
salt  marshes,  275 

Atlantic  Ocean,  earth- 
quakes, 40,  41 ;  volcanoes, 

Atlas  Mts,  elevation,  726 

Atmosphere,  5;  destruction 
by,  99,  1OO,  140,  146 

Atolls,  265 

Atops,  *557,  558 

Atractites,  673 

Atrypa,    588,   *6oi,   603 

Aturia,  737 

Aucella,  *7i5,  717 

Augite,  16,  18,  103,  267; 
285,  293,  296,  300,  409, 
413,421;  -andesite,  293, 
-andesite-porphyry ,  2931 


INDEX 


795 


-granite,    296;     -syenite, 

Basalt,  293,  299,  393,  406; 

Beyrich,  726 

297 

-breccia,  301;    columnar, 

Big  Horn  Mts,    *333  *356, 

Augitite,    293,    30O;     -por- 

369;       -obsidian,       293; 

*457 

phyry,  293 

-porphyry,  293  ;  -tuff,  301 

Binary  granite,  296 

Aulacoceras,  673 

Base-level  of  erosion,   1O1, 

Biotite,  15,  289,    293,   294, 

Auriferous  gravels,  749 

139,  438,  439,  442,  444, 

295,   296,   297,  408,  421; 

Au  Sable  Chasm,  "=142,  143, 

482,  490,  511,   512,  513; 

-andesite,  298 

442,  *468,  473 

local,  444,  446 

Birds,   656,   697,   720,    738, 

Austin  stage,  702 

Basement  complex,   534 

756;  flightless,  738 

Australasia,  Triassic,  667 

Basic  rocks,  292,  293,  294; 

Birkenia,  *589 

Australia,    Archaean,     539; 

metals  in,  428 

Bison,  787 

barrier   reef,    *262,    265; 

Basin    Ranges,    508;     ele- 

Bivalves, see  Pelecypoda 

Cambrian,      553;       Car- 

vation, 752,  783 

Black  Forest,  466,  467 

boniferous,     624;       Cre- 

Basins, interior  continental, 

Black  Hills,  505,  542,  564,  i 

taceous,     713;      Eocene, 

185,  203,  279;  lake,  215; 

679 

734;   Jurassic,  683;  Mio- 

ocean, *244;    of  folding, 

Black  River  stage,  560 

cene,    754;     Ordovician, 

328,  329 

Black  Sea,  deltas,  210 

567;        Permian,       644; 

Batholiths,    293,   401,   405, 

Blanco  stage,  724,  761 

Pleistocene,     772;      Silu- 

462,  514,  534 

Blastoidea,    547,    572,    586, 

,    rian,  584;   Triassic,  667 

Bathonian  series,  677 

602,   629,   655,   670 

Avalanches,   148,   150,   510 

Bays,    493,    494,    495,    496, 

Blastoidocrinus  ,  *56g,    573 

Aviculopecten,     *633,     634, 

499,  500,  501 

Block  mountains,  464 

651 

Beach,     246,     492;      coral, 

Blocks,     5;     volcanic,    80, 

Axes,    anticlinal,    327;     of 
crystals,  7  ;   of  folds,  331, 

265;    gravel,  *246,  *247; 
lake,  176,  *2i7;   inclina- 

387, 389 
Blue  Ridge,   504,  513 

411,  506 

tion  of,  325 

Bog     accumulations,     196; 

Azores,     53,     238;      earth- 

Beach-rock,    307  ;      -sand, 

iron  ore,  199 

quakes,  40 

304;     -wall,    *246,    247, 

Bogoslof  Ids,  68 

492 

Bokkeveld  beds,  599 

Baboon,  787 

Bear  Butte,  398,  *4oi,  460 

Bombs,   volcanic,   *8o,   81, 

Bacteria,   178,   197,  314 
Bactrites,  604 

Bears,  766,  787 
Beaufort  series,  666 

286,  389 
Bony   Fishes,  see  Teleostei 

Baculites,  *7i5,  718 

Beaver,  giant,  787 

Borax,  187,  226,  278 

Bad     lands,     *109,     *uo, 

Beaverdam  Creek,  488 

Boron,  409,  429 

*m,  149,  176,  451,  *75i 

Beavers,  747,  758 

Bottom-  set  beds,  213 

Baiera,  649,  669,  684 

Becraft  stage,  590 

.Bosses,  400 

Bajocian  series,  677 

Bedding,     horizontal     and 

Boulder  beaches,  252;  -clay, 

Bajuvaric  series,    658,  663 

oblique,  383 

232,  643,  644,  645 

Bakevellia,  *633,  651 

Bedding-planes,    126,   182; 

Boulders,    118,   304;    coral, 

Bala  series,  560 

obliteration  of,    408,  417 

263;    glacial,    161,    *229, 

Balearic  Ids,   500 

Beeches,  716,  735 

*23i,   232,   543,  599,  643, 

Balkan  Mts,  538 

Beechey,  Capt,  115 

644;     littoral,      246;      of 

Baltic  Sea,  181  ;  boulders  in, 

Beekmantown    stage,    560, 

glacial      streams,      *235, 

239;    deltas,    210;     dia- 

563 

237;  of  weathering,  *  1  04, 

strophism,  36 

Belemnitella,  *7i5,  718 

105 

Bananas,  735 

Belemnites,  *68g,  718 

Brachiopoda,  544,  547,  558, 

Bandai  San,  55 

Belemnites,  656,  673,  *689, 

573,   588,   603,   634,  651, 

Banding     of     veins,     423, 

690,    718,  723,    729,    736 

656,   671,   687,   717;  Ar- 

424 

Belemnoidea,     see    Belem- 

ticulata, 558;  Inarticu'lata, 

Baptanodon,  692 

nites 

558 

Barbadoes,  radiolarian  ooze, 

Bellerophon,  *633,  634,  651 

Brackish-water        animals, 

313 

Belly  River  stage,  702,  709 

274 

Barite,  425 

Belodon,  674,  *675 

Brachyura,    687,    717,   736 

Barium,    6;     sulphate,  426 

Belt  of  weathering,  98 

Bradyseism,  29 

Barrell,  J,  216 

Belt  terrane,  541,  544 

Brahmanian      stage,      658, 

Barriers,    marine,    253;     to 

Bennettiteae,  684,  714 

661 

migration,  529 

Benton  stage,   702,   708 

Brahmapootra     R,      delta, 

Bars,  201 

Bermuda,  sands,  190;  sand- 

210,  214 

Bartonian  stage,  724 

rock,  276 

Brahmatherium,  766 

Barus,  C,  71 

Betulties,  *7i4 

Branchiosaurus,  636 

796 


INDEX 


Brancoceras,  *6$o,  634 

Canter  ella,  *5$4 

Caucasus,    514;    elevation, 

Breadfruit,  756 

Camptonectes,  +685,  688 

726.  754 

Breccia,  187,  188,  196,  317; 

Canadian  series,  560 

Caulopteris,  668 

coral,    264;     fault-,  317, 

Canary  Ids,  53 

Cave  bear,  787 

*34i;  volcanic,  81,  300 

Canis,  788 

Cave,  deposits,  194;  earth, 

Bridger    stage,    724,     732, 

Canon,    of   Colorado,    144; 

196 

739 

of    Columbia,    483;      of 

Caverns,  ancient,  196;  lime- 

British Channel,  tidal  scour 

Gunnison,  *394,  485;  of 

stone,   127,   *i35;  wind- 

in,  174 

Snake,   483,    of    Yellow- 

cut, 122 

Brittle-stars,     see     Ophiu- 

stone,  *I28 

Caves,  194;   sea-,  174 

roidea 

Canons,  480 

Cavies,  787 

Brock,  R  W,  129,  130 

Cape     Fairweather     stage, 

Cayugan  series,  578 

Brcngniart,   677,     726,   734 

764 

Celebes,  lobate  coast  of,  501 

Bronteus,  573,  *576 

Cape  Verde  Ids,  53 

Cement,     calcareous,     105, 

Brontosaurus,  696 

Capillarity  in  soil,   124 

109,     no;    consolidation 

Brontotherium,  *746 

Capitan  stage,  637 

by,      276;       ferruginous, 

Bronzite,  16 

Caprotina,  717 

105,    106,    304;  of  sand- 

Brown coal,  314 

Caprotinae,  713 

stones,     105,     304;    sili- 

Bryozoa, 258,  575,  634,  651, 

Capture     of     rivers,     486, 

ceous,  105,  106,  304 

671 

*489 

Cementation,   409,  416 

Buchanan  stage,    775,    777 

Capulus,  589 

Cenomanian  series,  702 

Bumastus,  573,  *576 

Carbon,   6,    197,   314,   315, 

Cenozoic  era,  531,  722 

Bunter  Sandstein,  658,  659 

316 

Centipedes,  547,  632 

Burlington    substage,    610, 

Carbonaceous       accumula- 

Central     America,     earth- 

614 

tions,  314 

quakes,  40,  41,  50;  Oli- 

Burrowers,  179 

Carbonates,  104 

gocene,     742  ;     Pliocene, 

Buzzards,  738 
Byssonichia,  *574 

Carbonation    of    minerals, 
98,  1O2 

759;    volcanoes,  54,  58 
Centrosphere,  5 

Carbon-dioxide,  82,  98,  101, 

Cephalaspis,  605,  606 

Cables,    broken    by    earth- 

102,  191,   195,   197,  271, 

Cephalopoda,      559,      570, 

quakes,  41 

407,  4i5 

575,  589,  604,  634,  651, 

Ccenopus,  745 

Carboniferous,     531,     547, 

656,  671,  689,  717 

Cainotheres,  747 

609;     Lower,   610,    622, 

Ceratites,  673 

Calamites,  *626,  627,  648, 

623,    624;     Upper,    610, 

Ceratodus,  652,  673,  691 

649,  668 

615,  623,  624 

Ceratosaurus,  696 

Galas  coasts,  500,  501,  502   . 

Cardiaster,  *7i$,  717 

Ceraurus,  573,  *576 

Calcareous,    deposits,    191; 

Cardita,  671,  *737 

Cerithium,  671 

shoal-water,     245,     257; 

Caribbean  Sea,  311 

Chain,  mountain,  504 

materials,  302;   minerals, 

Carinthian  stage,  658 

Chalcedony,  12,    123,    194, 

19 

Carnivora,    729,    739,    745, 

309 

Calceola  beds,  590 

747.  758 

Chalk,  *311,  700 

Calciferous  stage,  613 

Caryocrinus,    *$&$,    586 

Chamberlin,    T     C,     368, 

Calcite,    n,    19,   302,   307, 

Cascade  Mts,  483;     eleva- 

533» 77i,  774 

408,  409,  413,  417,  418, 

tion,  68  1 

Champlain  subsidence,  780 

423,  425,  426,    520;    re- 
crystallization  of,  312,  417 

Cascades,  514 
Caspian    Sea,  227;    deltas, 

Changes,  geographical,  528; 
of   level,    29;     horizontal 

Calcium,  6;   carbonate,  19, 

2IO 

in  strata,  183;  vertical  in 

187,   188,   190,   191,    195, 

Cassidulus,  717 

strata,  183;    of  tempera- 

196, 223,  *224,  257,   264, 

Castor  oides,  787 

ture,  116,  1  80 

266,   267,    269,   270,   271, 

Casts,  *5i9 

Channels,     ancient      river, 

273,       276,       304,      307; 
chloride,   82,   225;   phos- 

Cataracts, glacial,  i6« 
Catastrophism,  527 

*iio,  207,  *209 
Charnwood  Forest,  440 

phate,  194;  sulphate,  2O, 

Catfishes,  718 

Chattahoochee   stage,-   724 

224,  257,  266 

Catopterus,  673 

Chautauquan    series,    590, 

Callipteris,  *649 

Cats,  757,  766,  787 

596 

Callovian  series,   67y,   679 

Catskill    Mts,   streams   of, 

Chazy  stage,  560,  563,  564 

Caloosahatchie   stage,    724 

487 

Cheirodus,  635 

Calymmene,  573,  *576,  587 

Catskill    stage,    590,    696, 

Cheirotherium,  674 

Cambrian,    531,    546,    548 

599 

Chemical  action,  by  organic 

Camels,  745,  747,  757,  766 

Caturus,  691 

matter,     178;    by    rain, 

INDEX 


797 


101;  by  sea- water,  *i73, 

1 74 ;      by      underground 

water,  126 
Chemical     deposits,     land, 

187;      lake,     220,     223; 

shoal-water,  266;  precipi- 

tates,  307 

Chemung  stage,  590,  596 
Chert,  12,  3O9,  313,  324 
Chesapeake  Bay,  33,  495, 

501;    estuary,  274 
Chesapeake  stage,   724 
Chestnuts,  716 
Chickasaw  stage,  724 
Chico  series,   702,   71O 
Chimaeroidei,  690 
Chipola  stage,  724 
Chlorine,  6 
Chlorite,  18,  129 
Chonetes,  *6oi,   603,    *633, 

634 

Chonoliths,  399,  403 
Choristoceras,  673 
Choristodera,       719,      723, 

729,  738 
Chronology,  geological,  321, 

385,  521,  525 
Cidaris,  631,  *68s,  686,  716 
Cimolestes,  721 
Cincinnati    anticline,    568, 

579,  612,  615 
Cincinnatian  series,  560 
Cinnabar,  194 
Circulation   of   matter,   24, 

97,  281 

Cirques,  glacial,  164,  510 
Civet-cats,  747,  758,  766 
Cladoselache,  *6o6 
Claiborne  stage,  724 
Clarke,  F  W,  6;   J  M,  522, 

523,  596 

Clathropteris,  668 

Clay,  103,  107,  131,  199, 
267,  269,  302,  304,  305, 
306,416,417;  brick,  306; 
concretions,  324;  in  lakes, 
218,  219,  220;  metamor- 
phism  of,  407;  oceanic 
red,  174,  245,  272,  274, 
279;  porcelain,  306;  pot- 
ters', 306;  rocks,  meta- 
morphism  of,  408 ;  settling 
in  brine,  223;  shoal- 
water,  256 

Clear  Fork  stage,  637,  640 

Cleavage,  mineral,  9,  11, 
411;  of  igneous  rocks, 
410;  slaty,  278,  *410, 
413,  43.3;  cause  of,  412; 
in  mountains,  506,  508 


Cliffs,  168,  492,  493;  in  hard 
rocks,  452;  of  fault- 
scarps,  348,  463,  *464, 
*46s 

Climacograptus,  *554,  *S7i 

Climate,  arid,  446;  Algon- 
kian,  543;  Carboniferous, 
624;  Cenozoic,  722;  Cre- 
taceous, 713;  Devonian, 
600;  effects  of,  on  conti- 
nental deposits,  240;  on 
deposition,  186;  on  ero- 
sion, 100 ;  on  marine  de- 
posits, 272,  273;  Eocene, 
735;  Interglacial,  777; 
Jurassic,  683;  Mesozoic, 
657;  Miocene,  759,  767; 
Oligocene,  747;  Ordo- 
vician,  567;  Palaeozoic, 
548;  Permian,  645; 
Pleistocene,  785,  786; 
Pliocene,  767;  pluvial, 
439;  Silurian,  584;  topo- 
graphical effects  of,  439, 
442,  445;  Triassic,  667 

Climatic  changes,  450,  529; 
causes  of,  785;  effect  on 
rivers,  491 

Clinometer,  326 

Clinton  stage,  578,  581,  588 

Ckmian  series,  578 

Clymenia,  604 

Clymenia  Limestone,  590 

Clypeastroidea,  686 

Coal,  196,  306,  314,  618; 
anthracite,  315,  418; 
bituminous,  315;  brown, 
314,  743;  cannel,  315; 
Cretaceous,  704,  706,  709, 
710,  713;  Jurassic,  682; 
Measures,  610,  617,  622, 
623;  metamorphism  of, 
407,  409,  418;  Miocene, 
752;  origin  of ,  6 1 8 ;  seams, 
199,  306,  320;  semibitu- 
minous,  315;  steam,  315; 
Triassic,  660,  664,  667, 
668 

Coast-ice,  166 

Coast  Range,  504;  elevation, 
681,  752,  759;  fault-val- 
leys, 467;  thickness  of 
strata,  505 

Coasts,  adjusted,  493,  495, 
501;  calas,  5OO,  502; 
determined  by  marine 
erosion,  493,  502;  by 
structure,  501;  by  sub- 
aerial  erosion,  500; 
faulted,  502;  flat,  492, 


493,  494,  501;  folded 
502;  irregular,  492,  495 
500,  501;  lobate,  492 
501;  old,  495;  rias, 
499,  5OI>  502J  serrate, 
493;  steep,  492,  493, 
494;  wear  of,  167;  youth- 
ful, 494 

Cobblestones,  304;  in  del- 
tas, 213 

Coblenzian  fauna,  invasion 

of,  593 

Coblenzian  series,  590 
Cobleskill  stage,  578,  583 
Coccosteus,  606,  *6o7 
Cochloceras.  673 
Cod,  718 
Ccelacanthus,  635 
Ccelenterata,  570,  602,  629. 

650,  670,  686,  716 
Cceymans  stage,  590 
Coke,  natural,  409 
Coleman,  A  P,  543 
Coleoptera,  671,  687 
Colorado  River,  136,    144, 

222,    241,   752 

Colorado  series,  702,  708 
Colouring  of  rocks,  22,  102 
Columbia  River,  483,  484, 

485 

Columbian  formation,    782 
Columnaria,  *56g,  572 
Columnar  jointing,  76,  *77, 

*78,  369, *402 
Comanche  series,  702,  703, 

705,  706 
Comatula,  686 
Combinations  of  crystals,  10 
Commentry,  coal-basin  of, 

619 

Comores  Ids,  54 
Como  stage,  680 
Compression,  lateral,  345, 

35i,  359,  36°,  362,  363, 

364,  365,  367,  411,  412, 
413,  433,  508>  consolida- 
tion by,  277;   genesis  of, 

365,  367 ;  of  igneous  rocks, 
396;    of  mountains,  505, 
508;    orogenic,  403;    re- 
currence of,  508;  second- 
ary, 366 

Compsognathus,  696 

Concentration,  calcareous, 
303;  of  iron,  187;  of 
material,  183;  of  metals, 
428;  siliceous,  303 

Concretions,      322,      *32$ 

Condylarthra,  729 


798 


INDEX 


Conemaugh  stage,  610 
Cones,  volcanic,   *83,   *84, 

*8s,  *86,  *87,  386,  459; 

buried    388;    denudation 

of,  385,  459 

Conformity,  377;  deceptive, 
380 

Conglomerate,  *305,  306, 
321;  basal,  381;  coral, 
264;  cross-bedded,  255; 
flint,  305;  granite,  305; 
limestone,  305 ;  meta- 
morphism  of,  408,  416, 
419,  .*42o;  modern,  266; 
quartz,  305 

Congo,  submarine  channel 
of,  34 

Coniferae,  628,  649,  655, 
668,  *669,  684,  714,  756 

Connection  of  America  with 
Eurasia,  724,  732,  733, 
734,  738,  743.  75°;  of 
North  and  South  Amer- 
ica, 732,  750,  766 

Conocardium,  603 

Consanguinity  of  rocks,  291 

Consolidation  of  sediments, 
97,  276 

Constellaria,  *57i 

Contact-zone,  408,  409,  429 

Continental  deposits,  181, 
184,  278 

Continental  shelf,  243,  *244 

Continents,  area  of,  *i85; 
height  of,  *i8s;  Per- 
mian, 645 ;  Eocene,  734 

Contortions,  332,    359,  506 

Contraction  of  earth,  51, 
367 

Conularia,  *S74,  577,  634 

Copper,  194,  427,  544; 

Coquina  rock,  *258 

Coral,  crystallization  of,  264, 
312;  limestone,  *2&3 ; 
mud,  270;  Rag,  677 

Corallian  series,  677 

Coralliochama,  717 

Coral-reefs,  34,  259,  *262; 
barrier,  265;  Devonian, 
594,598;  fringing,  265; 
Silurian,  582 ;  slopes  of, 
266 

Corals,  258,  259,  260,  *262, 
547,  548,  555,  572,  586, 
602,  629,  650,  655,  670, 
686,  716,  736;  reef- 
building,  261 

Cordaiteae,  *626,  628,  668 

Cordaites,  *626,  649 

Cordillera,  504 


Cordilleran    Glacier,    *773, 

C  upres  sites  >  684 

776,  778;  Sea,  594 
Cormorants,  721 

Cupressocrinus,  602 
Current-  bedding,  255 

Coryphodon,  739,  741 

Currents,  ocean,  167;  tidal 

Cotopaxi,  80 

17.3 

Cotylosauria,  654 

Cuvier,  G,  726,  734 

Country-rock,     391,      399, 

Cyathophycus,  *56g 

400,  423,  425,  429 

Cycadaceae,  669 

Cranes,  756 
Crater  Lake,  57,  *58 

Cycadales,    <U7,    628,    649, 
655,  668/669,  684,   714 

Crater-rings,  57,  84 

Cycadofilices,     600,     *626, 

Creep,  frost,  113,  *ii4 

628 

Creodonta,  729,    739,    741, 

Cycle,  arid,  439,  446;  com- 

745, 747 

plete,  512;    geographical, 

Crepicephalus,  *557 

438;    normal,  439,  448, 

Cretaceous,  490,   512,   531, 

449;  of  denudation,  512; 

655,    657,    7OO;     Lower, 

of  destruction  and  recon- 

702,  703,   704,  708,  711, 

struction,    279,    281;    of 

712;     Upper,    702,    705, 

river   development,   481  ; 

706,   707,  712 

of      rock-transformation, 

Crete,  30,  33 

415;    partial,  512 

Crevasses,  153,  164 

Cycles,   Appalachian,    511; 

Crinoidea,    259,    547,   572, 

on   sea-coasts,  494 

586,  602,  614,  631,  650, 

Cycloccras,  634 

655,  670,  686,   716;    Ar- 

Cyclonema,  *574,  589 

ticulata,     655,  670,    686; 

Cyclotosaurus,  674 

Camerata,  631,  655,  670 

Cymatonota,  *574 

Crioceras,  *68s,  689/717 

Cynoglossa,  649 

Crocodilia,    694,    719,    723, 

Cyprcea,  756 

738,  744,  745,  747 
Cross,  W,  647 

Cypresses,  736 
Cypridina  Slates,  590 

Cross-bedding,  *254,  *255, 

Cyrtina,  671 

322 

Cyrtoceras,  *574,   577,  604 

Crcssopterygii,     607,     635, 

Cyrtodonta,  *574 

673,  691 

Cyrtolites,  *574 

Crust,  earth's,  92,   93,  94, 

Cystoidea,    547,    555,    568, 

533,  539 

572,  586,  602,  629,  655, 

Crustacea,    259,    555,    656, 

670 

687,  717 

Cryptogams,  547,  600 

Dacite,     293,    298;     -por- 

Crystal forms,  7 
Crystalline    rocks,    disinte- 

phyry, 293 
Dakota     Sea,     594,     5955 

gration,     118;      faulting, 

stage,  702,  706 

344 

Ball,  W  H,  741,  749,  76i, 

Crystallites,  74,  295 

762 

Crystallization,  9,  284,  288, 

Dalmanella,  *57i,  573 

414 

Dalmanites,  587 

Crystals,  6  ;   compound,  1  1  ; 
in  lava,  75;  in  tuffs,  301; 

Daly,  R,  292,  385,  399,  514 
Dammarites,  *7i4 

in  veins,  424 

Dana,  J  D,  330,  591 

Ctenacodon,  698 

Danian  series,  702 

Ctenodus,  635 

Daonella,  671,  *672 

Cuba,     barrier    reef,     265; 

Dapedius,  *690,  691 

radiolarian  ooze,  313 

Darwin,  C,  179 

Cubical  system,  7 

Davis,  W  M,  448,  449,  483 

Cuboides-zone,  590,  595 

Dawsonoceras,  *$&$ 

Culm,  622,  623 

Dead  Sea,  185,  444,  469 

Cumberland     Basin,     579, 

Dean,  B,  605,  606,  607,  652 

582,  583,   591,  593,  595, 

674 

646 

Decapoda,  670,  687 

INDEX 


799 


Decomposition  of  rocks, 
97,  241;  of  silicates,  17, 
103;  organic,  178, 197,  241 

Deep  River  stage,  724,  75O, 

Deer,    757,    758,    766,    787 

Deformation,  cause  of,  358 

De  Geers,  32 

Degradation  of  land,  436 

Deiphon,  *58s,  587 

Delaware  Bay,  495,  501; 
estuary,  274 

Delaware  Mt   stage,   637 

Delaware  Water  Gap,  114, 
*ii5,  443,  453 

Deltas,  210,  *2ii,  216,  219; 
cross- bedding  in,  255; 
submarine,  213 

Dendrerpeton,  636 

Denudation,  97,  440;  ma- 
rine, 440,  501;  of  land, 

•  185,  1 86;  of  mountains, 
509;  subaerial,  442,  494 

Denver  stage,  702,  71O,  720, 
727  _  _ 

Deposition,  97,  98;  in 
deserts,  241;  '  in  fresh 
water,  210,  *2i2;  in 
lakes,  215;  in-  salt  water, 
210,  *2i2;  in  sea,  245; 
in  temperate  regions,  241 ; 
in  tropics,  242;  of  land- 
waste,  181;  on  land,  185, 
186 

Depression,  evidences  of, 
33;  effect  on  rivers,  482 

Depth,  as  a  controlling 
factor,  27,  359,  360,  413, 

433 

Derbya,  *633,  634 

Derivative  rocks,  105,  182, 
302 

Desert,  Colorado,  222;  Mo- 
have,  *447;  zones,  241 

Deserts,  circulation  of 
matter  in,  123;  of  North 
America,  449;  of  South 
Africa,  449;  talus  in,  317 

Destruction  of  rock,  97 

Destructive  processes,  98, 
100 

Devitrification,  9,  295,  296 

Devonian,  531,  547,  563, 
59O,  610 

Diabase,  293,  299,  393, 
*397;  jointing  of,  *37o; 
metamorphism  of,  420; 
weathering  of,  *37o 

Diadectes,  654 

Diagenesis,  276 


Diallage,  16 

Diamond,  1 1 ;  mines,  388 

Diastrophism,  28,  98,  425, 

436,     437,     482;      effect 

on  coasts,  493,  494,  501 ; 

on  denudation,   99,   438; 

on    deposition,    99,   439; 

epeirogenic,  29;  erogenic, 

28 
Diatoms,     194,    22O,    267, 

272,  275,  313,  314,  748, 


ill9 


Dibranchiata,  575,  656,  673, 

690 

Diceras,  688 
Diceratherium,  747 
Diclonius,  *72O 
Dicotyledons,  655,  714,  716 
Dictyonema,  *554,  555 
Dictyopyge,  673 
Dictyospongidae,  602 
Dicrocynodon,  699 
Dicynodon,  676 
Didelphops,  721 
Dielasma,  *633,  634 
Differentiation,    magmatic, 

290,  291,  292 
Dikellocephalus,  549,  558 
Dimetric  system,  7 
Dinaric  series,   658,  663 
Dinichthys,  607 
Dinosauria,  675,  695,  719, 

723,  727,  729 
Dinotherium,       758,      766; 

Sands,  764 
Diorite,     293,     298,      395; 

family,   293,   297;     -por- 
phyry, 293,  296 
Dip,  initial,  325,  *3&3,  364; 

of  fault,  340;    of  strata, 

326,  327,  *328,  333;    of 

veins,  423 

Dip-slopes,  *453»  454,  47^ 
Diplodocus,  696 
Diplurus,  673,  *674 
Dipnoi,  6O6,  635,  652,  691 
Diptera,  687 
Dirt-bed,  187 
Disintegration  of  rock,  97 
Dislocations,  96,  338,  345, 

404;  causes  of,  358 
Displacements,      433;       of 

coast-line,  30 
Dissolved     substances,     in 

rivers,  146;  in  sea,  269 
Distortion  of  crystals,  10 
Divides,  480;  shifting  of, 

486 

Dogs,  745,  747,  766 
Dolatocrinus>  602 


Dolerite,  299 

Dolomite,    2O,     266,    312; 

crystalline,  417 
Dolphins,  758 
Domes,     329,     384,      505; 

granite,    117,   *i2o,   474, 

*475 

Don  formation,  777 
Dorypyge,  *557 
Double  Mt  stage,  637,  64O 
Dovetailing      of      deposits, 

*257 

Downthrow,  340,  347,  350, 
366,  384,  469,  470,  471 

Downwarp,  29 

Drag,  342,  *3so,  *356 

Drainage,  adjustment  of, 
459;  changed  by  earth- 
quakes, 49;  by  joints, 
472;  epigenetic,  484;  in- 
herited, 484;  -level,  125, 
127;  lattice,  467;  of  arid 
regions,  446 ;  superim- 
posed, 484 

Drainage  system,  disinte- 
gration of,  448;  maturity 
of,  490 

Drift,  glacial,  *i59,  *i6o, 
232,  *233,  770 

Drift-sand  rock,  191,  276, 
317 

Driftwood  theory,  618 

Dromatherium,  676 

Drumlins,    235,    *237,    771 

Dry  mines,  125 

Dr'yolestes,  699 

•Ducks,  756 

Dunes,  *i89,  *i90,  317 

Dunkard  stage,  637 

Dust,  volcanic,  81 

Dust-storms,  189 

Dwyka  stage,  643 

Dyas,  641 

Dykes,    79,    83,    293,    391, 

*392,  *393»  *395,  397, 
400,  403,  407,  433,  460; 
sandstone,  *426,  427 

Eagle  Ford  stage,  702 

Eagles,  738,  756 

Earth,  internal  constitu- 
tion of,  90;  internal  me- 
chanics of,  368;  internal 
temperature  of,  91;  ori- 
gin of,  533;  specific 
gravity  of,  90 

Earthquakes,  28,  29,  36, 
95,  338,  404,  42 7;  pauses 
of,  50;  classification  of, 
42;  distribution  of,  *39, 


8oo 


INDEX 


*4o;  effects  of,  45;  in 
sea-bed,  41 ;  phenom- 
ena, 42;  tectonic,  42,  50; 
volcanic,  42,  5O 

Earthquake  waves,  93 

Earthworms,  work  of,  179 

Echidna,  698 

Echinocaris ,  *6oi 

Echinodermata,  258,  259, 
27°,  547,  548,  555,  572, 
586,  602,  629,  650,  655, 
670,  686,  716,  736 

Echinoidea,  572,  586,  602, 
631,  686,  716,  736 

Ecphora,  *756 

Edentates,  758,  787 

Edriocrinus,  *6oi,  602 

Elasmosaurus,  718 

Elephant,  hairy,  787 

Elephants,  741,  750,  757, 
766,  787;  frozen  car- 
casses of,  518 

Elevation,  effect  on  rivers, 
482,  483,  490;  evidences 
of,  30 

ElkMts,  399,  503 

Elms,  716,  735,  736 

Elotherium,  747 

Empire  stage,  724 

Encrinurus,  587 

Encrinus,  670 

Endoceras,  577,  589 

Endothyra,  629,  *63O 

Energy,  solar,  25;  terres- 
trial, 28 

Englacial  drift,  *i65,  166 

Enrichment   of   veins,   431 

Enstatite,  16 

Eobasileus,  *74o,  741 

Eocene,  710,  724,  726,  727, 
739 

Eocystites,  *554 

Eohyus,  739 

Eotomaria,  *574,  575 

Epidote,  17 

Epoch,  geological,  530 

Equisetales,  547,  600,  *626, 
627,668,684,  714,  735 

Equisetum,  668 

Equus  Beds,  782 

Era,  geological,  530 

Erian  series,  590,  595 

Erosion,  97,  98;  contem- 
poraneous, 382,  *3S3; 
glacier,  *i58;  lake,  *i75; 
river,  135;  sea,  167 

Erratics,  23O,  *232 

Eruptions,  fissure,  84;  vol- 
canic, 54 

Eruptive  rocks,  284,  290 


Eryops, 
Escarpments,  *453,  476;  re- 
cession of,  454,  455 
Eskers,  234,  *236,  237,  771 
Esopus  stage,  590,  594 
Estheria,  670 
Estuaries,  210,  274,  482 
Estuarine  deposits,  274 
Eucalyptocrinus,  *585,   586 
Euechinoidea,  631,  655,  670 
Eugeniacrinus,  686 
Eumicrotis,  *685 
Euomphalus,  604,  634, 
Eupachycrinus,  631,  *633 
Europe,     Algonkian,     543 ; 
Archaean,  538;  Cambrian, 
552;  Carboniferous,  622; 
Cretaceous,      712;     De- 
vonian,     598;      Eocene, 
733  >  Jurassic,  682 ;  Mio- 
cene,     752;      Oligocene, 
743;      Ordovician,      566, 
568;  Paleocene,  728;  Per- 
mian,   641;    Pleistocene, 
772,  784;  Pliocene,  763; 
Silurian,    583 ;     Triassic, 
658 

Eurylepis,  635 
Eurynotus,  635 
Eurypterida,  544,  547,  573, 
582,  *587,  588,  603,  632, 
650,  656 

Eurypterus,  *587,  603 
-Ewrys/0W7/es,  *574,  577 
Eutaw  stage,  702,  708 
Eutrochocrinus,    *630,    631 
Exfoliation,     *117,     *n8, 

*ii9,  *i20,  474,  *475 
Exogyra,  688;  *7i5,  717 
Extrusive  rocks,  284 

Facetted  pebbles,  161 

Facies,  changes  of,  *523 

Fairbanks,  H  W,  445,  447 

Falkland  Ids,  600 

False  Coal  Measures,  613 

Fans,  alluvial,  202,  325 

Fasciolaria,  *76s 

Fault-blocks,  5,  366,  384, 
464;  tilted,  464,  502,  503, 
508,  509;  tilting  of,  367 

Fault-breccia,  317,  *341 

Fault-plane,  341 

Fault-rock,  341 

Fault-scarps,  45,  *46,  47, 
48,  *348,  *349,  35i, 
436,463,  *464,  *465,  497; 
denudation,  469;  dis- 
sected, 463 ;  reappearance 
of,  471;  reversed,  469 


Fault-systems,  464,  465,  466, 
467 

Fault- valleys,  467 

Fault-zone,  424 

Faults,  5,  45,  338,  344,  413, 
432,  506;  dip-,  345, 
*349,  350,  351 ;  cause  of, 
365,  366;  compound, 
346;  horizontal,  49,  345, 
351,  *352;  in  igneous 
rocks,  396;  normal,  49, 
*339,  *34o,  345,  346, 
351,  366,  *468,  508; 
oblique,  345,  350; 
on  bedding-planes,  *343 ; 
pivotal,  345,  352,  *3545 
radial,  345;  reversed, 
345,351,  366;  step-,  347, 
*348;  strike-,  345,  346, 
*348,  350;  topographical 
effects,  463 ;  trough-, 
*346,  347;  vertical,  340, 
*344 

Faunal  provinces,  *522 

Faunas,    547 

Favosites,  *$&$,  586 

Fayol,  M,  619 

Feather  stars,  see  Crinoidea 

Felsenmeer,  510 

Felsite,  293,  295,  296; 
metamorphism  of,  420 

Felsitic  texture,  285,  294 

Felspars,  13,  19,  103,  104, 
129,  136,  267,  289,  290, 
293,  296,  299,  300,  304, 
408,  400,  414,  416,  421; 
lime-soda,  13,  72;  soda- 
lime, 298;  potash,  13,  72 

Felspathic  rocks,  107 

Felspathoids,  14,  289,  290, 
297 

Fenestella,  634 

Ferns,  547,  586,  600,  625, 
648,  649,  655,  668,  684, 

7U.735 

Ferrar,  H  T,  772 

Ferro-magnesian  minerals, 
72,  289,  294,  295,  296, 
297,  298 

Ferrous,  carbonate,  102, 
103,  105,  192,  199,  220; 
compounds,  102;  sul- 
phate, 431 

Ferruginous  accumulations, 
314;  precipitates,  309 

Filicales,  see  Ferns 

Fire-clay,  198, 199,  306,  618 

Fishes,  548,  606,  608,  635, 
652,  673,  690,  718,  723, 
736;Saurodont,  718,  *7ig 


INDEX 


80 1 


Fissility,    410,    *4ii,    413, 

433,    5°6>  5°8;  cause  of» 

412 
Fissures,  125,  126,  888,423; 

earthquake,  *43,  96,  427; 

relation  to  volcanoes,  54 
Fjord  coasts,  496,  501,  502; 

depression  of,  497 
Fjords,  496,  *497,  *49§ 
Flagstones,  304 
Flamingoes,  756 
Flexures,  complicated,  335; 

of  strata,  330,  331 
Flint,    12,    309,    313,  408; 

concretions,  324 
Flood-plain    deposits,  *uo, 

205,  278,  507 
Flood-plains      2O3,      *204, 

*235,  443 

Flocds,  *2oo,  *2oi,  203 
Floor,     pre-Archsean,     539, 

540 

Floras,  547 

Florida  island,  742,  752 
Florida  stage,  724 
Florissant  beds,  752,  756 
Flow,  lines  of,  295 
Flowage,  shell  of,  359,  508 
Flowage  and  fracture,  shell 

of,  360 

Fluorine,  409,  429 
Fluorite,  n,  20,  429 
Fluviatile  deposits,  199 
Flysch,  734,  744 
Folded  strata,  compression 

OI>  3S9J    forms  in,  456 
Folds,    326,  327,  413,  432, 


508;  asymmetrical,  *331 
*332»  S°S.  !°8;  cause  of, 
358;  classification  of,  330; 
closed,  *33i,  332,  359, 
505,  508,  512;  cross,  335; 
fan,  333,  506;  inclined, 
*33J.  *332,  359,  5°8;  in 
igneous  rocks,  396;  in- 
verted, 332,  359,  508;  iso- 
clinal, 333,  *335;  mono- 
clinal,  *335,  *336;  open, 
*33i)332,  357,  505,  508; 
overturned,  *33i,  332, 
*333,  364,  505,  .5*2;  re- 
cumbent, *33 1 ,  332, *334 ; 
regular,  505;  simple,  358; 
surface,  362;  symmetri- 

cal>  *33i,  358,  5°5,  5°8; 
synclinal,  508;  undulat- 
ing, 332;  upright,  *33i, 
5°S,  5°8 

Foliation,  412,  413,  414 

Fools'  gold,  21 


Footprints,  ibssil,  206-  251, 

255,  ^75 

Foot-Wcu',  *339,  34O,  351 
ForaminLera,  258,  260,  267, 

260,  27c,  272,  273,  *3ii, 

312,  313,   555,   570,  629, 

650,  684,  716,  736 
Fordilla,  *554 
Foreland  of  mountains,  502, 

506 
Foreset  beds,  213,  216,  255, 

325. 

Forestian  stages,  775 

Forests,  buried,  33 

Forms,  crystal,  10 

Fort  Pierre  stage,  702,  709 

Fort  Union  stage,  724,  727 

Fossa  magna,  467 

Fossils,  96,  184,  408,  516; 
destruction  of,  408,  417; 
embedding,  516;  in  lava, 
82 ;  in  metamorphic 
rocks,  406;  modes  of 
preservation,  518 

Fox  Hills  stage,  702,  709 

Fox  R,  folding  in  bed  of,  362 

Fracture,  shell  of,  360 

Fractures,  338,  432 

Fragmental  products,  vol- 
canic, 70,  79,  286,  300, 
389 

Fragmental   texture,    286 

Fredericksburg  stage,  702 

Fresh- water  lakes,  216 

Fritsch,  A,  652 

Frondicularia,  *7i5 

Frost,  action  of,  131,  164, 
239 ;  in  polar  regions,  115 

Frost-action,  113,  131,  164, 
180,  239,  510 

Funafuti,  coral  reef  of,  34, 
266 

Fusibility  of  igneous  rocks, 
71,  287 

Fuson  stage,  702,  704 

Fusulina,  312,  629,  *&33 

Fusulina  Limestone,  623 

Fusus,  717 

Gabbro,    293,     299,     400; 

family,  293,  298,  369,  395 

-porphyry,  293 
Gallinaceous  birds,  756 
Gangamopleris,  649 
Ganges,    delta,    210,     214; 

material  carried  by,  147 
Gangue,  426,  430;  minerals, 

426,  429 
Gannister,  199 
Gannister  secies,  610 


Ganoidei,    608,     673,    691 

Garnets,  17,  304,  408, 
*42i,  429 

Gas,  marsh,  row,  316;  nat- 
ural, 316 

Gases,  magmatic,  407,  429; 
volcanic,  70,  80,  82,  95 

Gastropoda,  558,  575,  589, 
603,  634,  651,  656,  671, 
688,  717,  736 

Gay  Head  Sands,  759 

Geanticline,  330 

Gedinnian  series,  590 

Geikie,   A,    546,     661,    772 

Geikie,  J,  774,  784 

Genesee  stage,     590,    595, 

596 

Geology,  i ;  dynamical,  4, 
23;  historical,  4,  516; 
physical,  4;  physiograph- 
ical,  4,  435;  structural, 
4,  280,  281;  tectonic, 
280,  281 

Geomorphogeny,  435 

Geomorphology,  4,  435 

Georgian  epoch,  549 

Geosyncline,  330,  504,  507, 
512,  647 

Gervillia,  *68$ 

Geyserite,  192,  194,  309 

Geysers,  134;  deposition 
by,  309 

Giant  granite,  296 

Giant  kettles,  137 

Giant's  Causeway,  369 

Gilbert,  C  H,  68 

Gilbert,  G  K,  219,  221,  398 

Gingko,  736 

Gingkoaceae,  628,  649,  669, 
684 

Giraffes,  766 

Girty,  G,  612,  614,  617 

Glacial  deposits,  227; 
epoch,  769;  ice,  152; 
erosion,  *i58;  marks, 
118,  *ii9,  *i6o,  *i6i, 
*i62;  pebbles,  229,  *23o; 
stages,  772,  784;  trans- 
portation, 164 

Glaciation,  Algonkian,  543, 
785;  Cambrian,  548,  552, 
785;  Devonian,  548,  599; 
Permian,  *i6i,  *23i, 
*233,  548,  641,  643,  *644, 
785;  Pleistocene,  769 

Glaciers,  149;  Alpine,  156; 
continental,  157;  hanging, 
*i53,  157;  maximum 
thickness  of,  152,  771; 
motion  of,  152;  pied- 


802 


INDEX 


mont,   157;  valley,  *i5o, 

J57 

Glacio-fluvial  deposits,  *234 

Glass,  acid,  293 ;  amor- 
phous, 9;  volcanic,  74, 
*75,  284,  288,  294,  295, 
301 

Glassy  rocks,  9 

Glassy  texture,  284,  288, 
294,  392 

Glauconite,  19,  267,  269,313 

Glauconitic  beds,  702 

'Globigerina,  270,  *554,  716 

Glossopteris,  649,  *65o 

Glossopteris  Flora,  649,  654 

Glyptocrinus,  *56g 

Glyptodonts,  758,  787 

Glyptostrobus,  *737 

Gnathodontia,  674 

Gneiss,  413,  418,  *4i9,  421, 
515;  biotite-,  418;  con- 
glomeratic, 419,  *42o; 
dioritic,  419;  granitic, 
419;  hornblende-,  418; 
jointing  of,  *37i,  *372, 
476;  of  complex  origin, 
420;  syenitic,  419 

Gold,  194,  428,  429 

Gomphoceras,  604 

Gondwana  Land,  666 ; 
system,  643,  644,  661,682 

Goniatites,  604,  634,  651 

Goniograptus,  *57i 

Gordon,  C  H,  419 

Gorges,  river,  443,  480,  482, 
483 

Gossan,  431 

Graded  streams,  481 

Graham  Id,  67,  173 

Grammysia,  603 

Granatocrinus ,  69 

Grand  Canon  of  Colorado 
R,  144,  *378,  442 

Grand  Canon  series,  544 

( >ranite,  *287,  293,  *  296, 
400,  401,  *4O4;  disinte- 
gration of,  102,  103; 
family,  293,  294;  joints 
of,  369,  474;  metamor- 
phosed, 419;  -porphyry, 
393,  296 

Granitite,  296 

G/anitpid  texture,  285,  *28j 

Graphite,  409,  417,  418, 
540,  544 

Graptolite  zones,  572 

Graptolites,  547,  555,  568, 
570,  586,  602 

Grass,    protection    by,  176 

Grasses,  735,  754 


Gravel,  279,  304,  320; 
in  deltas,  213;  littoral, 
245,  *246,  *247 ;  of  gla- 
cial streams,  *235,  237; 
old  river,  207 ;  river,  *2oo, 
428;  shoal-water,  245, 
252.  273 

Great  Basin,  203,  226,  444, 
464,  467,  491,  508,  509 

Great  Basin  Sea,  612 

Great  Lakes,  218;  history 
of,  779 

Great  Oolite,  677 

Great  Plains,  absence  of 
intrusions,  403 ;  Carbo- 
niferous, 610;  Devonian, 
596;  Ordovician,  565 

Greenbrier  stage,  610,  613 

Greenland,  Carboniferous, 
623;  diastrophism,  36; 
fjord  coast  of,  496 ;  ice- 
sheet,  *iS5,  *is6,  157, 
165;  temperate  floras  in, 

529 
Green    River    stage,     724, 

731 

Green  sand,  269,  312 
Grenville  series,  535 
Griffif  hides,  632 
Grooves,  glacial,  *i62 
Ground  ice,  166 
Ground  mass,  285,  286 
Ground    Sloths,    758,    766, 

787 
Ground    water,    98,     124, 

478;   level  of,  125,   431 
Group,  geological,  530 
Gryphcea,  *68$,  688,717 
Guadalupian  series,  637 
Guano,  194 

Guelph  stage,  578,  581,  597 
Gulches,  young,  *479 
Gulf  of  Mexico,  basin,  216, 

*244;      deltas     in,     210; 

limestone  banks,  259,  311 
Gulfs  of  faulting,  501 
Gulf  Stream,  167 
Guh\  738,  756 
Gunnison  R,  485 
Gymnospermas,    547,    602, 

628,  649 

Gymnotoceras,  *672,  673 
Gypidula,  *6oi,  603 
Gypsum,  n,  2O,  224,  226, 

278,  308,  312 
Gyroceras,  651 

Hackett,  A  E,  71,  104 
Hade  of    fault,    49,    *339, 
34O,  347,  348,  354 


Hadrosaurus,  720 

Haematite,  21,  187,431,  581 

Halobia,  671 

Halysites,  +585,  586,  602 

Hamilton  stage,   590,  595 

Hancpx,  F,  387 

Hanging  wall,  *339,  34O, 
351,  *3S2,  354,  *3S6- 

Haploceras,  713 

Haplophragmium,  *7i5 

Hardness  of  minerals,  10 

Harpoceras,  689 

Hartz  Mts,  466 

Hawaian  Ids,  40,  266 

Hayford,  J,  94,  95 

Headlands,  493,  494,  495 

Heat,  consolidation  by,  277; 
in  metamorphism,  414, 
radio-active,  368;  vol- 
canic, 86,  88 

Heave,  *339,  34O 

Heave-faults,  351 

Heaves,  352 

Heavy  spar,  425 

Hebertella,  +571 

Heilprin,  A,  51,  60 

Helderbergian  series,  590, 
591,  593,  597. 

Heliolites,  586 

Heliophyllum,  *6oa 

Helix,  *737 

Helvetian,  724,  775 

Hernias  pis,  588 

Henry  Mts,  399,  503 

Herculaneum,  55 

Herrings,  718 

Hesperornis,  721 

Heteroceras,  *7i5,  718 

Heteropods,  271 

Hexacoralla,  650,  655,  670, 
686 

Hexagonal  system,  7 

Hickories,  754,  765 

Himalayas,  antecedent  riv- 
ers, 484;  Archaean,  538; 
elevation,  724,  754;  Per- 
mian, 643;  rainfall,  108; 
Trias,  66 1 

Hip  par  ion,  757 

Hipponicharion,  *557 

Hippopotamus,  766,  787 

Hip  pur  ties,  717 

Hobbs,  W  H,  49 

Hog-backs,  *4$6 

H olaster,  717 

Hollows,  wind-made,  448 

Holmia,  *5=;6,  558 

Holocystites,  +585,  586 

Holoptychius,  *6o7,  608 

Holothuroidea,  631 


INDEX 


803 


Homalodotheria,  759 
H omalonotus ,  603 
Hoplites,  717 
Horizontal     displacements, 

*46,  *47,  *48 
Horizontal  strata,  forms  in, 

451 

Hormotoma,  *574,  575 

Hornblende,  16,  18,  72, 
103,  267,  289,  293,  294, 
296,  297,  300,  408,  413, 
418,  419,  421;  -andesite, 
298 ;  -gabbro,  299  ;  -gran- 
ite, 296,  297;  -schist,  421 

Hornfels,  408 

Hornstone,  309,  408 

Horses,  739,  744,  745,  747i 
757,  766,  787 

"Horses,"  in  coal-seams, 
382 

Horse-shoe  Crabs,  see  Xi- 
phosura 

Horsetails,  see  Equisetales 

Horsetown  series,  702,  705 

Horst,  *347,  384,  466; 
mountains,  466 

Horton  sandstone,  613 

Hovey,  E  O,  59 

Hudson  R,  467;  drowning 
of,  499;  estuary,  274; 
submarine  channel,  140 

Hudson  River  series,  565 

Hudson's  Bay,  raised 
beaches,  32 

Humboldt,  A  v,  677 

Humous  acids,  102,  197 

Huntington,  775 

Huronian  series,  535,  541 

Hyaenas,  766,  787 

Hyanodon,  745 

Hydaspian  stage,  658 

Hydration  of  minerals,  98, 
'102 

Hydrocarbons,  306,  315, 
316 

Hydrochloric  acid,  82 

Hydrochcerus,  787 

Hydro-fluosilicic  acid,  409 

Hydrogen,  6,  197,  315,  316 

Hydroid  Corals,  547,  572, 
586 

Hydrosphere,  5 

Hydrozoa,  555 

Hymenoptera,  687 

Hyoltihes,  *554,  555 

Hyopotamus,  747 

Hypabyssal  rocks,  284,  294 

Hyperodapedon,  674 

Hypersthene,  16;  -gabbro, 
299 


Hypothesis,     Nebular,    88, 

Intrusions,   403,    425,   429; 

532;  Planetesimal,  533 

energy     of,      405;      me- 

Hypothyris, 595,  *6oi,  603 
Hypsocormus,  *6()i 

chanics  of,    401 
Intrusive  rocks,  284 

Hyracodon,  745 

Intrusive  sheets,  77,  79 

Hyracotherium,  739 

Invertebrata,  547,  723,  735, 

Hystricomorpha,  758 

756»  765 
Inyo  Mts,  elevation  of,  783 

Ibis,  738,  756. 

Iowa-Missouri       coal-field, 

Ice,      8;       deposits,      227; 

620 

structure    of,    151. 

lowan  stage,  775,  777 

Iceberg  deposits,  238. 

Iron,  6,  104,  199,  292,  427, 

Icebergs,  151,  167. 

43  1  ;  deposition  in  tropics, 

Ice-fields,  157;  -foot,    166; 

104,    187,  242;  meteoric, 

-sheet,   107,  143 

273;    native,  91;  precipi- 

Ichthyornis, 721. 

tates  of,  309;  as  cement, 

Ichthyosauria,     674,     692, 

105,   276,   304;    chloride, 

*693,  718,  723,  729. 

82;    deposits,    192;   -hat, 

Ichthyosaurus,  *693. 

431;     in     marble,     417; 

Idiomorphic   particles,    285 

minerals,  21,    102,    129; 

Idonearca,  *7i5,  717. 

-ore,     *36o,     427-9    432, 

Igneous  agencies,  26,  38 

bog,  199,  309,  314;  lake, 

Igneous  rocks,  4,   26,    129, 

220,     223,    309;      -ores, 

283,    506;    classification, 

Archaean,  540;  pre-Cam- 

292;  destruction  of,  103, 

brian,    544;    oxide,     72, 

302;      Devonian,     597; 

192,      199,      269,      276, 

joints  of,    76,   369,   374; 

oxides,     294,    298,    300, 

metamorphism    of,    413; 

302,409,429;  pyrites,  21, 

minerals  of,  290;  Ordo- 

white,   22;   sulphide,   21, 

vician,      566;      Silurian, 

267,    429;     surface     de- 

583 ;  Triassic,  665. 

posits  of,  279,  432 

Iguanodon,  720 

Irregulares,  736 

Ill&nus,  587 

Isalco,  58,  67 

Illinoian  stage,  775,  777 

Isastrcea,  686 

Ilmenite,  21,  289 

Ischia,  earthquake,  50 

Inclined  strata,  land  forms 

Ischypterus,  673 

in,  452 

Islands,  493,  496;  volcanic, 

Inclusions,  *4O4,  405 

65 

Indiana-Illinois     coal-field, 

Isometric  system,  7 

620 

Isopoda,  603,  687 

Indian  earthquake,  427 

Isostasy,  94 

Indian  Swallows,  756 

Isostatic  adjustments,  514 

Indus,  sediment  from,  214 

Isotelus,  573,  *576 

Infusorial    earth,   220,  313 

Isotropic  substances,  8 

Injected  bodies,  391,  404 

Isthmus  of  Panama,  750 

Injection,  409,  413 

Inlier,  384;    faulted,  384 

Jackson  stage,  724 

Inoceramus,  *7i5,  717 

Jakutian  stage,  658 

Insecta,   547,  573,  588,  603, 

Jan  Mayen,  53 

632,  651,   656,  671,  687 

Japan,  earthquakes,  40,45; 

Insectivora,  729 

fault-systems,    467  ;      lo- 

Interglacial  stages,  772,  775, 

bate   coast,    501;    volca- 

784 

noes,  54,  55 

Interior   basins,   arid,    205  ; 

Jaspilite,  *36o 

pluvial,  205 

Java,  volcanoes,  54 

Interior  Sea,  546,  551,  563, 

Jellyfish,.  Cambrian,  555 

568,  579,  583,  594,  610, 

Jersey  an  stage,  775 

617,   621,   639, 

Joannites,  *6^2,  673 

Intratelluric   crystals,    285, 

John  Day  stage,  724,  743, 

289 

757 

804 


INDEX 


Joint-blocks,  102,  136,*  172, 

405,  473,  476 

Joints,  5,  76,  126,  278,  369, 
*379;  cause  of,  374; 
columnar,  *77,  *78,  369, 
*388  5*402,474;  compres- 
sion, 375;  diagonal,  374; 
dip,  374,  376;  irregular, 
*388;  master,  373,  376, 
472,  473;  oblique,  374; 
strike,  374,  376;  tension, 
374;  topographical  effects 
of,  467,  *468,  473 

ordan  valley,  185,  469 

orullo,  66 

ura  Mts,  478,  505 

urassic,  655,  657,  663,  677 

uvavian  stage,  658 

Kames,  234,  *236,  237,  771 
Kansan  stage,  775,  777,  784 
Kaolin,  129,  306 
Kaolinite,  19,  103,  218,  302, 

3°5 

Kaolinization,  186 
Karibogas,   227 
Karroo    system,    644,    645, 

666,  683 

Kaskaskia  stage,  610,  614 
Kayser,  £,35,  54,  268,  600, 

623,  728 
Keewatin     Glacier,     *773, 

776,  778,  779 
Keewatin  series,  535 
Kellaway  Rock,  677 
Kemp,  J  F,  290,  292,  315, 

418,  419,  465, 
Kenai  stage,  724,  742,  748 
Keokuk  substage,  610,  614 
Kettle  moraines,  230,  *23i 
Keuper,  658,  660,  665,  666 
Keweenawan  series,  535 
Kilauea,  *62,  63,  *67,    69, 

*7«»  *73 

Kimmeridge  Clay,  677 
Kimmeridgian,  677 
Kinderhook  stage,  610,  614 
Kirunga,  54 
Kittatinny   peneplain,   512, 

708 

Klamath  Mts,  68 1. 
Knife-edges,  119,  164,  510 
Knight,  C  R,  693,  695,  719, 

740,  746,  760 

Knoxville  stage,  702,  7O4 
Koninckina,  671 
Kootanie  stage,  702,  704 
Krakatoa,  56,  *57,  63,  80, 

82 
Kutorgina,  *554 


Labradorean  glacier,  *773, 
776 

Labradorite,  13,  14,  298, 
299 

Labyrinthodon,  674 

Lacertilia,  see  Lizards 

Laccoliths,  293,  397,  *398, 
*399,_403,  460 

Lacustrine  deposits,   215 

Ladinian  stage,  658 

Lcslaps,  720 

Lafayette  stage,  762 

Lagoons,  coral,  266;  depo- 
sition in,  308 

Lake  Agassiz,  778 

Lake  Algonquin,  780;  Bon- 
neville,  *2i8,  *219,  *22i, 
222,  491,  782;  Champlain 
781 ;  Chicago,  780 

Lake  deposits,  215;  chemi- 
cal, 220,  223;  in  polar  re- 
gions, 241;  in  temperate 
regions,  241;  mechanical, 
216,  222;  organic,  220 

Lake  Erie,  779,  780,  781 ; 
Huron,  216,  779,  780; 
Iroquois,  780,  781 ;  La- 
hontan,  223,  782;  Michi- 
gan, 779,  780;  Ontario, 
*i6o,  *i75,  217,  779,  780, 
781;  Saginaw,  780;  Su- 
perior, 216,  218,  779,  780; 
Warren,  780;  Whittlesey, 
780 

Lakes,  174,  215,  480,  783, 
"alkali,"  *226;  barrier, 
215;  .  deltas  in,  210; 
erosion,  215;  filling  of, 
217;  formation  of,.  96; 
fresh- water,  216;  ice-bar- 
rier, 235,  237;  in  tropics 
279;  peat  in,  197;  salt, 
205,  220;  salt  in,  278; 
tectonic,  215;  temporary, 
216;  volcanic,  57,  *58,  215 

Lakota  stage,  702,  704 

Laminae,  10,  319 

Lanarkia,  *588 

Land-bridges,  529 

Land  sculpture,  451 

Landslips,  45,  96,  131,  164, 
1 80 

Land-surfaces,  buried,  440, 
484;  submergence  of,  495 

Land,  waste  of,  180 

Langhian  stage,  724 

Lapilli,  81,  286 

Lapworth,  560 

Laramie  stage,  702,  709, 
710,  720,  72' 


Lasanius, 

La  Soufriere,  59 

Lateral   erosion    by   rivers, 

i39 
Latente,  104,  187,  242,  269, 

279 

Laurels,  735,  756 
Laurentian  granites,   535 
Laurentide    glacier,     *773, 

776,  778,  779 
Lava,  55,  58,  61-65,  67,  *6g 

*70,  *7i,  82,  83,  91,  95, 

535;   acid,  71,  72;  ascen- 

sive  force  of,  86,  89,  403 ; 

basic,  71,  72;    columnar, 

76,  *77,  *78,  369,  *3«8; 
irregularly  jointed,  *388; 
origin    of,    86,    88;    pla- 
teaus, 436,  459 ;  plug,  387  ; 
submarine,     79;     succes- 
sion of,  79 

Lava  flows,  293,  387,  *388, 
*389,  459,  491;  meta- 
morphism  by,  407;  tex- 
ture of,  397 

Layers,  182,  319 

Lead,  427;  ore,  432 

Lemming,  787 

Lemuroidea,  729,  739,  741, 
745 

Lepidodendrids,  600 

Lepidodendron,  625,  *626, 
648,  649 

Lepidodiscus,  *56g 

Lepidoptera,  687 

Lepidosteus,  691 

Lepidotus,  673,  691 

Leptana,  573,  588 

Leptolepis,  691 

Leptomitus,  ^554 

Leucite,  A4,  289,  293,  297, 
299;  -basalt,  293,  299; 
-rocks,  293 ;  -syenite,  293 

Level,  changes  of,  29,  96; 
of  no  variation,  91 

Levis  channel,  564 

Lias,  677,  678,  682 

Libbey,  W,  62,  69,  72,  73, 

77,  83,  87,  155,  156,  165, 
240 

Lichas,  *585,  587,  603 
Life,  Algonkian,  543;  Ar- 
chaean, 540;  Cambrian, 
553;  Carboniferous,  624; 
Cenozoic,  723;  Creta- 
ceous, 714,  721;  Devo- 
nian,600;  Eocene,  735; 
Jurassic,  684;  Mesozoic, 
655;  Miocene,  754;  Or- 
dovician,  568;  Oligocene, 


INDEX 


805 


744;  Palaeozoic,  547;  Pal- 
eocene,  729;  Permian, 
648;  Pleistocene,  785; 
Pliocene,  764;  progression 
of,  521;  Silurian,  584; 
Triassic,  667 
Lignite,  314,  315,  730,  742- 

744,  764 

Ligurian  stage,  724 

Limburgite,  293,  30O;  -por- 
phyry, 293 

Limestone,  19,  194,  310; 
chemically  formed,  279, 
307;  coral,  *263,  311; 
crinoidal,  312;  crystal- 
line, 312,  417;  crystalliza- 
tion in,  310;  estuarine, 
275;  foraminiferal,  312; 
fresh-water,  311;  magne- 
sian,  266,  312;  marine, 
310;  metamorphism  of, 
407,  408,  414,  420,  429; 
organic,  310;  shell,  *258, 
312;  shoal-water,  273; 
solution  of,  107,  127; 
terrestrial,  187,  242 

Limestone  banks,  259,  *26o, 

279,  3IO>  311 
Limestone  regions,  springs 

in,  132;   streams  of,  *i35 
Limestone  Shales,  610 
Limit    of    perpetual    snow, 

149 
Limonite,  21,  220,  241,  409, 

43  * 

Limulus,  687 
Linarssonia,  *554,  558 
Lingulella,  558 
Lingulepis,  *$$4,  558 
Lions,  787 
Lipari  Ids,  63,  79 
Liparite,  295 
Liriodendron,  *7 1 4,  7 1 6 
Lithodomus,  31 
Lithological  similarity,  534 
Lithosphere,    5 ;     segments 

of,  368;  shells  of,  27 
Lithostrotion,  629,  *63O 
Litopterna,  759,  *76o,  787 
Little  Sun-Dance  Hill,  398, 

*4<DO,  460 

Littoral  deposits,  245,  246 
Littoral  zone,  181 ;   area  of, 

247 

Live  Oaks,  756 
Livingstone  stage,  702,  71O, 

727 
Lizards,  692,  718,  723,  738, 

745 
Llamas,  745,  757,  766,  787 


Llandeilo  series,  560 

Mammals,    548,    656,    676, 

Llanvirn  series,  560 

698,   721,    723,  729,   738, 

Load,  modifying  effects  of, 

743-747,   757,    758,   765/ 

364,  413,  433 

786,  788 

Lockport   stage,    578,   581, 

Mammoth,  787 

588,  597 

Man  of  Spy,  788 

Lodes,  429 

Man,  origin  of,  788 

Loess,  *188,  241,  317 

Manasquan  stage,  702 

Lopkophyllum,  629 

Manganese,    6;     ore,    428; 

Lorraine  stage,  560,  564 

oxide,  273 

Loup  Fork  stage,  724,  75O, 

Mangrove  tree,  177 

757. 

Manlius  stage,  578,  583 

Lowville  stage,  560 

Manticoceras,  *6oi 

Loxonema,  634,  671 

Maples,  716,  735,  756, 

Ludlow  series,  578 

Maps,  geological,  561 

Lung  Fishes,  see  Dipnoi 

Marattiaceas,  625,  668 

Lutetian  stage,  724 

Marble,  408,  417,  418,  515; 

Lycopodiales,      547,      600, 

onyx,  307 

625,   *626,    648,   668 

Marcasite,  22 

Lyell,  C,  79,   109,  142,  726 

Marcellus  stage,  590,  595 

Lyosoma,  *68$ 

Marginella,  *7&5 

Lytoceras,  *685,  689,  713 

Marine  deposits,  181,  243, 

*268;    classification,  244, 

McGee,  W  J,  112 

245;     modern    compared 

Maclurea,  *574,  575 

with  ancient,  273 

Macroseisms,  42,  49,  50 

Marl,  306;    in  lakes,  220; 

Macrot&niopteris,  *668 

shell,  220,  311 

Macrura,  670 

Marmots,  747,  758 

Magma,  284,  285,  288,  397, 

Marshall  series,  613 

399;       ascending,      291; 

Marshes,  salt,  275 

ascension  of,  403;    chem- 

Marsupials, 758,  788 

ical  composition  of,  288; 

Marsupites,  716 

dioritic,  297;  fluidity  of, 
403;  fusion  of    rock    by, 

Martin,  46 
Martinique,  59,  61 

291,  404;     gabbro,    298; 

Masaya,  58 

granitic,      296;     syenitic, 

Mascarene  Ids,  54 

296;    universal,  292,  539 

Massive  rocks,  284 

Magmas,    407;     acid,    407; 

Master-stream,  486 

ascensive    force  of,    401; 

Mastodons,  758,   766,  786, 

as    agents  of  dislocation, 

787 

366,  403;  basic,  395,  407; 

Mastodonsaurus,  674 

heat    from,  414;     meta- 

Matawan  stage,  702,  708 

morphic,  415 

Mato  Tepee,  369,  399,  *402> 

Magmatic  segregation,  428 

460 

Magnesia,    104,    199,    292, 

Matopos  Hills,  *ii7,  *n8 

294,  298,  300,  306,  312 

Matthew,  G  F,  344 

Magnesium,  6 
Magnesium  carbonate,  312; 
chloride,    225,    266;   sul- 

Maturity in  arid  cycle,  447  ; 
topographical,  438 
Mauch  Chunk   stage,  610, 

phate,   224,    226 

613 

Magnetite,    21,  289,     290, 

Mauna  Loa,  50,  62,  74,  *83 

296,  297,  298,  300,  304, 

May  Hill  series,  578 

408,  409 

Meandering     of     streams, 

Magnolias,  735,  756,  765 
Maidenhair       Tree,        see 

140,  *i4i 
Meanders,  intrenched,  483 

Gingkoaceae 

Mechanical    deposits,   303; 

Malaspina     glacier,    *154, 

in  lakes,  216,  *22i,  222; 

157,  235,  *239 

land,  187 

Malay  Ids,    Archaean,    538 

Mecklenburgian  stage,  775 

Malocystites,  *$6g 

784 

8o6 


INDEX 


Medina  stage,  560,  565, 
578,  581 

Mediterranean,  267;  deltas, 
210;  earthquakes,  40; 
fault-scarps,  47 ;  vol- 
canoes, 54 

Medlicottia,  651 

Meekella,  *633,  634 

Meekoceras,  *6j2,  673 

Megaceros,  787 

Megalonyx,  787 

Megalosaurus,  696 

Megalurus,  691 

Megatherium,  787 

Melonites,  *63o,  631 

Meniscoessus,  721 

Menaspis,  652 

Merced  series,  724,  761 

Mercury  sulphide,  194 

Meristina,  *58$ 

Mesas,  451,  452,  459,  476 

Mesohippus,  745 

Mesonacis,  *557 

Mesosaurus,  652 

Mesozoic  era,  531,655,  722 

Messinian  stage,  724 

Metallic  carbonates,  426; 
oxides,  .426;  sulphides, 
426 

Metals,  concentration  of, 
432 ;  native,  426 

Metamorphic  rocks,  5,  283, 
317,  415;  crystalline, 
410;  foliated,  418;  non- 
foliated,  415;  resistance 
to  weathering,  515 

Metamorphism,  27,  406, 
540;  causes  of,  413;  con- 
tact, 407,  413,  414,  4!6, 
420;  dynamic,  409,  414, 
415,  420,  433,  5°6»  5°8, 
514;  regional,  407,  409, 
413,  416 

Metamynodon,  745 

Meteoric  iron,  273 

Meteorites,  24,  273,  533 

Methane  series,  316 

Mexican  mountains,  707 

Mica,  15,  18,  72,  103,  290, 
304,  408,  409,  416,  419; 
-schist,  421;  -syenite, 
297;  -trachyte,  297 

Mice,  747,  758 

Michigan  coal-field,  620 

Microcline,  13,  14 

Microconodon,  676 

Macrocrystalline    texture, 
285,  288 

Microdiscus,  *557,  558 

Microlestes,  676 


Microseisms,  42,  50 

Middle  Coal,  610 

Midway  stage,  724,  727 

Millstone  Grit,  610,  615, 
620,  680 

Milne,  J,  42 

Mineral  veins,  194,  423 

Mineralizers,  287,  288,  414 

Minerals,  6,  282 ;  accessory, 
290;  alteration  of,  i2q, 
290;  essential,  290;  fi- 
brous, 10 ;  massive,  10;  of 
igneous  rocks,  290,  302; 
of  sedimentary  rocks,  302  ; 
original,  290;  physical 
properties,  8;  rock-form- 
ing, 4,  6,  11;  secondary, 
290;  volcanic,  72  272 

Miocene,  724,  726,  748 

Mississippi  embayment, 
708,  730,  748 

Mississippi,  delta  of,  33, 
211,  213 

Mississippi  R,  140,  147, 
1 80,  200,  481,  484;  delta, 
33,  211,  213;  material 
carried  by,  146,  147,  216 

Mississippi  Valley,  Carbo- 
niferous, 610;  Palaeozoic, 
545;  thickness  of  beds, 
505 

Mississippian  series,  610, 
612,  614 

Mitchill,  B  N,  229 

Mitra,  756,  *765, 

Mock-orange,  777 

Mohawkian  series,  560, 
564 

Moisture  as  agent  of  meta- 
morphism,  414 

Mollusca,    258,    259,    270, 

547,  558,  *574,  575,  6°3, 
634,   651,   656,  671,  688, 
717,     736;    Arctic,     767, 
771;  fresh-water,  220 
Monkeys,     729,     739,    745, 

766 

Monmouth  stage,  702 
Mono  Lake,  224,  *465 
Monoclinal      folds,     *335, 

*336,  354,  456 
Monoclinic  system,  7 
Monodonius',  720 
Monocotyledons,  655,  684 
Monomerella,  *$8$,  588 
Monometric  system,  7 
Monongahela  stage,  610 
Monopteria,  *(>33,  634 
Monosymmetric  system,  7 
Monotremata,  698 


Montana  series,  702,  709 
Monte    Diablo   range,    up 

heaval,  763 

Monte  Nuovo,  31,  *66 
Monte  Somma,  83,  *85 
Monterey  series,  724,  749, 

761^ 

Montian  stage,  724 
Montlivaultia,  686 
Monument  Park,  in,  *ii2 
Moraines,      164;      ground, 
165,   *228,   232;     kettle, 
23O,  *23i;    lateral,   164, 
227;    medial,  *i56,   164, 
228;  terminal,  *i5o,  166, 
228,  *229 

Morrison  series,  680,  704 
Mosasauria,  718,  *7i9,  723, 

Mosses,  197 

Motion,  horizontal,  367; 
pivotal,  367 

Moulds,  *5i8 

Mt.  Blanc,  *i48,  "=150 

Mt.  Hood,  83 

Mt.  Pelee,  *6o,  61 

Mt.  Rainier,  83,  387 

Mt.  Shasta,  83,  *84,  387 

Mountain  Limestone,  613 

Mountain  range,  504;  chain, 
504;  system,  504 

Mountain  ranges,  ancient, 
511;  conformity  in,  505; 
degradation  of,  510;  fold- 
ing in,  505,  508;  geo- 
logical date  of,  509; 
granite  core  of,  401,  514; 
origin  of,  507;  thickness 
of  strata,  504;  uncon- 
formities in,  505;  youth- 
ful, 511 

Mountains,  503 ;  block, 
464,  503 ;  denudation  of, 
509;  Horst,  466;  lac- 
colithic,  398,  460,  503 ;  of 
folding,  503 ;  pyramidal, 
476;  residual,  *445,  449 ; 
synclinal,  458,  510;  table, 
451,  459,  503;  volcanic, 
436,  503 

Mud,  367,  305,  306;  blue, 
218,  245,  267,  274; 
coral,  245,  259,  263; 
felspathic,  306;  green, 
245,  269;  littoral,  245, 
247;  playa,  278;  red, 
245,  269,  274;  shoal- 
water,  245,  254,  255; 
volcanic,  245,  269 

Mud  cracks,  *2O6,  249 


INDEX 


807 


Mud  flats,  274 
Mudstone,  306 
Miiller,  G,  641 
Mullet,  718 
Multituberculata,  698,  721, 

729 
Murchison,    R,    549,    560, 

,578,  .59°,  637 
Murch^son^a)  671 
Murex,  717 
Murray,  J,    245,   266,    268, 

270,  271 

Muschelkalk,  658,  659 
Muscovite,     15,    296,    421; 

-granite,  296 
Musk-ox,  771,  787 
Mustelines,  766 
Myacites,  *68$ 
Myalina,  *6^,  634,  651 
My  odes,  787 
Mylodon,  787 
Myophoria,  671 
Myrtles,  735 


"523, 


Naosaurus, 

Naples  Fauna,  *522,  *< 

596 
Nassa, 
Natica,  * 
Nautiloidea,  575,  604,  634, 

671,  689,  736 
Nautilus,  575,  651,  718 
Nebula,  532,  533 
Necks,  volcanic,  385,  *386, 

399,.  459. 

Negative  displacements,  30 

Neocene,  727 

Neocomian,  762,  712 

Neogene,  727 

Nepheline,  14,  289,  293, 
297,  299;  -basalt,  293, 
299;  -syenite,  293,  297; 
-syenite-porphyry,  293 

Nerinea,  *68s,  688 

Nesodon,  758 

Neudeckian  stage,  775 

Neumayr,  M,  683 

Neuroptera,  603,  632,  687 

Neuropteris,  586,  649 

Nevada  trough,  566, 582, 597 

Neve,  151 

Newark  series,  658, 664,  678 

Newfoundland  Glacier, 
*773,  ™ 

Newman  stage,  613 

New  R,  490 

New  Scotland  stage,  590 

New  York,  Devonian,  590; 
Ordovician,  560;  Silu- 
rian, 578 


New  Zealand,    Cretaceous, 

Obsidian  Cliff,  *78,  288,  369 

713;    Eocene,  734;    fjord 

Ocean  basins,  5 

coast    of,    496;     geysers, 

Oceans,     area     of,     *i85; 

192;    Miocene,  754;    Or- 

depth  of,  *i85 

dovician,  567;    Permian, 

Odontocephalus,  *6oi,  603 

645;      Pleistocene,     772; 

Offset,  341,  *349,  350,  351 

Trias,  667 

Ohio  shale,  596 

Niagara  R,  137,  143,  781 

Oil-fields  of  U.S.,  316 

Niagaran  series,   578,   582, 

Old  age,  in  arid  cycle,  448; 

583 

of     rivers,     481;      topo- 

Nickel, 429 

graphical,  438 

Nicobar  Ids,  volcanoes,  54 

Oldham,  R  D,  44 

Nile,  241;    delta,  214 

Old    Red    Sandstone,    598 

Niobrara  stage,  702,  708 

Olenellus,  *556,  558 

Nitrogen,  197;    fixation  of, 

Olenellus  Fauna,  549 

178 

Oligocene,  724,  726,  741 

Nodules,    322,    324;     iron, 

Oligoclase,  13,  14,  294 

187;     manganese,     273; 

Oligoporus,  631 

phosphatic,  194 

Oliva,  *737 

Non-marine  sediments,  181 

Olivine,  17,  289,  290,  293, 

Norfolkian  stage,  775 

298,    299,    300;     -basalt, 

Norian  stage,  658 
Norite,  209 

293,  299;  -diabase,  293; 
-gabbro,  293,  299;  -gab- 

North  America,  Algonkian, 

bro-porphyry,  293;  nick- 

541;        Archaean,       536; 

eliferous,  429 

Cambrian,      549,      *55o; 

Omosaurus,  696 

coal-fields,      619;       Cre- 

Oneida conglomerate,  565, 

taceous,        700,        *7oi; 

578 

Devonian,      591,      *592; 

Onondaga  stage,  590,  594, 

Eocene,     729;     Jurassic, 

597 

*662,  678;    Lower   Car- 

Onychocrinus, 631 

boniferous,     610,     *6n; 

Onyx  marble,   307  ;   Mexi- 

Miocene, 748;  Oligocene, 

can,  307 

741;      Ordovician,     561, 

Oolite,  264,  307,  677 

*5&2;      Palaeozoic,     546; 

Ooze,     diatom,    245,    272, 

Paleocene,  727;  Permian, 

274;    foraminiferal,    245, 

*6i6,    638;     Pleistocene, 

*27O,     274,      311,      712; 

772,  *7.73..7755  Pliocene, 

gteropod,  245,  *271;  ra- 

759;  Silurian,  579,  *58o; 

iolarian,  245,    272,  274 

Tertiary,       724,       *725; 

Oozes,    organic,    279,    310; 

Trias,    657,    661,    *662  ; 

siliceous,  313 

Upper         Carboniferous, 

Ophicalcites,  417 

615,  *6i6 

Ophileta,  574 

North  Sea,  174;  deltas,  210 
Nolhosaurus,  675 

Ophioglossaceae,  625 
Ophiuroidea,  572,  686 

Notostylops  beds,  728,  787 

Opisthoptera,  *574 

Notre  Dame  de  la  Garde, 

Opossums,  758 

statue  of,  6  1 

Orbitolites,  312,  736 

Nova  Zambia,  623 

Orbulina,  *SS4 

Nudeocrinus,  *6oi 

Ordovician,  531,   541,   56O, 

Nullipores,  260,  263,  270 

597,  598 

Nummulites,  312,  734,  736 

Ore  deposits,  427 

Nunataks,  *155,  165 

Oreodonts,  745,  747,  757 

Ores,  enriched,  431  ;  formed 

Oaks,  716,  735,  736,  756 

by  surface    waters,   432; 

Obelisk,  Egyptian,  118 

in  veins,  426;    iron,  309, 

Oblique  system,  7 

428,    432;   lead,   432;  of 

Obsidian,  74,  *75,  *78,  293, 
294;     devitrification    of, 

contact      metamorphism, 
429;  of  magmatic  segre« 

296 

gation,    428;    origin    of 

8o8 


INDEX 


430  ;   oxidized,   431;   un- 

Palaeechinoidea, 631,  655 

altered,  431;   zinc,  432 

Palaeogene,  726 

Organic  accumulations,  309 
Organic  agencies,  1  76  ;  ero- 

Palceohatteria, 653 
Pal&oniscus,  635 

sion  by,  1  80 
Organic  deposits,  in  lakes, 

Pal&osyops,  741 
Palaeotheres,  747 

220;   shoal  water,  257 

Palaeozoic    era,    512,    531, 

Orinoco  R,  205,  269 

545,  655,  656,  722 

Oriskanian  series,  590,  591, 

Palaeozoic      rocks,      shoal- 

593 

water,  545;  thickness,  545 

Oriskany  stage,  590 

Paleocene,  724,  726,  727 

Ornithomimus,  720 

Palisades,    114,    299,    395, 

Ornithorhynchus,  698 

*396,  *397,  *46i,  467,  665 

Ornithosloma,  719 

Palms,  716,  735,  *736 

Orontes,  valley,  469 

Palustrine  deposits,  196 

Orthis,  *57i,  573,  603 

Pampas,  189,  222 

Orthoceras,  *574,  577,  604, 

Panthers,  757 

634,  651,  671 

Paradoxides,      552,      *557, 

Orthoceratites,  570,  589 

558 

Orthoclase,  n,  13,  19,  103, 

Paradoxides  Fauna,  549 

293,  294,  296,  297,  419 

Paraguay  R,  205 

Orthoptera,  603,  632,  687 

Paramorphic  minerals,  413 

Orthorhombic  system,  7 

Parana  stage,  764 

Orthothetes,  603 

Parasuchia,  674 

Osage  stage,  610,  614,  631 

Pareiasaurus,  654 

Osborn,  H  F,  653,  693,  695, 

Parrots,  756 

719,  740,  746,  751 

Partings  in  coal,  617 

Ostracoda,    558,   573,   632, 

Passarge,  449 

670 

Patagonian  stage,  754 

Ostracodermata,   577,   589, 

Patapsco  stage,  702 

604,  635 

Patuxent  stage,  702 

Ostrea,  *685,  688,  *7i5,  717, 

Pawpaw,  777 

*737 

Peat,    197,   314,    3T55     in 

Oswegan  series,  578 

lakes,  220;    in  polar  re- 

Otozamites, *66g 

gions,    241;     in    tropics, 

Otters,  757,  758 

242 

Ouachita  Mts,  639;  647 

Peat-bog  theory,  619 

Ouachita  range,  504,  505 

Peat-bogs,    196,    278,    310; 

Outcrop,  326;    affected  by 

ancient,  315,  320,  382 

faults,   *348,   *349,   350; 

Pebbles,  136,  304,  305,  322; 

of  mineral  veins,  431 

beach,  172;    compressed, 

Outliers,  383;  faulted,  384 

416,    419,    *42o;     coral, 

Overlap,  *38i 

263;     glacial,    161,    229, 

Overwash  plain,  234,  771 

*230,    232;     river,    201; 

Owls,  738,  756 

sheared,      375;        wind- 

Ox-bow  lakes,  140,  *i4i 

carved,  123 

Oxen,  766 

Peccaries,  747,  758,  766,  787 

Oxford  clay,  677 

Pecopteris,  649 

Oxfordian,  677 

Pecten,  671,  *737 

Oxidation  of  minerals,  98, 

Pediomys,  721 

102 

Pegmatite,    296,  394 

Oxygen,  6,    98,    101,    197, 

Pelagic  deposits,  245,  269; 

3r5 

fauna,    555;     organisms, 

Oyster  banks,  275 

523 

Pelecypoda,  558,  575,  589, 

Pachyana,  739 

634,  651,  656,  671,  688, 

Pacific,  atolls,   265;    earth- 

736 

quakes,    40  ;     volcanoes, 

Pelicans,  738,  756 

Palceasterina,  *$6g 

Peltoceras,  *68s,  689 
Pelycosauria,  *653 

Penck,   A,    180,    185,    49.1 

495.  501,  515,  552,  775 
Peneplain,  444,  *445,  48^ 
487,  490;    dissected,  445, 
451,  482,  511,  512;  reele- 
vated,  458,  482,  511 
Peneplanation,  514 
Pentacrinus,  *685,  686 
Pentameridae,  588 
Pentamerus,  *585,  588 
Pentremites,  629,  *63o 
Peorian  stage,  775,  777 
Perched  blocks,  230,  *232 
Percolating  water,  194 
Peridotite,   293,  300;   fam- 
ily, 293,  300;  -porphyry, 

293 

Periods,  geological,  527,  530 

Peris phinctites,  689 

Perissodactyla,  729,  739, 
744,  745,  747,  758 

Perlite,  293,  295 

Perlitic  structure,  295 

Permian,  531,  547,  637,  666, 
667 

Petalodus,  *63<D 

Petraia,  *$6g,  572 

Petrifactions,  520 

Petrography,  292 

Petroleum,  316, 

Phacops,  587,  *6oi,  603 

Phalangers,  758 

Phenocrysts,  285,  288,  289. 
295,  296,  299 

Philippi,  E,  659 

Philippine  Ids,  volcanoes,  54 

Phillipsaslraa,  602 

Phillipsia,  632,  *633,  650 

Pholadomya,  *685 

Phonolite,  2^3,  297;  colum- 
nar, 399,  *4O2 

Phonolite-porphyry,  293 

Phosphate  deposits,  194 

Phosphatic  nodules,  194 

Phosphorus,  6 

Phragmoceras,  589,  604 

Phyllite,  416,  421 

Phyllocarida,  558,  573,  603 

Phylloceras,  689,  713 

Phyllograptus,  *57i 

Phyllopoda,  632,  670 

Physiography,  4,  435 

Piedmont  glaciers,  157 

Pigs,  739,  747,  7°6 

Pillars,  384,  493;  ram,  in, 

*II2 

Pines,  735,  736 
Pimtes,  684 

Pipes,  127;  volcanic,  *387, 
388 


INDEX 


809 


Pisolite,  307 

Pistazite,  17 

Pitch  of  folds,  *328,  330, 
^374,  478 

Pitchstone,  293,  295 

Pithecanthropus,  766,  788 

Placenticeras ,  717 

Placers,  428 

Plagiaulax,  698 

Plagioclase,  13,  293,  297, 
298;  acid,  296 

Plains,  alluvial  436;  coast- 
tal,  437,  492,  493;  of  sub- 
marine origin,  436,  437, 
440,  441;  rock-floored, 
449;  of  subaerial  origin, 
441 ;  truncated  rock,  448, 
449;  volcanic,  436 

Plaisancian  stage,  724 

Planorbis,  *737 

Plants,  547,  555,  570,  600, 
625,  648,  655,  668,  684, 
714,  723,  728,  735,  748, 
754,  764,  786 

Plaster  of  Paris,  20 

Plasticity  of  rocks,  359 

Plateau  province,  elevation 
of,  752,  763,  783 

Plateaus,  volcanic,  436,  483, 
484 

Platform,  wave-cut,  440, 
494 

Platinum,  428 

Platyceras,  *6oi,  604 

Platycrinus,  602,  631 

Plalyostoma,  589 

Platystrophia,  *57i,  573 

Playas,  216 

Plectambonites,  *57i,  573 

Pleistocene,  33,  762,  769 

Plesiosauria,  675,  694,  718, 
723,  729 

Plesiosaurus,  *6g4 

Pleur acanthus,  635,  *652 

Pleurocystites,  *56g 

Pleuromya,  *685 

Pleurophorus,  *633,  651 

Pleur  otomaria,  *633,  634, 
*68s,  688 

Pliauchenia,  757 

Plication,  333,  *334,  355, 
359,*36i,  *362,  364,  505, 
508 

Pliny,  55 

Pliocene,    724,    726,    759 

Pliosaurus,  694 

Plucking,  by  streams,  136; 
glacial,  *i58,  *i59,  164 

Plutonic  bodies,  composite, 
391;  injected,  391;  mul- 


tiple, 391;    simple,   391; 

subjacent,  391,  400 
Plutonic  ^  rocks,   284,   290; 

forms  in,  460 
Pluvial  climates,  205,   241, 

278,  439, 

Pocket-gophers,  747 
Pocono  stage,  610,  613 
Poebrotherium,  747 
Polandian  stage,  775 
Polar  regions,  91 ;  continen- 
tal   deposits,     240,    278; 

denudation,   446;  marine 

deposits,  252,  273 
Polar  seas,  deposits,  273 
Polish,  glacial,  #158,  *161 
Polymastodon,  729 
Pompeii,  *56 
Ponderosa  Marls,  702 
Popanoceras,  651 
Poplars,  716,  735,  736,  754, 

756 

Populus,  714 
Portage  stage,  *522,  *523, 

590,  596,  -597 
Port  Ewen  stage,  590 
Portheus,  718,  *7ig 
Portlandian,  677 
Portland  stage,  677 
Porphyritic  texture,  76,  285, 

*286,  294 

Positive  displacements,  30 
Potash,  104,  136 
Potassium,  6,  294 
Pot-holes,  137,  *i38,  *i39; 

glacial,     164;     wind-cut, 

122 
Potomac  R,  482,  488,  490; 

gorge,  443;  488 
Potomac  series,  678,  7O2 
Pottsville   stage,    610,   615, 

621,  680 

Pourtales  Plateau,  *26o 
Pozzuoli,  30,  33,  66 
Pre-Cambrian     eras,     531, 

534,   *537;   classification 

of>  535 

Precipitates,  chemical,  307 
Preliminary  tremors,  *38 
Pressure,  consolidation  by, 

277;    in   metamorphism, 

414 

Prestwich,  J,  90 
Prestwichia,  632 
Primary  group,  724 
Primary  rocks,  283 
Proboscidea,  757,  758 
Procamelus,  757 
Prodromites,  *63O,  635 
Produced,  603 


Productidae,  651 

Productus,  *63O,  *633,  634 

Proetus,  587,  632 

Proganosauria,  652 

Protapirus,  745 

Protection  of  soil  by  vege- 
tation, 100,  108  ;  111,  176 

Proterosauria,  653 

Proterosaurus,  653 

Protoceras,  747,  757 

Protohippus,  757 

Protorthis,  *554 

Protospongia,  *554 

Protowarthia,  *574,  575 

Protylopus,  745 

Provinces,  Cambrian,  550; 
Devonian,  600 ;  faunal, 
*522;  petrographical,  290 

Pseudodiadema,  716 

Pseudomonotis,  *633,  651, 
671,  *672 

Pseudomorphs,  11,  424,  520 

Pteranodon,  719 

Pteraspis,  605,  606 

Pterichthys,  *6o5,  606 

Pterinea,  *574,  *6oi,  603 

Pteropods,  271,  589,  634 

Pterosauria,  676,  *696,  719, 
723,  729 

Pterophyllum,  669 

Pterygometopus,  573 

Pterygotus,  587,  603 

Ptilodus,  721,  729 

Ptychites,  651 

Ptychoceras,  718 

Ptychoparia,  *557 

Ptyonius,  636 

Puerco  stage,  724,  728 

Pugnax,  *633 

Pumice,  81,  293,  295,  *39o; 
in  deep  sea,  273 

Pumiceous  texture,  285 

Purbeck,  677 

Purpura,  756,  *765 

Purpurina,  688 

Pyramidal  system,  7 

Pyrenees,  514;  Archaean 
of,  538;  upheaval  of,  753 

Pyrite,  21,  194,  289,  431, 
520 

Pyroclastic  rocks,  286,  389, 
"300 

Pyropsis,  *7i5 

Pyroxene,  429 ;  -andesite, 
298;  -trachyte,  297 

Pyroxenes,  16,  289,  290, 
293.  297,  298,  300,  409, 
418 

Pyroxemte,  293,  300  J  -por- 
phyry, 293, 


8io 


INDEX 


a 


uail,  738 

uartz,  n,  13,  72,  103, 
104,  105,  123,  172,  194, 
246,  267,  289,  290,  293, 
294,  295,  296,  298,  302, 
3°4,  3°S,  3°6>  3°9.  4o8, 
409,  416,  419,  421,  425, 
426;  -diorite,  293,  298; 
-diorite  -  porphyry,  293 ; 
-porphyry,  293,  296,  393; 
-trachyte,  295 

Quartzite,  408,  *4ii,  415, 
416,  421,  515 

Quaternary,  531,  723,  724, 
761,  768 

Rabbits,  747,  758 

Raccoons,  745 

Radio-activity,  88,  368,  432 

Radiolaria,  267,  272,  313, 
570,  684 

Radiolites,  717 

Rafinesquina,  +571,  573 

Rain,  destruction  by,  101, 
1 1 6,  1 80;  prints,  207, 
*25i,  275,  322;  wash, 
278;  water,  101,  124 

Rancocas  stage,  702 

Ransome,  352 

Raritan  stage,  702 

Rats,  758 

Rays,  690,  732 

Recent  age,  769,  788 

Receptaculites,  *s6g 

Recession  of  spring-heads, 
132 

Reconstruction  of  rock,  97 

Red  Beds,  640,  647 

Red  colour  of  desert  de- 
posits, 242 

Red  R,  480 

Red  Sea,  500;    sun  cracks, 

Redwood,  756 

Reef  rock,  264 

Regimen  of  a  river,  139 

Regular  system,  7 

Reid,  J  A,  346,  354,  367 

Reindeer,  771,  787 

Relief,  101,  442;    effect  on 

marine  deposits,  243 
Renard,  245,  267,  268,  270, 

271 

Renssellaeria,  603 
Reptilia,  548,  652,  656,  674, 

692,  718,  723,  738 
Republican    R  stage,    724, 

761 

Requienia,  717 
Reservoirs,  volcanic,  86,  87 


Residual  accumulations,  186 

Resorption  of  crystals,  285, 
289 

Reticularia,  *63o,  634 

Reusch,  552 

Rhabdoceras,  673 

Rhsetic  stage,  658,  660,  665, 
666,  682 

Rhamphorhynchus ,  *6g6 

Rhaphistoma,  *554 

Rhine,  delta,  210;  fault- 
valley,  467 ;  sub-lacustrine 
channel,  141 

Rhine-Belgium,  Devonian 
section,  590 

Rhinoceros,  frozen  carcasses 
of,  518;  hairy,  787 

Rhinoceroses,  741,  744,  745, 
>47,  7.57,.  766,  787 

Rhodocrinus,  631 

Rhombic  system,  7 

Rhone,  216;  delta  of,  213; 
sub-lacustrine  channel  of, 
141 

Rhus,  *755 

Rhynchoccphalia,  692,  719 

Rhynchonella,  671,  687, 
717 

Rhynchotrema,  *57i,  575 

Rkynchotreta,  *$85,  588 

Rhyolite,  293,  295,  296; 
-breccia,  301;  -obsidian, 
295;  -porphyry,  293; 
-tuff,  301 

Richmond  stage,  560,  565 

Richthofenia,  651 

Ridges,  anticlinal,  456, 
*457,  *4$8;  mountain, 
504;  of  hard  rocks,  452, 
*458,  482;  synclinal,  457 

Rift  Valley,  469 

Rigidity  of  rocks,  362 

Rill  marks,  249,  *252,  322 

Ripley  stage,  702,  708 

Ripple  marks,  *248,  *249, 
*25o,  255,  322,  507 

Ripples,  wind,  *i8g 

River  channels,  sub-lacus- 
trine, 141 ;  submarine, 
34,  14° 

River  deposits,  199;  behind 
barriers,  253;  in  estu- 
aries, 275;  of  polar 
regions,  240;  in  temper- 
ate regions,  241;  in 
tropics,  242 

River  gravels,  old,  207 

River  mud,  499 

River-system,  evolution  of, 
488,  *489 


Rivers,  477;  accidents  to 
491;  adjustment  of,  485; 
aged,  481;  "alkali,"  146; 
antecedent,  483,  485; 
consequent,  478,  485; 
constructional  work,  443 ; 
cross-bedding  in,  255; 
denuding  work  of,  442; 
drowned,  482 ;  erosion 
by,  135,  1 80;  mature, 
480;  rejuvenated,  482; 
revived,  483;  salt,  146; 
subsequent,  485 ;  super- 
imposed, 484,  485 ;  trans- 
verse, 487 ;  youthful,  480 

River  waters,  dissolved  sub- 
stances, 146,  222 

Roches  moutonnees,  161, 
*640,  769 

Rochester  stage,  578,  581  - 

Rock-flour,  229 

Rock-forming  minerals,  4, 11 

Rock-powder,  161 

Rock-slides,  *i29,  *i30, 
131,  180 

Rock  terraces,  210 

Rocking  stones,  122,  232 

Rocks,  4,  282;  classifica- 
tion of,  282;  igneous,  4, 
26,  283,  318,  385,  406, 
433;  massive,  284,  318, 
385;  metamorphic,  5, 
283,  317,  406,  407,  415; 
pyroclastic,  286,  3OO; 
resistance  of,  100,  443; 
sedimentary,  5,  182,  283, 
302,  318,  407;  stratified, 
5,  182,  318;  unstratified, 
3i8,385,433 

Rocky  Mt.°,  504;  Archaean, 
538;  batholiths,  401; 
Carboniferous,  620;  De- 
vonian, 597;  elevation, 
711,  763;  glaciers,  152, 
157;  thermal  springs,  134 

Rodentia,  729,  739,  741, 
747,  758,  766,  787 

Rogers,  A  W,  233 

Romingeria,  *$6<) 

Rondout  stage,  578,  583 

Roots,  fossil,  618 

Rosebud  stage,  750,  751 

Rothliegendes,  641,  642 

Rotten  rock,  107;  stone,  107 

Rudistes,  713,  717 

Ruminants,  '739.  747,  757 

Running  water,  destruction 
by,  99,  124 

Run-on,  124 

Russell,  I  C,  668 


INDEX 


811 


Sabre-tooth  cats,  745,  747, 

certain  origin,    184;   rip- 

Sea-wave, earthquake,  44 

757.   787 

ple-marked,  *249;  wind- 

Seas,  enclosed,  181  ;  deposits 

St    Elias    Alps,    154,    235; 

sculptured,  *I2I 

in  land-locked,  243 

upheaval,  763 

Sandwich  Ids,  62,  83 

Seatstone,  618 

St  Helena,  53 

Sangamon  stage,  775,  777 

Seaweeds,  547;  as  limestone 

St  Lawrence  R  ;  submarine 

Sanidine,  13,  295 

makers,  312;  calcareous, 

channel  of,  34 

Santa  Cruz  stage,  754,  758 

258;  Coralline,  263,  670 

St  Louis  stage,  610,  614 

Santa  Maria,  58 

Secondary  group,  724 

St  Peter  stage,  564 

Santorin,  67 

Secondary  rocks,  182,  302 

St  Pierre,  destruction  of,  61 

Saportcea,  649 

Secretary  Birds,  756 

St  Vincent,  59,  61 

Saratogan  epoch,  549 

Sedgwick,  A,  549,  578,  590 

Sagenites,  *6j2,  673 

Sarmatian  Sea,  753,  764 

Sediment,     settling    of,     in 

Salenia,  716 

Sassafras,  714,  716 

fresh   water,    210,    *2i2; 

Salina  stage,  578,  582,  584, 

Savile  Kent,  262,  263 

in  salt  water,  210,  *2i2, 

597 

Saxonian  stage,  775,  784 

223 

Salisbury,  R  D,  533,  771,  774 

Scanian  stage,  775 

Sedimentary  rocks,  disturb- 

Salix, *737 

Scaphites,  *7i5,  717 

ances     of,    432  ;     joints, 

Salmon,  718 

Scarboro  formation,    777 

371 

Salopian  series,  578 

Scaur      Limestone      series, 

Sediments,  consolidation  of, 

Salt,    187,    222,    226,    278; 

610,  613 

276;      continental,      181; 

deposition  of,  224,  *225; 

Scelidosaurus,  696 

marine,  181;   nonmarine, 

desert,  242;    rock,  308 

Schimper,  726 

181;  original  horizontal- 

Salt  Lake,  219,  222,  225 

Schist,  *404,  515;  chlorite, 

ity   of,    324;    original   in- 

Salt lakes,  205,  220,  242 

421;  graphite,  421;  horn- 

clination, 325,  363;  river, 

Salt  Range,  643,  661 

blende,   421;   mica,   408, 

203 

Salton  Sink,  136,  220,  222 

412,   416,   *421;   quartz, 

Sedimentation,      horizontal 

Salts,  278 

420;  talc,  421 

changes  in,  321,  *322;  in 

San  Francisco  earthquake, 

Schistosity,  412 

shoal-water,       318;       ia 

*38,  *47,  *48 

Schists,     crystalline,      420; 

oceanic  abysses,  318;  ver- 

Sand,   104,    107,    136,    199, 

jointing  of,   476 

tical  changes  in,  *32o 

207,  279,  304,  306,  417; 

Schizambon,  *57i 

Segregation  in  strata,  324 

beach,    172,   304;   calca- 

Schizoneura, 649 

Seismic  regions,  *39,  *4o 

reous,    259,    317;    coral, 
263;  cross-bedded,  *254; 

Schizopoda,  603 
Schlcenbachia,  717 

Seismicity,  40 
Seismograph,  37 

desert,  242,  304;  ejected 

Schoharie  Creek,  capture  of, 

Selachii,  589,  606,  635,  652, 

by  earthquakes,  43,  427; 

487 

690,  718 

felspar,  247;  glacial,  229; 

Schoharie  stage,  590,  594 

Selenite,  20,  308 

green,  245,  269;  in  deep 

Schrcederoceras,  *574,  575 

Selkirk  Mts,  514 

sea,    269;    littoral,    245, 

Schuchert,  C,  563,  565 

Selma  stage,  702,  708 

246;  olivine,  247;  ripple- 

Schwagerina,  629 

Seminula,  *633 

marked,  *248;  river,  200, 

Scoriaceous    texture,     285, 

Semionotus,  673 

*202,    304;    shoal-  water, 

288 

Senecan  series,  590,  596 

245,  254,   273;   volcanic, 

Scoriae,  72,  73,   76,   78,  81, 

Senonian  series,  702 

247  ;     wind-blown,     189, 

83,  293,  396',  407 

Septarium,  323 

3°4,  3J7 

Scorpions,  588,   632 

Sequoia,  736,  756,  765 

Sand-bars,  river,  201 

Scott,  D  H,  628,  669 

Serapeum,  30,  *3i 

Sand-craters,  45 

Screw-pines,  736 

Sericite,  15 

Sand-grains,     beach,     317; 
blown,  190,  317;  desert, 

Scythic  series,  658,  663 
Sea,  as  place  of  accumula- 

Series, geological,  530 
Serpentine,    19,    129,    30C 

242;  river,  317 

tion,  243;  cross  bedding 

418 

Sand-grouse,  756 

in,    255;   destruction   by, 

Shale,  3O6,  311,  314,  320 

Sandstone,    105,    109,   304, 

167,    440  ;    precipitation 

321,     406,       416,     *426: 

306,  311,  316,  321,  *388, 

in,  309 

arenaceous,    306;      bitu- 

*389 ;    argillaceous,    304, 

Sea-caves,  493;  -cliff,   493; 

minous,     306;     jointimr, 

306;  cross-bedded,  *255; 
decomposition     of,     105, 

-coasts,  492;  -floor,  pre- 
cipitous, 41 

*373>        *3795        rippk- 
marked,     *25o;      saline 

*io6;    joints,  371;  meta- 

Sea-lilies,  see  Crinoidea 

308 

morphism    of,    408,  416, 

Sea-urchins,  see  Echinoidea 

Sharks,  see  Selachii 

420;      micaceous,      304; 

Sea-water,  chemical  action, 

Shastan  series,  702 

modern,  266,  276;  of  un- 

*i7£. 124 

Shattuck,  G,  762 

812 


INDEX 


Shawangunk  grit,  565,  582 

oozes,    313;    precipitates, 

cene,      772;        Pliocene! 

Shearing-planes,    375,  412, 

309  ;  rocks,  304 

764;    river  deposits,  203, 

425 

Silicic  acids,  12 

205,    242;   Silurian,    584; 

Sheets,      contemporaneous, 

Silicon,  6,  12 

Triassic,  666 

389,    396;    igneous,    433; 

Sill  tunnel,  137 

South  Shetland  Ids,  754 

inclusions  in,  396;  inter- 

Sills,  293,    394,  *398,  *399, 

Sphagnum,  197,  198 

bedded,     389;     intrusive, 

403,  407;  fusion  of  strata 

Sphenophyllales,  600,  *626, 

see  Sills;  lava,  389 

by>    395  ;     topographical 

638 

Shelf,  continental,   267 

effects,  460,  *46i 

Sphenopteris,  *64g 

Shell,    of   cementation,    98, 

Silurian,  531,  547,  560,  578, 

Spatangoidea,  686 

126,     127;      of    flowage, 

597,     598,    610;     Lower, 

Spiders,  547,  632 

359,     376;    of    fracture, 

560;    Upper,  578 

Spirifer,    *585,    588,    *6oi, 

360,     376;     of     fracture 

Silver,  428 

603,  *63<D,  *633,  634 

and     flowage,     360;      of 

Simeto  R,  141 

Spiriferidas,  588 

weathering,  98,   126,   127 

Sinclair,  W  J,  370,  378,  379, 

Spiriferina,  688 

Shell-banks,  259,  310;  -lime- 

475 

Spits,    lake,    218;     marine, 

stone,   *258,   312;  -marl, 

Sink-holes,  *i27 

253 

220,    311;     -rocks,    194; 

Sinter,  calcareous,  307;  sili- 

Spitzbergen, 599,  623;  frost 

-sand,  190,  258,  264 

ceous,  192,  3O9 

action,  115 

Shells,  of  lithosphere,  98 

Sivatherium,  766 

Sponge-spicules,.267,  313 

Shenandoah  R,  488 

Siwalik  Hills,  Pliocene,  764 

Spongida,    555,  '  570,    586, 

Shenandoah  peneplain,  513 

Skaptar  Jokul,  79,  82 

602,  629,  686,  716 

Sheridan  stage,  782 

Slate,   105,  107,   408,    416, 

Spring  deposits,  *191,  307 

Shingle,  246,  3O5 

420,  421;  cleaved,  411 

Springs,    124,    131;    chaly- 

Shirane, 55 

Slickensides,   343  ;   horizon- 

beate,     192;     deposition, 

Shoal  water,  243  ;  deposits, 

tal,  *353J  vertical,  *342, 

307;    fissure,    132,    *i33; 

245,  252,  266,  507 
Shore-lines,  lake,  176,  219 

*343 
Slip  of  strata,  *363,  364 

ferruginous,  276;  hillside, 
131,  *i32;  mineral,  194; 

Shrinkage  cracks,   249 
Shrinkage  of  earth,  368 

Slope  of  fault,  340 
Smith,  J  P,  663,  683 

thermal,  133,  430 
Spruces,  735 

Sicilian  stage,  724 

Smith,  Wm,  677 

Spurr,  J  E,   126,  338,  423, 

Siderite,  21,  520 

Snake  R,  483;  484,  485 

429 

Sierra    Nevada,    164,    464, 

Snakes,  718,  723,  738 

Squirrels,  747,  758 

*46s,     506,     510,    -514; 

Snow  line,  149;  slides,  149; 

Stacks,  384,  493 

batholiths,      401  ;      com- 

structure, 151 

Stage,  geological,  530 

pression,  505;    elevation, 

Soapstone,  19 

Stalactite,  195,  307 

681,       752,       759,      763, 
783  ;      fault  -scarps,    464, 

Soda,    104,    136,    278,    296, 
297;  -granite,  296 

Stalagmite,  195 
Stanton,  T  W,  680 

*4§5;    fault-valleys,  467; 

Sodium,  6,  294;  carbonate, 

Starfishes,  see  Asteroidea 

frost      action    in,      115; 

187,  226;    chloride,  226; 

Stauroceph^.lus,  *585,  587 

folding     of,      707;      gla- 

sulphate, 187,  226 

Steam,  288;  in  metamorph- 

ciers,    157,    *i58;     ther- 

Soil,   103,  *io6,    107,  186, 

ism,    414;    volcanic,    55, 

mal  springs  of,  134 

240,        242,      27.8,      279, 

60,    62,    63,    64,    65,    86, 

Sierra  Teras,  45 

306,    618,    619 

89,  90,  95,  430 

Sigillaria,   *626,   627,   648, 

Solar  system,  origin  of,  86, 

Steatite,  19 

649,  668 

88,  532  _ 

Stegocephalia,     636,     652, 

Silica,  12,  71,  104,  105,  276, 

Solidification,  magmatic,287 

*653,  656,  674 

277,  287,   288,  289,    292, 

Solution,  magmatic,  289 

Stegosaurus,  696 

293,  294,   296,    298,   313, 

Sonora  earthquake,  45 

Stenotheca,  *554 

407,  416,  520;  amorphous, 

Sorting  power,  of  water,  182, 

Step-faults,  347,  *348,  466 

194,      309;     chalcedonic, 

183,    245,    302,    319;    of 

Stephanoceras,  680 

194;    hydrated,    12,    194; 

wind,  182,  183,  319 

Steppe  fauna,  786' 

in  sea-water,  272 

South    America,    Archaean, 

Stereosternum,  652 

Silicates,  12,  103,  174,   297; 

539;  Carboniferous,  624; 

Stictoporella,  *57i 

complex,  302  ;  decomposi- 

Cretaceous,     711;      De- 

Stigmaria, 627 

tion  products  of,   17;  f  er- 

vonian,      599  ;       Eocene, 

Stocks,   400;  topographical 

ro-magnesian,   289,    294- 

734;         Jurassic        68  1; 

effects,  460 

298 

Miocene,     754;     Ordovi- 

Stomatopoda,  687 

Siliceous,  deposits,  191,  220, 

cian,  567;  Paleocene,  728; 

Stomatopora,  *57i 

303,  3O4;  materials,  302  ; 

Permian,     645;    Pleisto- 

Straparollus, *6j3 

INDEX 


813 


Stratification,  182,  319; 
irregular,  202,  322;  of 
deltas,  213;  of  estuarine 
deposits,  274;  of  flood- 

E'ain    deposits,    205;     of 
ke     deposits,     218;     of 

marine      deposits,      245 ; 

of    river    deposits,     202 ; 

of  tuffs,   301;   planes  of, 

182;  regular',  205,  322 
Stratified  crystalline  rocks, 

227 
Stratified  rocks,  5,  182,  318; 

disturbances  of,  277,  324; 

joints    of,    476;     relative 

ages  of,  321 
Stratum,  319 
Streams,  see  Rivers 
Stream  tin,  428 
Streptelasma,  572 
Streptis,  588 
Striae,   glacial,   *i6o,   *i6i, 

*344;       of     slickensides, 

*342,  *343,  35T>*353 
Strike,  of  strata,  326,  *32S, 

329  ;  of  veins,  423 
Strike-faults,  345,  346,  *348, 

35° 

Stringocephalus,  603 
Stringocephalus  Limestone, 

59° 

Strobilospongia,  *56g 

Stromatopora,  555,  586 

Stromboli,  55,  63,  75,  79, 
285 

Slrombus,  756 

Stropheodonta,  *6oi,  603 

Strophomena,  *57i,  573 

Stropho stylus,   *585,    589 

Styliolina,  604 

Stylodon,  699 

Stylonurus,  587,  603 

Subaerial,  agents,  442  ;  part 
of  coast,  492,  493 

Sub-Aftonian  "stage,  775, 
776,  784 

Subcrustal  magma,  52,  367, 
403 

Subjacent  bodies,  404 

Sub-lacustrine  river  chan- 
nels, 141 

Submarine,  earthquakes, 
41 ;  part  of  coast,  492,  493 ; 
river  channels,  140;  vol- 
canoes, 53 

Subsidence,  effect  on  deltas, 
210,213;  effect  on  littoral 
deposits,  248 

Subsoil,  107,  186 

Substage,  geological,  530 


Substitution,         molecular, 

424, 520 
Subterranean  agencies,   26, 

28,    401,    4335    streams, 

135 

Succession        of        organic 

groups,   524,  525 
Suess,  E,  469,  645 
Suessonian  stage,  724 
Suphides,  enriched,  431 
Sulphur,  6 
Sulphur  dioxide,  82 
Sulphuretted  hydrogen,  82 
Sumatra,  56;   volcanoes,  54 
Sumatran  earthquake,  47 
Summary,     of     destructive 
action,     180;     of    recon- 
structive  processes,   278; 
of    subterranean    agents, 
95;  of  structural  geology, 

432 
Sun  cracks,  *206,  241,  249, 

*253,  255,  274,  278,  507 
Superficial  agencies,  26 
Superheated     water,     409, 

414;  deposition  from,  394 
Superposition,      order      of, 

321,  521,  525  -> 
Surface  agencies,  26,  97 
Susquehanna   R,  482,   490, 

Swamp,  Great  Dismal,  197, 

"=198 
Swamp  deposits,  196;  water, 

194 

Swamps,  ancient,  320,  617 
Sycamores,  735 
Syenite,   293,  297;   family, 

293,  296;  -obsidian,  297; 

-porphyry,  293,  296 
Synclines,   327,   *328,   333, 

358,  384,  *4io,  436,  456, 

457,    478,    510;     faulted, 

349;      joints     in,      374; 
Synclinorium,  *33O 
Syndyoceras,  757 
Syringopora,  586 
Syringothyris,  *63o,  634 
System,   geological,   530 
Systemodon,  739 
Systems  of  crystal  forms,  7 

Table  mountains,  451,  459 
Table  Mountain  sandstone, 

584,  599 

Tachylyte,  293,  299 
Taconic  Mts,  567;  system, 

5°4 

Taeniodonta,  729 
Taniopteris,  668 


Talc,  n,  18,  129 

Talchir  stage,  643 

Talus,  *m,  113,  *ii5,  117, 

187,  278,  317,  452,   510; 

in  deserts,  242,  317 
Tancredia,  *685 
Tapirs,  739,  741,  744,  787 
Tarr,  R  S,  46 
Tataric  stage,  642,  654 
Taxites,  684 
Tejon  series,  724,  730 
Teleosaurus,  694,  719 
Teleostei,     608,    656,     691, 

718 

Telerpeton,  674 
Temboro,  82 
Tennessee    R,    history   of, 

490 
Tension,  of  rocks,  345,  351, 

36S»    366»    367J    genesis, 

365 

T entaculites ,  *585,  589 
Terataspis,  *6oi,  603 
Terebratella,  *7i5,  717 
Terebratula,  634,  671 

*7IS.  71; 
Terebratulidae,  603 
Termites,  179 
Terraces,     cut    and    built, 
217;      lake,      176,     217, 
*2i9;    river,   *i89,    207, 
*2o8,     *2O9,   443;     rock, 
210;   sea-cut,    171,  *i72, 
441 

Terrestrial  deposits,    186 
Terrigenous  deposits,   245, 

257,  269;  minerals,   270 
Tertiary,  531,  723,  724 
Testudinata,  see  Turtles 
Tetrabelodon,  757 
Tetrabranchiata,  575 
Tetracoralla,  572,  655,  670 
Tetragonal  system,  7 
Tetragraptus,  *57i 
Texture,  74,  284,  288 
Thalattosuchia,  695 
Thames,  estuary,  210 
Thanetian  stage,  724 
Thecidium,  671 
Thecosmilia,  686 
Theosodon,  *76o 
Theralite,  293 
Theriodontia,  654,  676 
Therocephalia,  676 
Thetys,  645,  661,  683 
Thlaodon,  721 
Throw,     45,.  *339,     *340, 
341;      horizontal,     *339J 
stratigraphic,  *339,  *34o. 
341 


814 


INDEX 


Thrust,  experimental,  *365 

Thrusts,  *i2g,  344,  345, 
354,  413,  432,  *47i,  505, 
508;  cause  of,  364;  fold, 
345,  *355,  *356;  scission, 
345.  355;  surface,  345, 
*357,  *358;  topographi- 
cal effects  of,  471 

Thujites,  684 

Thuringian  Forest,  466 

Tide,  173,  210,  243,  274 

Till,  232,  770 

Tillodonta,  729,  741 

Tilting  of  fault-blocks, 
.367 

Timber-line,  510 

Time,  classification  of  ge- 
ological, 530;  divisions 
of,  528 

Tin,  427;  oxide,  428 

Tirolic  series,  658,  663 

Tirolites,  673 

Titanichthys,  607 

Titanium,  6,  429 

Titanotheres,  739,  741,  744, 
745,  +746,  747 

Titanotherium,  745 

Tittmann,  94,  95 

Tongrian  stage,  724 

Tongues,  plutonic,  400 

Topaz,  ii 

Topography,  construc- 

tional, 436,  437;  deter- 
mined by  arrangement 
of  rocks,  436,  451;  effect 
on  deposition,  186;  gla- 
cial, *438,  443;  tectonic, 
436,  456;  volcanic,  436, 

*437»  45Q 
Topset  beds,  213 
Toronto   interglacial    beds, 

Torosaurus,  *720 

Torrejon  stage,  724,  728 

Tortonian  stage,  724 

'loxaster,  717 

Toxodontia,  758,  787 

Trachyceras,  673 

Trachyte,  293,  297;  -por- 
phyry, 293 

Tracks,  of  land-animals, 
206,  251,  275;  of  worms, 

*256 

Transportation,  97,  98; 
183,  447;  glacier,  164; 
ice,  1 66;  river,  144; 
wind,  123 

Transporting  power  of  run- 
ning water,  144,  199 

Trap,  299 


Traquair,  588,  589,  606,  607 
Travertine,  191,  *ig2,  3O7, 

Turtles,  675,  692,  718,  723 
738 

*3o8 

Tuscaloosa  series,  702 

Tree-line,  509 
Tremadoc  series,  560 

Twin,  polysynthetic,  n 
Twinning  of  crystals,  1  1 

Trematis,  *57i 
Trematonotus,  *$85,  589 

Tylopoda,  745 
Tylosaurus,  *ji() 

Tremolite,  16 

Typhis,  *765 

Tremors,  of  earth,  37 
Trenton   stage,    560,    564, 

Typotheria,  758,  787 
Tyrannosaurus.  720 

,597 

Tyrrell,  J,  778 

Irwrthrus,  573,  *576 

Triassic,  531,  637,  642,  648, 

Uinta   Mts,   452,  505,  506, 

655,  657,  682 

542,  564,  706;    elevation 

Tributaries,  480;  extension 

of,  711 

of,  486;   subsequent,  485 

U  intacrinus  ,  716 

Triceratops,  *72O 

Uinta  stage,  724,  742,  744 

Triclinic  system,  7 

Uintalherium,  741,  744 

Triconodon,  699 

Uitenhage  beds,  713 

Trigonia,  *68s,  *688 

Ulrich,  E  O,  563,  565 

Trigonoccras,  634 

Ulsterian  series,  590 

Trigonolestes,  739 

Uncompahgre  R,  485 

Trilobita,  547,  555,556,572, 

Unconformities,    526,    527; 

*5?6,  587,  602,  631,  650, 

obliteration  of,  505,  54? 

656 
Trimerellidae,  588 

Unconformity,   376;    angu- 
lar,   *377,    *378,     *379, 

Trimetric  system,  7 

*38o;  without  change   of 

Trinity  stage,  702,  703 

dip,  380,  *38i 

Trinucleus,  573,  *576 

Underclay,  618,  619 

THplecia,  *57i 

Underground     waters,     de- 

Tripoli powder,  220 

struction  by,  124,  1  80 

Tristan  d'Acunha,  53 

Undertow,  247 

Trochoceras,  *s85,  589 

Undina,  691 

Trocholites,  577 

Upper    Barren    Measures, 

Trochonema,  *574,  575 

638 

Tropical    seas,  deposits  in, 

Upthrow,    340,    347,    351, 

274 

354,  366,  469 

Tropics,     continental     de- 

Upwarp, 29,  365,  483 

posits,  242,  279;  glaciers, 

Utica  stage,  560,  564,  597 

151;   rain  action  in,   104; 

soil-  water,  187 

Val  d'Arno  stage,  764 

Tropidoleptus,  *6oi,  603 

Valentian  series,  578 

Tro  piles,  *6j2,  673 

Valley  glaciers,  *i5o 

Trough-fault,     *346,     347, 

Valley  train,  234,  771 

384,  467 

Valleys,  anticlinal,  457,  458; 

Tufa,  calcareous,  *224,  307 

canoe-shaped,    328;     de- 

Tuffs,   62,    81,    277,    3O1, 

termined  by  faults,  467; 

387,     390,     406;      meta- 

dip,  455;   drowned,   501; 

morphism    of,    420;     rq- 

glacial,    162,   *i63,   444; 

crystalLzation     of,     296; 

glaciated,  497;    hanging, 

subaqueous,  301 

163,  444;     in  soft  rocks, 

Tulip  Trees,  765 

455,     482;     longitudinal, 

Tully  stage,  590,  595 

455,  478,  482,  487,  499, 

Tundras,  241 

504;     of     folding,     499; 

Tunnels,  lava,  *72,  74 

over-deepened,  497;  river 

Turbarian  stage,  775 

made,   139,   140;    strike, 

Turbo,  *765. 

455;      submarine,     496, 

Turonian  series,  702 

500;     submarine   glacial, 

Turrilites,  718 

164;      submerged,     494; 

Turtle  Mt,  *iagt  *i^° 

synclinal,  436,  456,  478; 

INDEX 


8lS 


tectonic,      469  ;       trans- 

Volcanic rocks,    284,    385; 

verse,  455,    482;    trench- 

Cambrian,  552  ;  Cenozoic, 

like,  442;   U-shaped,  162, 

722;   Carboniferous,  621, 

*i6v,    wide,  443 

623  ;      Cretaceous,      707, 

Van  Hise,  C  R,  27,  98,  413 

711,712;  Devonian,  597, 

Van   Ingen,    G,    138,    204, 

599;    Eocene,   741;   land 

211,  253,  372,  376,  389, 
396,  468 
Vapours,  in  plutonic  rocks, 

forms  in,  459;    Miocene, 
749,  752;  Oligocene,  743, 
Ordovician,      566;      Per- 

407 ;  magmatic,  249  ;  min- 

mian, 641,  645;  Pleisto- 

eralizing, 287,   288,  407; 

cene,  762,  763;  Pliocene, 

volcanic,  70,  82 

783;  Silurian,  583;  Trias- 

Vegetable      accumulations, 

sic,  665 

196;  in  temperate  regions, 

Volcanoes,    52,    286,    385; 

241;  in  tropics,  242,  279 

distribution,    52;      erup- 

Vegetable matter,  197;   de- 

tions, 54;    intermittency, 

cay  of,  314 

86,    90;     new,    66;   sub- 

Vegetation,   protection    by, 

marine,  53,  65,  273 

100,  108,  in,  176,  *i77, 

Volsella,  *68s 

*  1  7  8  ;     topographi  cai   ef- 

Voltzia,  649,  *66g 

,     fects,  445 

Valuta,  717 

Veins,  banded,  423;    com- 

Volutolithes, *737 

plex,    424;    fissure,    423, 
425;    gold-bearing,    431; 

Vosges  Mts,  466 
Vulcanism,  28 

granite,   *395;    intrusive, 

Vulcano,  79 

*394,    *395,    399,     400; 

Vultures,  738 

metalliferous,   426,    429, 

433  >  metalliferous,  forma- 

Waagenoceras, *633,  651 

tion  of,  430;  metamorphic, 

Waders,  721,  738 

408,   425;     mineral,    194, 

Walchia,  649,  669 

423,  433;   mineral,  alter- 

Walcott, C  D,  541 

ation  by  weathering,  43  1  ; 

Waldheimia,  687 

of  replacement,  424;  sedi- 

Wales, Ordovician  of,  560; 

ment-filled,  *426 

Silurian  of,  578 

Vein-stuff,  426 

Walnuts,  735,  754,  765 

Velocity   of    streams,    136, 

Walther,  J  W,  242 

i37,  i99 

Warping,  29,  374;  effect  on 

Veniella,  *7i5,  717 

drainage,  490 

Verbeek,  82 

Warren  R,  779 

Vertebrata,    547,   577,   589, 

Warsaw  substage,  610 

604,  633,  652,  673,  690, 

Wasatch     Mts,     504,  542, 

718 

564,    _  597,      679,      706; 

Vesicular  texture,  285 

elevation     of,    711,     763, 

Vesuvius,   50,   52,   55,   *64, 

783;      fault-scarp,     464; 

65,  *7o,  83,  *85,  763. 

thickness   of   beds,     505 

Vicksburg  stage,  724 

Wasatch  stage,    724,    731, 

Victoria    Falls,    144,    472, 

739. 

473,  *474 

Washita  stage,  702,  703,  706 

Vitulina,  *6oi,  603 

Waste  of  land,  annual,  180 

Vogt,  J  H,  428 

Water,    8;   erosion   in  des- 

Volborthella, 559 

erts^     447,      449;       ex- 

Volcanic activity,  causes  of, 

pansion  on  freezing,  113; 

86 

penetration  of,  in  earth, 

Volcanic,    cones,    83;     de- 

J25 

nudation,    84,    386,   459; 

Waterfalls,  480 

earthquakes,  42,  50;  lakes, 

Water  Hogs,  787 

57,  215;  necks,  385,  *386, 

Water-lime  substage,  582 

pipes,  *387,  388  ;  plateaus, 
85,  129,    436,  459;  prod- 

Watershed, 480 
Waters,  alkaline,  192,   194; 

ucts,  69,  273,  286,  300 

magmatic,  430  ;  meteoric, 

430;   swamp,  194;    ther- 
mal, 128,  433 
Wave    erosion,    167,    *i68, 

*i6q,   *i7o,   *i7i,  *i75, 

493.  501 

Wave  marks,  248,  *25i 
Wave  motion  in  solids,  38 
Waverly  series,  613 
Waves,  compressional,   38; 

distortions!,  38;  normal, 

38;     of    distortion,      38, 

93;   transverse,  38 
Wealden,  702,  712,  720 
Weasels^  745,  747,  757,  758 
Weathering,    100;   shell  of, 

98 
Weight,    consolidation    by, 

276 

Wenlock  series,  578,  583 
West    Indies,  earthquakes, 

40;    raised  beaches,  32; 

volcanoes,  54 
Whales,  758 
White,  D,  615 
White,  I  C,  320,  649 
White  ants,  179 
White  Mts,  landslip,  131 
White  River  stage,  724,  742, 

Whitfieldella,  588 
Wichita  Mts,  566 
Wichita  stage,  637,  639,  642 
Wild  Dog,  788 
Williams,  H  S,  594 
Willis,  6,188,328,331,355 

363,  364,  366,  513 
Wflliston,  S  W,  680 
Willows,  716,  735,  736 
Wind   deposits,    188,    241; 

erosion,    119,    180,   447, 

448,  449,  510 
Wind-gaps,  488,  489 
Wind  transportation,  183 
Wind     River     stage,    724, 

731,  739 

Windsor  stage,  613 
Wisconsin  stage,  775,  778 
Witteberg  beds,  624 
Wood,  315 
Woodcock,  738 
Woodpeckers,  756 
Woodward,  A   Smith,  605 

606,  607,  690,  691 

Xenoliths,  *404,  405 
Xiphodonts,  747 
Xiphosura,  587,  632,  687 

Yarmouth  stage,  775,  777 
Yellow  Sea,  269 


8i6 


INDEX 


Yellowstone      Lake,      487; 

River,    129,  216,  487 
Yews,  736 
Yoredale  scries,  610 
Yosemite  Valley,  117,  *i20, 

*47S 

Young,  R  B,  161,  644 
Youth,  in  arid  cycle,  446; 

in  topography,  "438 


Yucatan  Bank,  259 
Yucatan,  modern  limestone, 
260 

Zacanthoides,  *557 
Zambesi,   gorge,    144,    472, 

*473 

Zamites,  669 
Zanclodon,  675 


Zechstein,  641,'  642 

Zeolites,  18 

/.euglodon,  741 

/Vine,  427;  ores,  429,  433 

Zittel,  K  v,  696 

ZHK-lclla,  *569 

Zoisite,  17 

Zone,  geological,  530 

Zygospira,  *s7i,  575 


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